156 Evolutionary Anthropology

ARTICLEA Theory of Human Life History Evolution:
Diet, Intelligence, and Longevity
HILLARD KAPLAN, KIM HILL, JANE LANCASTER, A. MAGDALENA HURTADO

Human life histories, as compared to those of other primates and mammals,
have at least four distinctive characteristics: an exceptionally long lifespan, an
extended period of juvenile dependence, support of reproduction by older postreproductive individuals, and male support of reproduction through the provisioning of females and their offspring. Another distinctive feature of our species is alarge brain, with its associated psychological attributes: increased capacities forlearning, cognition, and insight. In this paper, we propose a theory that unites andorganizes these observations and generates many theoretical and empirical predictions. We present some tests of those predictions and outline new predictions
that can be tested in future research by comparative biologists, archeologists,
paleontologists, biological anthropologists, demographers, geneticists, and cultural anthropologists.

Hillard Kaplan is Professor at the University of
New Mexico. His recent research and publications have focused on integration of life history theory in biology and human capital theory in economics, with specific emphases on
fertility, parental investment, and aging in developed, developing, and traditional settings.
He has also conducted fieldwork with native
South Americans and southern Africans. Email: hkaplan@unm.edu.
Kim Hill is an Associate Professor at the University of New Mexico. He studies humanbehavioral ecology with a focus on life history
theory, foraging patterns, sexual division oflabor, food sharing, and the evolution of cooperation. He has carried out fieldwork in fivedifferent South American hunter-gatherer ortribal horticulturalist populations in the past
23 years. E-mail: kimhill@unm.edu.
Jane Lancaster is Professor of Anthropology
at the University of New Mexico. Her research and publications are on human reproductive biology and behavior especially human parental investment; on women’s
reproductive biology of pregnancy, lactationand child-spacing; and on male fertility andinvestment in children. She edits the quarterly
journal, Human Nature, which publishes research in human evolutionary ecology.
E-mail: jlancas@unm.edu.
A. Magdalena Hurtado is Associate Professor
in the Department of Anthropology, University of New Mexico. She has done research
on a wide range of problems in human behavioral ecology and evolutionary medicine
among the Ache, Hiwi, and Machiguenga of
Lowland South America. She is Co-Directorof the Native Peoples and Tropical Conservation Fund, University of New Mexico. Email: amhurtad@unm.edu.

Our theory is that those four life
history characteristics and extreme
intelligence are co-evolved responses
to a dietary shift toward high-quality,
nutrient-dense, and difficult-to-acquire food resources.
The following logic underlies our
proposal. First, high levels of knowledge, skill, coordination, and strength
are required to exploit the suite of highquality, difficult-to-acquire resources
humans consume. The attainment of
those abilities requires time and a significant commitment to development.
This extended learning phase, during
which productivity is low, is compensated for by higher productivity during
the adult period and an intergenerational flow of food from old to young.
Because productivity increases withage, the investment of time in acquiring
skill and knowledge leads to selection
for lowered mortality rates and greater
longevity. The returns on investments
in development occur at older ages.
This, in turn, favors a longer juvenile
period if there are important gains in
productive ability with body size andgrowth ceases at sexual maturity.
Second, we believe that the feeding

niche that involves specializing onlarge, valuable food packages promotes food sharing, provisioning ofjuveniles, and increased grouping, allof which act to lower mortality during
the juvenile and early adult periods.
Food sharing and provisioning assist
recovery in times of illness and reducerisk by limiting juvenile time allocation to foraging. Grouping also lowers
predation risks. These buffers against
mortality also favor a longer juvenile
period and higher investment in othermechanisms to increase the life span.
Thus, we propose that the long human life span co-evolved with lengthening of the juvenile period, increased
brain capacities for information processing and storage, and intergenerational resource flows, all as a result of
an important dietary shift. Humans are
specialists in that they consume only
the highest-quality plant and animal resources in their local ecology and rely
on creative, skill-intensive techniques to
exploit them. Yet the capacity to develop new techniques for extractive foraging and hunting allows them to exploit a wide variety of different foods
and to colonize all of earth’s terrestrial
and coastal ecosystems.
We begin with an overview of thedata on which the theory is based: acomparative examination of huntergatherer and chimpanzee life-history
traits and age profiles of energy acquisition and consumption. The data
show that hunter-gatherers have alonger juvenile period, a longer adult
lifespan, and higher fertility thanchimpanzees do. Hunter-gatherer children are energetically dependent onolder individuals until they reach sexual maturity. Energy acquisition rates
increase dramatically, especially forEvolutionary Anthropology 157

ARTICLEman foragers and chimpanzees; Table
1 compares a variety of life-history
traits of the two species. The huntergatherer data come from studies on
populations during periods when they
were almost completely dependent on
wild foods, having little modern technology and no firearms, no significant
outside interference in interpersonal
violence or fertility rates, and no significant access to modern medicine.
The chimpanzee data are compiled
from all published and unpublishedsources that we are aware of. Becauseof small sample sizes at individualsites, mortality data were combined tocreate a single synthetic life table andsurvival function that combines alldata for wild chimpanzees.8

Figure 1. Survival curves for forager populations were derived from sources listed in notes fortable 1. Chimpanzee mortality is from a synthetic life table combining all mortality data from
Bossou, Gombe, Kibale, Mahale and Tai.8

males, until mid-adulthood and stayhigh until late in life.
We then present both theoretical andempirical tests of our theory. For thetheory to be correct, a model of natural
selection must show that mortality
rates, the length of the juvenile period,and investments in learning co-evolvein the ways predicted by the theory.
Building on existing models of life-history evolution,1–5 we develop such amodel. The results of our analysis confirm the theory’s predictions. Those theoretical tests are followed by empiricaltests. In order for our theory to be correct, we must demonstrate that: Humans do, in fact, consume more skillintensive, difficult-to-acquire, highquality foods than do chimpanzees andother nonhuman primates; Difficulty ofacquisition explains the age profile ofproduction for both humans and chimpanzees; Men play a large role in supporting human reproduction; The foraging niche occupied by humans
lowers mortality rates among juveniles
and adults relative to corresponding
rates among chimpanzees and othernonhuman primates. We present strong
evidence in support of the first three
propositions and suggestive evidence insupport of the fourth.

We then examine the evolution of
the primate order to determine whether
the same principles invoked in ourtheory of hominid evolution explain
the major primate radiations. We thenconsider the fundamental differences
between our theory and the “grandmother hypothesis” recently proposed by Hawkes, Blurton Jones andO’Connell.6,7 We conclude with a listing of the new and unique predictions
derived from our theory.
Our theory is not the first to proposethat high-quality foods, extractive foraging, and hunting are fundamental tohuman evolution. However, it is thefirst to do so with a specific model ofnatural selection that unifies the evolution of life history, brain and intelligence, diet, and age profiles of food production and consumption. As a result, it
organizes existing data in a new way
and leads to a novel set of predictions.

DIFFERENCES BETWEEN THE
LIFE-HISTORY TRAITS OF
HUNTER-GATHERERS AND
CHIMPANZEES
Mortality, Fertility, and Growth
Figure 1 shows the differences between the life spans of traditional hu-

Despite the fact that thehuman juvenile andadult periods are longer
than those ofchimpanzees and thathuman infants are larger
than chimpanzee infants
at birth (about 3 kg
versus 2 kg), huntergatherer womencharacteristically havehigher fertility than do
chimpanzee females.

The data suggest that hunter-gatherer children have a higher rate of survival to age 15 (60% versus 35%) andhigher growth rates during the first 5
years of life (2.6 kg/yr versus 1.6 kg/yr)
than do juvenile chimpanzees. Chimpanzees, however, grow faster between
ages 5 and 10, both in absolute weightgain (2.5 kg/yr for chimps versus 2.1
kg/yr for humans) and proportional
weight gain (16% per year for chimps
versus 10% per year for humans) (Table1). The early higher weight gain for humans may be due to an earlier weaningage (approximately 2.5 years for hunter-gatherers versus 5 years for chimpanzees) and parental provisioning of158 Evolutionary Anthropology

ARTICLETABLE 1. LIFE HISTORY PARAMETERS OF HUMAN HUNTER-GATHERERS AND CHIMPANZEES
Probabilityof Survival
to Age 15

Group

ExpectedAge ofDeath at15 (years)

Mean Age at
First
Reproduction(years)Mean Age at
Last
Reproductionb(years)InterbirthIntervala(months)

Mean
WeightAge 5 (kg)

Mean
WeightAge 10 (kg)

15.7
15.5
15.5
14.2
18
16.4
14
16
15.7

25.9
27
20
21.2
29.8
33.6
19.5
22.5
24.9

10

21

10

24

10

22.5

HumansdAche female
Ache maleHadza femalee
Hadza male
Hiwi femalef
Hiwi male!Kung femaleg!Kung male
Forager meanc

0.61
0.71
0.58
0.55
0.58
0.58
0.6
0.56
0.60

58.3
51.8
54.7
52.4
51.3
51.3
56.5
56.5
54.1

19.5

42.1

37.6

20.5

37.8

45.1

19.2

37

41.3

39.0

41.3

19.7

Chimpanzees
Bossou femaleh
Bossou maleGombe
femalei
Gombe male
Kibale femalej
Kibale male
Mahalefemalek
Mahale maleTai female
Tai malel
Chimpanzee
mean

51
0.545

32.7

0.439
0.805
0.408

28.6
35.6
40.6

0.193
0.094
0.35

23.8
24
29.7

14.1

64.6

68
14.6

72

14.3

69.1

14.3

27.7**

66.7

aMean interbirth interval following a surviving infant.
Age of last reproduction for chimpanzee females was estimated as two years prior to the mean adult life expectancy.
c
The forager mean values were calculated by weighting each forager study equally. The chimpanzee mean mortality is from asynthetic life table using data from all five sites listed.8,138dAche: Demographic and weight data from Hill and Hurtado.10eHadza: Demographic data from Blurton Jones and colleagues.16 Weight data from Blurton Jones (personal communication).
f
Hiwi: Demographic data from Hill and Hurtado unpublished database collected on the Hiwi foragers from reproductive-history
interviews conducted between 1982 and 1991 using the same methodology published in Hill and Hurtado.10g!Kung: Demographic and weight data from Howell.83
hBossou: Data from Sugiyama.139
i
Gombe: Data on mortality from Hill and coworkers,8 and Pusey and Williams (personal communication). Gombe data on fertility
from Pusey,140 Tutin,141 and Wallis.142 Weights from Pusey.140
jKibale: All data from Wrangham (personal communication). Mortality data in Hill and coworkers.8kMahale: Data from Nishida, Takasaki, and Takahata.143lTai: Data from Boesch and Boesch.20bhighly processed foods. Among humans, the slow growth during middle
childhood is intriguing. According to
the allometric growth law, mammalian
growth can be described by the equation dw/dt ⫽ Aw0.75 (where change inweight per unit of time is expressed as afunction of a growth constant, A, andweight, w, to the 0.75 power). Most
mammals show a yearly growth constant of about 1, whereas the mean primate value for A is about 0.4.9 Huntergatherer children between the ages of 5
to 10 years are characterized by ex-tremely slow growth, with A being approximately 0.2.
Chimpanzees spend less time as juveniles than humans do: Female
chimpanzees give birth for the firsttime about 5 years earlier than dohunter-gatherer women. In natural
habitats, chimpanzees also have amuch shorter adult life span than humans do. At age 15, chimpanzee life
expectancy is an additional 15 years,
as compared to 39 more years for human foragers. Importantly, women
spend more than a third of their adultlife in a postreproductive phase,whereas very few chimpanzee females
survive to the postreproductive phase.
The differences in overall survival andlife span are striking (Fig. 1). Less
than 10% of chimpanzees survive to
age 40, but more than 15% of huntergatherers survive to age 70. These naturalistic observations are also consistent with data on maximum life spans.
The maximum life span of humans is
between 100 and 120 years, depending upon how it is calculated, which isabout two times longer than the max-

Evolutionary Anthropology 159

ARTICLEversus 2 kg), hunter-gatherer womencharacteristically have higher fertility
than do chimpanzee females. The
mean interbirth interval between offspring when the first survives to thebirth of the second is more than 1.5
times longer among wild chimpanzees than among modern huntergatherer populations. These numbers
lead to an interesting paradox. Life
tables from modern human foragers
always imply positive growth (see Hilland Hurtado,10 chapter 14), whereas
the chimpanzee numbers presented
here imply slightly negative population growth rates. Chimpanzee negative population growth may be a real

Adult men acquire
much more food than
do those in any otherage-sex category.
Although the patterns formen seem consistent for
all three societies,
Hadza children andpostreproductive
women appear toacquire substantiallymore food than do their
Ache and Hiwi
counterparts.

Figure 2. Daily energy acquisition data are recorded by individual among the Ache andHiwi. Thus, the age and sex of each acquirer is known for every day sampled. Mean
production for 5- or 10-year age intervals (y value) was calculated from raw data bysumming all calories produced over the sample period by individuals in that age-sex classand dividing by the total sample of person days monitored for individuals in that category.
This was plotted at the mean age of person days sampled (x value) in the category
analyzed. Hadza production levels are given for various juvenile age categories, for all
adult men combined (no age breakdown), and for all reproductive women and all womenof postreproductive age combined (no age breakdown). All values are calculated as
described in the notes for Tables 2 and 3.

imum adult chimpanzee life span (approximately 60 years for captive populations).
Despite the fact that the human ju-

venile and adult periods are longer
than those of chimpanzees and thathuman infants are larger than chimpanzee infants at birth (about 3 kgfeature of recent habitat destruction
and other human intrusion, or “natural” mortality rates may have been
overestimated due to the inclusion ofdeaths from viral epidemics such as
ebola and polio (see Hill and coworkers8 for a discussion).
To summarize, hunter-gatherers
have a juvenile period that is 1.4 times
longer than that of chimpanzees and amean adult life span that is 2.5 times
longer than that of chimpanzees. Theyshow higher survival at all ages after
weaning, but lower growth rates during middle childhood. Despite alonger juvenile period, slower growth,160 Evolutionary Anthropology

ARTICLEand a longer life span, hunter-gatherer women achieve higher fertility
rates than do chimpanzee females.

The Age and Sex Profile ofEnergy Acquisition

Figure 3. The mean expected daily energy consumption per individual for each age-sex
category of each foraging group was estimated by first multiplying all the age-sex specificproduction rates for that foraging group times the proportional representation of thatage-sex category as expected from the survival curves and summing expected production
across all age categories. This total expected production for the group was then divided by
the expected total number of individuals (as determined by the survival curve) at all ages
times their proportion of a standard consumer. This procedure assumes that all populations
are in steady state and that the proportional representation of each age category isdetermined by the probability of surviving to that age, and gives consumption per standard
consumer in each group. This gives the mean consumption for a standard consumer in thegroup. Daily expected consumption for individuals in various age-sex categories is estimated by multiplying the proportion of a consumer represented by each age class times
the mean consumption of a standard consumer. Kaplan17 provides a detailed descriptionof these calculations and how proportional standard consumers were determined for eachage-sex category. A standard consumption rate of 1 is assigned to young adult males andfemales; children begin at a consumption level of 0.3 that of a standard consumer. Daily
energy acquisition for age-sex categories is calculated as described for Figure 2 andaveraged across the Ache, Hiwi, and Hadza, weighting each group equally.

Data on food acquisition by age andsex category exist for only three modern foraging populations. Ache andHiwi food production was directly
monitored by weighing all food produced by those in different age andsex categories throughout mostmonths of various years. (See Hill andcoworkers11 and Hurtado and Hill12
for definitions, methodology, andsampling plan). Hadza women’s andchildren’s plant-food acquisition was
estimated indirectly from samples of
in-patch return rates for different fruitand root resources over various age orsex classes during part of the wet season and part of the dry season of various years. (For details, see Hawkes
and coworkers,6 and Blurton Jones,
Hawkes, and O’Connell13,14). These
data were combined with sample estimates of time spent foraging and frequency across days to estimate daily
food acquisition.13,14 Hadza men’s
food acquisition from hunting wasmeasured directly by weighing alllarge game brought to camp.15
Although there is some cross-cultural variation, all three societies
show similar patterns. Hunter-gatherer children produce little food compared to adults (Fig. 2). In the latejuvenile period, daily food acquisition
rates rise dramatically, especially formales. These rates continue to increase until mid-adulthood for males
in all three groups and even longer for
Hadza and Hiwi females. Adult men
acquire much more food than dothose in any other age-sex category.
Although the patterns for men seemconsistent for all three societies,
Hadza children and postreproductive
women appear to acquire substantially more food than do their Acheand Hiwi counterparts. But total foodconsumption estimates for the Hadza
may be unrealistically high, since thedata suggest per capita consumptionof about 3,400 calories per day (Tables
2, 3). That is 126% of the mean daily
caloric consumption of the Ache, despite the fact that 10-year-old HadzaEvolutionary Anthropology 161

ARTICLEchildren weigh only 78% of the weightof 10-year-old Ache children, andadult Hadza women weight only 89%of the weight of Ache women. Thoseestimates are derived by assuming an
age-structure consistent with the
Hadza life-table.16 If the dependency
ratio in the camps studied by the
Hadza researchers was greater than
expected by the life table, as Blurton
Jones (personal communication) believes this to be the case, the per-capita consumption estimates would be
reduced accordingly and would be
more realistic.
Figure 3 shows the mean daily energy consumption and acquisition
rates for all three hunter-gatherer societies as compared to the rates for
chimpanzees of the same age and sex.
The food-consumption rates of forager children and adults is estimatedfrom body weight and total group production.17 Chimpanzee energy acquisition, while not measured directly,
can be estimated from body size andcaloric requirements, since very little
food is transferred between age-sex
categories after weaning. Daily foodacquisition and consumption are virtually the same for chimpanzees fromthe juvenile period onward. The human consumption-acquisition profileis strikingly different from that ofchimpanzees, with chimpanzee juveniles acquiring considerably more energy than forager children do until
about the age of sexual maturity. No
children in any forager society produced as much as they consumed until they reached their mid- to lateteens. Thus, human juveniles, unlike
chimpanzee juveniles, have an evolutionary history of dependency on
adults to provide their daily energy
needs. This can be appreciated by realizing that by age 15 the children in
our forager sample had consumed
over 25% of their expected life-timeenergy consumption but had acquired
less than 5% of their life-time energy
acquisition.
The area in Figure 3 where foodacquisition is greater than consumption (where the solid line for eachspecies is above the dotted line) represents surplus energy provided during the later part of the life span.
These averaged data imply that
hunter-gatherer men provide mostof the energy surplus that is used tosubsidize juveniles and reproductive-aged women. Although based on
averaging only three societies, thistrend can be confirmed by comparing the food-acquisition rates ofadult males and females from a sample of ten hunter-gatherer societiesin which food acquisition has beenmeasured with a systematic sample(Table 2).

Our proposal is that theshift to calorie-dense,
large-package, skillintensive food resources
is responsible for theunique evolutionary
trajectory of the genus
Homo. The key elementin our theory is that thisshift produced coevolutionary selection
pressures, which, in turn,operated to produce
the extreme
intelligence, longdevelopmental period,
three-generational
system of resource
flows, and exceptionallylong adult life
characteristic of our
species.

A THEORETICAL TEST: WOULD
NATURAL SELECTION ACTUALLY
PRODUCE THE COEVOLUTIONARY EFFECTSPROPOSED BY THE THEORY?
Our proposal is that the shift to calorie-dense, large-package, skill-intensive food resources (Fig. 4) is responsible for the unique evolutionary
trajectory of the genus Homo. The key

element in our theory is that this shift
produced co-evolutionary selection
pressures, which, in turn, operated to
produce the extreme intelligence, longdevelopmental period, three-generational system of resource flows, andexceptionally long adult life characteristic of our species. We envision
two important effects of the change infeeding niche.
First, a long developmental period,
parental provisioning, and a large
brain are necessary foundations of theskill-intensive feeding niche, andtherefore are products of selection asa result of entry into that niche. Ourview is that human childhood is elongated by including a period of very
slow physical growth, during which
the brain is growing, learning is rapid,
and little work is done. This is followed by adolescence, during which
growth is accelerated so that the brain
and body can function together in thefood quest. Early adulthood is a timefor vigorous work during which resource acquisition rates increase
through on-the-job training. Thus, investment in this life history involves
three important costs: low productivity early in life, delayed reproduction,
and a very expensive brain to growand maintain. The return from thoseinvestments is delayed, with extremely high productivity occurring in
the middle and latter portions of theadult period. That return increases
with lengthening of the adult life spanbecause the return is realized over agreater period of time. Second, we
propose that the shift in the humanfeeding niche operated directly to
lower mortality rates because it increased food package size, which, inturn, favored food sharing, provisioning, and larger group size. Another indirect effect was that the added intelligence and use of tools associatedwith the feeding niche also loweredpredation rates.
Underlying our theory is the hypothesis that these two effects produce co-evolutionary processes oflarge magnitude. Holding all else constant, ecological changes that increase
the benefits of a long developmental
period and a concomitant increase inlater adult productivity not only produce selection pressures to delay theonset of reproduction, but also pro-

162 Evolutionary Anthropology

ARTICLETABLE 2. PRODUCTION OF ENERGY BY MEN AND WOMEN IN FORAGING SOCIETIES
Daily Adult Production in Caloriesa

Ongeb
Anbarrac
Arnhemd
Achee
Nukakj
Hiwig!Kung1,h!Kung2,i
Gwif
Hadzak
a

men
women
men
women
men
women
men
women
men
women
men
women
men
women
men
women
men
women
men
women

Mean Daily
Total% Total Adult
Calories% Total Adult
Protein

348

81
52
79
379
8
251
636
976
1500
0
156
82
974
3169

4000
1021
2742
1174
4578
2012
5590
1055
4556
2988
3489
916
3221
3864
6409

79.7
20.3
70.0
30.0
69.5
30.5
84.1
15.9
60.4
39.6
79.2
20.8
45.5
54.5
Ⰷ50

94.8
5.2
71.8
28.1
93.0
7.0
97.1
2.9
98.6
1.4
93.4
6.6
44.7
55.3

0
0
0
1304

0
3200
841
0

2412
3200
8089
4397

43.0
57.0
64.8
35.2

78.7
21.3
94.1
5.9

Meat

Roots

Fruits

Other

3919
0
2662
301
4570
0
4947
32
3056
0
3211
38
2247
0
6409

0
968
0
337
0
1724
0
0
0
0
2
713

0
1
0
157
0
37
6
47
0
2988
121
83

348

1612
0
7248
0

800
0
0
3093

Edible portion and caloric values were taken from individual studies when available. Otherwise we assumed vertebrate meatat 85% edible, the Ache measured average for animals, and used the following conventions for calories/100 g edible: mammals
150; roots 150; fruits 70; fish 120. When not specified, protein was assumed at 20% by weight for meat and 2% for roots and fruits.
bOnge: Data come from Bose.144 We assumed that all food is produced by adults and that men and women make up equal
percentages of the reported population. Caloric values (p. 156) and edible portions are taken from Meehan.145 We assumed
that males got all pigs, turtles, fish, and honey, whereas females acquired all crabs, bivalves, and plant products. Total caloric
intake seems very low, but the Onge are the smallest foragers in this sample and had very low fertility.
c
Anbarra: Data come from Meehan145; diet is found in Tables 29 –32. It is assumed that women collected 85% of shellfish (p. 125)
and that men obtained only birds, fish, mammals, and some shellfish (p. 149). Total person days of consumption are in eachtable. Women’s production days come from Table 27. We assumed an equal number of production days for men.
dArnhem: Arnhem land data are from McArthur150 (pp. 127–128 and p. 138). It is assumed that adults acquired all food, that men
obtained only vertebrate meat and honey, and that women acquired all other resources.
e
Ache: Data come from all observed foraging trips between 1980 and 1996 on which KH, HK, and/or MH were present. Data prior
to 1984 were published in Hill and coworkers.11 Subsequent data come from forest trips between 3 and 15 days long when nearly
all foods consumed were acquired from the forest. All foods were weighed on site and the edible portion was calculated fromrefuse samples collected after consumption. Caloric values were determined as previously published. Total production of fruits
was estimated by multiplying measured collection rates for different age categories times the time spent collecting by eachindividual. We have made two important modifications of 1984 data because of new field measures: 1) We now estimate theedible portion of wild honeycomb to be only 35% by weight; 2) The edible portion of palm starch is estimated at only 6% byweight, with the caloric value of the edible portion being 3,920 cal/kg. These corrections and new production data havelowered previously published estimates of daily caloric intake.
fGwi: Meat production per hunter day is averaged from Silberbauer’s146 one-year observations of a band including 20 men andTanaka’s147 180-day observations (p. 111) of 10 men. We estimate Silberbauer’s band to contain 20 men and 24 women
because there were 80 individuals, 46.5% of whom were male and 55% were adult (p. 286, 287). For live weight meat, we assume
85% edible weight containing 1,500 cal/kg. Plant production for adult women is estimated at the observed per-capita
consumption, 800 g/consumer day times 80/24 (the ratio of the total population to adult women) times 1,500 cal/kg raw plant,
times 80% collected by women147 (p. 70). Men are assumed to have produced 20% of the plant calories. Meat consumption percapita is the average from Tanaka147 (p. 70) and Silberbauer146 (p. 446). We assumed that Tanaka’s raw weights are 85% edible;
we also assume 1,500 cal/kg edible meat for both studies. Plant consumption is reported to be 800 g/person in both studies147(p. 70),146 (p. 199). We assume that this is equally split between roots and melons, with a mean caloric value of 1,500 cal/kg raw
weight. Man days hunting are reported for both studies, but calculations of the sample size of women’s production andper-capita consumption are not specified in either study.
gHiwi: Data come from a sample of days between 1985 and 1988 when KH and MH resided with the Hiwi and weighed all foodproduced by a sample of camp members. Details of calculations of edible portion and food value are published in Hurtado and
Hill.12,148h!Kung1: All data on adult production and per-capita consumption are from Lee42 (pp. 260 –271). Women’s plant production(non-mongongo) was assumed to be evenly split between roots and fruits.
i!Kung2: Data are from Yellen43 as calculated in Hill149 (pp. 182–183). Edible portion and caloric value are the same as in Lee.42
Only hunting data are recorded. In order to estimate per-capita consumption, adult men and women are assumed to compriseequal percentages of the band members. The percentage of the diet from meat is calculating assuming total consumption of
2,355 calories per person day, as per Lee.42

Evolutionary Anthropology 163

ARTICLETABLE 2. (CONTINUED)
j

Nukak: Data come from Politis151 (chapter 4). We assume that all food was produced by adults and that men and women makeup equal percentages of the population. Edible portions and caloric values for foods come from similar Ache resources. Fruits
show edible portions varying from 21% for fruits brought in and weighed with the stalk to 40% for fruits without the stalk collectedin baskets. Caloric values of fruits ranged from 600 cal/kg for sweet pulpy fruits to 1,430 cal/kg for oily palm fruits. Other resourceswere equivalent to common Ache and Hiwi resources.
k
Hadza: Data on the daily caloric production of children are from Blurton Jones, Hawkes, and O’Connell.14 We assumed that 61%of the calories produced come from fruit and the remainder from roots, as for youngest girls.6 Daily production of women takenfrom Hawkes, O’Connell, and Blurton Jones.6 We multiplied in-patch rates by time foraging for each season (both in Table 1), andthe proportion of time in patch (60% root, 66% berry) (Hawkes personal communication and Hawkes, O’Connell, and Blurton
Jones,6 p. 350), equally weighting production in dry and wet seasons. Ekwa roots are calculated as 88% edible (Hawkes, personalcommunication) and 850 cal/kg edible (Hawkes, O’Connell, and Blurton Jones,6 p. 691). Fruits are assumed to be 50% edible andhave a caloric value of 2,500 cal/kg edible42 (pp. 481, 484 for grewia sp. berries). Meat acquisition is 4.89 kg/day for adult men(over age 18) and assumed to have a caloric value of 1,500 cal/kg with the discounting for the edible portion.7 Honeyproduction was assumed to be 0.78 kg/man-day for males over age 1815 (p. 86), with 35% edible and 3,060 cal/kg (as for theAche). All food production by age and sex category was weighted by the probability of survival to that age for the Hadza,16then divided by the total of all survival probabilities to obtain the expected per-capita consumption. The estimate of total
per-capita consumption is very high, probably in part because actually sampled camps contained more juvenile consumers
than the life table implies (Hawkes, personal communication). However, we cannot correct the estimate of daily consumption
without a complete age-sex breakdown of the sampled camps, which currently is not available.
lChimpanzee diet: We use the Gombe diet from Goodall76 (Fig. 10.1). The absolute amount of meat in the diet is from Wrangham
and Riss.22 Kibale plant percentages are from Wrangham, Conklin-Brittain, and Hunt.97 The Kibale meat percentage is from
Wrangham and coworkers.152 The Mahale diet is taken from Hiraiwa-Hasegawa.25 The absolute amount of meat in the diet wascalculated from Uehara,153 assuming adult prey at 13 kg and juvenile prey at 6 kg, on average, the percentage of adult prey
was taken from Stanford.154 For Tai forest chimpanzees, the absolute amount of meat in the diet was calculated from Boesch
and Boesch20 (Table 7.4).

duce selection pressures to invest
more in survival during both the juvenile and adult periods. At the sametime, ecological changes that lower
mortality rates during the juvenile
and adult periods also produce selection pressures that favor a longer juvenile period if it results in higher
adult productivity. If both types ofchange occur (increased payoffs fortime spent in development and lower
mortality rates), great changes in bothmortality rates and time spent in development may result. Furthermore, if
those changes are accompanied bylarge increases in productivity after
adulthood is reached, we expect additional increases in time spent in development and in survival rates. Our proposal is that the skill-intensive feeding
niche, coupled with a large brain, isassociated with a significant amountof learning during the adult period.
To test this hypothesis, we developed a model to determine whether or
not natural selection would actually
result in co-evolution of the developmental period and the life span. Thismodel builds on two bodies of theory,
life-history theory in biology and human-capital theory in economics.
Life-history theory is based on thepremise that organisms face tradeoffs in the allocation of their time andeffort. Gadgil and Bossert2 offered thefirst explicit treatment of allocations
trade-offs with respect to reproduction and longevity. They postulated
that during the life course selection
acts on the allocation of energy to. . .we developed amodel to determine
whether or not natural
selection would actually
result in co-evolution of
the developmental
period and the life span.
This model builds on twobodies of theory, lifehistory theory in biology
and human-capitaltheory in economics.

each of three competing functions: reproduction, maintenance, and growth.
Energy allocated to reproduction will
necessarily reduce the quantity available for maintenance and growth.

Maintenance and growth may be seen
as investments in future reproduction,
for they affect both the probability
that an organism will survive to reproduce in the future and the amount ofenergy it will be able to harvest andtransform into reproduction. Thus,
one fundamental trade-off is between
current and future reproduction.
Human-capital theory in economics18,19 is designed to analyze investments in education and training
through the course of life. Central tothis theory is the notion of foregone
earnings: Time spent in education andtraining reduces current earnings inreturn for increased earnings in the
future. The economic trade-off between current and future earnings isdirectly analogous to the trade-off between current and future reproduction in biology.
Charnov,1,9 building on earlier workon optimal age at first reproduction,
developed a mathematical model ofthe trade-off between growth and reproduction for mammals. His model
is designed to capture determinate
growth, in which an organism has twolife-history phases after attaining independence from its parents. These
are a prereproductive growth phase inwhich all excess energy, remaining after maintenance requirements havebeen met, is allocated to growth and a

164 Evolutionary Anthropology

ARTICLE

Figure 4. The feeding ecology of humans and other primates.

reproductive phase in which all excessenergy is allocated to reproduction.
By growing, an organism increases
its energy capture rate, and thusincreases reproductive rate. During
adulthood the fundamental trade-off
is between the expected length of the
reproductive span (which is shorter
with each additional year spent growing, because of the increased probability of dying before reproducing)
and the adult reproductive rate (which
is higher with every year spent growing because of increased energy
stored in the form of adult bodymass). The model predicts the amountof time mammals will grow before
switching to the reproductive phaseby selecting the time that maximizes
expected energy for reproduction overthe life course.
Here we extend Charnov’s model inthree ways. First, we broaden the concept of growth from body size alone toinclude all investments in development. Development can be seen as aprocess in which individuals and theirparents invest in a stock of embodied
capital, a term that generalizes theconcept of human capital to all organisms. In a physical sense, embodied
capital is organized somatic tissue. In
a functional sense, embodied capital
includes strength, immune function,
coordination, skill, knowledge, andsocial networks, all of which affect the
profitability of allocating time andother resources to alternative activities such as resource acquisition, defense from predators and parasites,
mating competition, parenting, andsocial dominance. Because such stocks
tend to depreciate with time due tophysical entropic forces and direct assaults by parasites, predators, andconspecifics, allocations to maintenance efforts, such as feeding, cell repair, and vigilance, can also be seen as
investments in embodied capital. In
our model, the energy capture rate increases with embodied capital (that is,
time spent in development).
Second, in Charnov’s model, organisms have no control over mortality
rates, which are exogenously determined; the only variable of choice forthe organism is age at first reproduction, or time spent in development. In
our model, the organism can exercise
control over mortality rates by allocating energy to mortality reduction. We
include this second choice variable todetermine if time invested in development and energy allocated to mortality reduction co-evolve. Our theory
predicts that increased investment oftime in development due to the exploitation of difficult-to-acquire, highquality resources selects for increased
longevity or lower mortality rates.
Third, our model allows learning tocontinue after physical growth hasceased to analyse the effects on theincrease in return rates from foraging
during the adult period (see Figs. 2
and 3) as a consequence of “on-thejob” training.
Thus, the model has two choicevariables upon which selection canact: age at first reproduction, a proxy
for time spent in development andphysical growth, and allocation of energy to lowering mortality. It also has
three ecological parameters: factors
affecting the pay-offs to investments
in development, factors affecting mor-tality rate, and the growth in productivity after adulthood due to learning.
The formal model is presented inBox 1.
The six main results of the mathematical model confirm the co-evolutionary selection pressures predictedby the theory. Ecological factors increasing the productivity of investments in developmental embodied
capital (in the context of the present
theory, a skill-intensive foraging niche)
increase both time spent as a juvenile
and investments in mortality reduction. Ecological factors that lower
mortality rates increase both timespent in development and investment
in mortality reduction. Finally, thegreater the growth rate in production
during the adult period, due to large
brains and “on-the-job” training, themore it pays to invest in development
and mortality reduction. These results
all show that investments in development and investments in mortality reduction and longevity co-evolve.

EMPIRICAL TESTS OF
THE THEORY
Composition of the Diet
Figure 4 illustrates our proposal
about the differences between the diets of nonhuman primates and humans. While the diets of nonhumanprimates vary considerably by species
and by local ecology, the inverted triangle in Figure 4 represents thegreater importance of lower quality,easier-to-acquire foods in the diets ofmost nonhuman primates (excluding
insectivores). The upward-pointing
triangle in Figure 4 represents thegreater importance of large-packagesize, nutrient-dense, difficult-to-acquire foods in human diets.
Table 3 presents data on the diets often foraging societies and four chimpanzee communities for which caloric
production or time spent feeding weremonitored systematically. As far as weare aware, this is a complete sample of
the available data. The diet is subdivided into vertebrates, roots, nuts andseeds, other plant parts (such asleaves, flowers, and pith) and invertebrate resources. The diets of all modern foragers differ considerably fromthat of chimpanzees. Measured in calories, the major component of forager

Evolutionary Anthropology 165

ARTICLEBox 1. A Formal Model of Natural Selection on Age at First Reproduction and
Investments in Mortality Reduction
The model treats two phases of the life course, the juvenile andadult periods. The juvenile period begins after the high mortality
phase associated with infancy and weaning, is dedicated to
growth and development, and lasts a variable amount of time, t,
upon which selection acts. During this time, all energy is invested
in either embodied capital (growth and learning) affecting futureenergy production, P, or in reducing mortality rate, ␮. The two
choice variables during the juvenile period are its length, t, and the
proportion of energy invested in mortality reduction, ␭ (implyingthat {1 ⫺ ␭} is the proportion allocated to growth and learning).
The adult production of energy at the end of the juvenile period,
Pa, is determined by t, ␭, and the combined ecological effects ofthe environment and the technology of production, captured by
the vector, ␧, which is assumed to increase energy production(i.e., ⭸Pa/⳵␧ ⬎ 0). We can think of this as composed of two
functions, ⌸(t, ␧), which captures the growth in production due tolearning, growth, and development, and ⌿(␭), which represents
the proportional loss in production due to investments in mortality
reduction during the juvenile period. Thus, we have Pa ⫽ p(t, ␭;␧) ⫽ ⌸(t, ␧)⌿(␭).
The second period is reproductive. During this period, growth inbody size ceases and all excess energy is allocated to reproduction. Production grows at some constant rate, g, due to the effectsof learning. Thus, production at some age, x, after adulthood, Px,
is Paeg(x⫺t).
During both phases, the instantaneous mortality rate, ␮, remains constant, at a level determined by the amount of energy
production diverted to mortality reduction, ␭, and by ecology
factors affecting mortality (such as density of predators and diseases), ␪, which is assumed to increase mortality rates (i.e., ⳵␮/⭸␪ ⬎ 0). Thus, ␮ ⫽ ␮(␭, ␪). The net energy available for reproduction at age x during the adult period, Pr, x, will be equal to totalenergy production times the proportion allocated to reproduction,
Pr,x ⫽ (1 ⫺ ␭)Paeg(x⫺t).
The basic logic of the model is that an organism will maximize
fitness by maximizing its lifetime energy allocated to reproduction.
By increasing the length of the growth and development phase, t,
the adult rate of energy capture increases, but the expected lengthof the reproductive period, R, decreases. This decrease resultsfrom the fact no energy is allocated to reproduction during the
growth and development phase, though the organism is still exposed to mortality. Allocations to mortality reduction also have
opposing effects. Allocations to growth during the juvenile period
and to reproduction during the adult period are reduced by allocations to mortality reduction. Yet lowered mortality increases thelength of the reproductive period by increasing the probability ofreaching reproductive age and by increasing the expected timefrom reproductive age to death. Thus, R is determined by both tand ␭. If we consider only individuals who survive infancy, R isequal to expectation of adult reproductive years lived, given theprobability of dying at each age, x. Thus, the expected number ofadult years during which energy is allocated to reproduction is R ⫽⬁r(t, ␮(␭, ␪)) ⫽ 兰x⫽t
(x ⫺ t)␮e⫺␮xdx ⫽ 1/␮e⫺␮t, where 1/␮ is the
expected adult life span conditional on reaching age t and e ⫺ ␮t is
the probability of reaching age t, conditional on having survived
infancy. The expected energy production during the adult period is⬁e ⫺ ␮t(1 ⫺ ␭)Pa 兰x⫽te(g⫺␮)(x⫺t)dx, which is equal to e ⫺ ␮t(1 ⫺⫺1␭)Pa(␮ ⫺ g) for ␮ ⬎ g (which we assume to hold true, because
expected lifetime income otherwise would be infinite).

Selection is expected to optimize t and ␭ so as to maximize
lifetime energy allocated to reproduction. Thus, we have the following maximization problem:
Max W ⫽ e ⫺ ␮ 共␭, ␪ 兲t 关1 ⫺ ␭ 兴P a 共t, ␭ , ␧兲共 ␮ 共 ␭ , ␪ 兲 ⫺ g兲 ⫺ 1 .

(1)t,␭, ␮

Partially differentiating the fitness function, W, with respect to t
and ␭, respectively, the following first-order conditions for an
optimum are obtained:⭸P a ⫺ 1
P ⫽␮
⭸t a

(2)

⭸␮
⭸P ⫺ 1
共t ⫹ 共 ␮ ⫺ g兲 ⫺ 1 兲 ⫽ ⫺P ⫹ 共1 ⫺ ␭ 兲 ⫺ 1
⭸␭
⭸␭ a

(3)

and
⫺

Equation 2 for optimal t, taking ␮ as given, replicates Charnov’s1 result. Optimal t occurs at the age when the proportional
increase in adult production due to a small increase in timespent in development (the left hand side of equation 2) is equal
to the proportional loss in the probability of reaching adulthood(the right-hand side). The benefits and costs are measured interms of proportions because fitness is a product of the probability of reaching adulthood and the production rate as anadult.
Equation 3 concerns optimal allocations to mortality reduction,
given time spent in development. The left-hand side of the equation is the benefit of a small increase in investment in mortality
reduction. It is the proportional increase in reaching adulthood,plus the proportional increase in the adult lifespan, adjusted forthe growth in income due to learning, g. The right-hand side is thecost, the proportional loss in production due to increased investment in mortality reduction. This proportional cost is two-foldbecause allocations during the juvenile period reduce the growthrate and therefore reduce P a (the first term), while allocations tomortality reduction during the adult period reduce the proportion of adult production allocated to reproduction (the secondterm).
Differentiating equations 2 and 3 with respect to the ecologicalparameters, ␧, ␪, and g, we derive our six main analytical results,
confirming the co-evolutionary effects predicted by our theory. ⭸t/⭸␧and ⳵␭/⳵␧ are both positive. This means that ecological factors increasing the productivity of investments in developmental embodied
capital, as indexed by ␧, not only increase time spent in development
but also increase investments in mortality reduction. ⭸t/⭸␪ and ⳵␭/⳵␪
are both negative, meaning that exogenous or extrinsic increases inmortality, as indexed by ␪, reduce both time spent in development
and investments in mortality reduction. Conversely, ecological factors that lower mortality rates increase both time spent in development and investment in mortality reduction). Finally, ⭸t/⭸g and ⭸␭/⭸g
are both positive, showing that the greater the growth rate in production during the adult period due to “on-the-job” training, as indexed by g, the more it pays to invest in development and mortality
reduction. (A formal proof of the six results was developed by Arthur
Robson and is available from the authors). These results all show thatinvestments in development and investments in mortality reduction/
longevity co-evolve.

980
242
0
0
0
20
1243
0

calories/person day
meat
roots
seeds, nuts
fruitsother plant
invertebrate
Total
Non foragedd
690
150
1365
150
0
0
2355
trace

0.26
0.15
0.21
0.15
0.00
0.00

trace

1602

0.59

928

!Kung

b

417
600?
0
600?
0
0
1617?
0

0.30
0.40?
0.00
0.40?
0.00
0.00

?

Gwi

48
30
0
15
0
6
15
36
48

1940
1214
0
621
0
255
4030
trace

1.10
1.62
0.00
0.54
0.00
0.24

?

Hadza

79
19
0
0
0
2
0.0
21.9
78.0

75
8
0
4
0
12
4.0
20.3
75.7

77
19
0
0
0
3
0.6
30.1
69.4

41
0
0
40
0
20
20?
40?
40

75
15
0
5
2
3
4.6
21.6
73.7

29
6
58
6
0
0
4.9
63.4
31.7
?
?
68.0

68?

26?
37?
0
37?
0
0
37?
37?
26.0

1.5
0.0
5.1
60.2
29.3
3.9
94.2
3.8
2

Gombe

Sampleb
kg/person dayc
meat
roots
seeds, nuts
fruitsother plants
invertebrate

meat
roots
seeds, nuts
fruitsother plant
invertebratecollected
extracted
hunted

1350
268
0
82
36
57
1793
626

0.97
0.44
0.00
0.29
0.04
0.02

!Kung
866

Dietary percentage of foraged foods

764
0
0
747
0
375
1886
378

0.62
0.00
0.00
0.86
0.00
0.35

Hiwi
4756

meat
roots
seeds, nuts
fruitsother plant
invertebratecollected
extracted
hunted

2126
0
0
22
255
308
2712
trace

1.36
0.00
0.00
0.61
0.96
0.11

941

Ache Nukak
3645

2

Feeding
time
percentage

1821
456
0
10
3
67
2357
0

1.34
0.43
0.00
0.01
0.00
0.09

78
0
0
1
9
11
0.8
24.3
74.9

Arnhem276

1

For information on methods of calculation and data sources, see footnotes, Table 2.
Person days sampled, including all men women and children as equal consumers.
cThis is the weight of the edible portion for meat and field weight for all other resources.
d
Intake of nonforaged foods in calories/person day when measured or reported.

a

0.58
0.07
0.00
0.15
0.00
1.01

0.59
0.21
0.00
0.00
0.00
0.01

822
93
0
44
0
127
1085
1116

3654

Onge Anbarra

1256

Sampleb
kg/person dayc
meat
roots
seeds, nuts
fruitsother plants
invertebrate

Hunter-Gatherers

TABLE 3. DIET OF MODERN HUNTER-GATHERERS AND CHIMPANZEESa

0.9
0.1
0
78.5
21.3
0
99.1
0
0.9

Kibale

0.03

Gombe

0.01

Mahale

2.5
0
0
57.7
33.4
6.4
91.1
6.4
2.5

MahaleKibaleChimpanzees

2.1
1.4
7
67.6
15.5
5.6
92.3
5.6
2.1

Lope

0.04

Tai

166 Evolutionary Anthropology

ARTICLEEvolutionary Anthropology 167

ARTICLE

diets is vertebrate meat. This ranges
from about 30% to 80% of the diet in
the sampled societies, with most diets
consisting of more than 50% vertebrate meat (equally weighted mean ⫽60%). The emphasis on vertebratemeat would be even more clear if any
high-latitude foraging societies were
included in the sample. In contrast,
chimpanzees spend only about 2% of
their feeding time eating meat. Unfortunately, the diet of wild primates is
not usually expressed in calories, as isthat of human foragers. Field workers
studying nonhuman primates use
time spent feeding on specific foods as
the closest approximation of energy
acquired and rarely either measure ingestion rate or calculate calorie intake. The absolute intake of meat per
day also varies tremendously: Chimpanzee per capita meat intake is estimated at about 10 to 40 g per day,while human meat intake ranges fromabout 270 to 1,400 g per person perday. Although it is true that chimpanzee males eat much more meat thando females and juveniles,20 –22 we conclude that, in general, members of foraging societies eat more than ten
times as much meat as do chimpanzees.
The next most important food category in our forager sample is roots,
which make up an average of about
15% of the energy in the diet and were
important in about half the societies
in our sample. In contrast, the chimpanzee diet is primarily composed ofripe fruit, which accounts for over
60% of feeding time. Only two foraging societies ate large amounts of ripe
fruit, the Gwi San of the Kalahari
desert, who consume melons for water and nutrients during much of theyear, and the Nukak of Colombia, who
extensively exploit tropical palm
fruits, which, however, are difficult toacquire. Other plant products are an
important secondary food for chimpanzees, making up about 25% of observed feeding time. This category is
unimportant for the foragers in our
sample.
The data suggest that humans specialize in rare but nutrient-dense resource packages or patches (meat,roots, and nuts), whereas chimpanzees specialize in ripe fruit and plantparts with low nutrient density. These

differences in the nutrient density offoods ingested are also reflected in human and chimpanzee gut morphology
and food passage time. The chimpanzees gut is specialized for rapid processing of large quantities and lownutrient, bulky, fibrous meals.23
However, a stronger contrast is apparent when we consider how the resources are obtained. We have categorized all foods into three types.
Collected foods are those that can be

The absolute intake ofmeat per day also
varies tremendously:
Chimpanzee per capita
meat intake is estimatedat about 10 to 40 g perday, while human meatintake ranges fromabout 270 to 1,400 g perperson per day.
Although it is true thatchimpanzee males eat
much more meat thando females andjuveniles, we conclude
that, in general,
members of foraging
societies eat more thanten times as much meat
as do chimpanzees.

obtained and eaten simply by gathering them from the environment. Extracted foods are non mobile but areembedded in a protective context
from which they must be removed.
Such foods may be underground, in
hard shells or associated with toxins.
Hunted foods include mobile resources that must also be extracted
and processed before consumption.
Collected resources include fruits,
leaves, flowers, and other easily acces-

sible plant parts. Extracted resources
include roots, nuts and seeds, mostinvertebrate products, and plant parts
that are difficult to extract, such as
palm fiber or growing shoots. Hunted
resources include all vertebrates andsome mobile invertebrates.
Table 3 shows a breakdown of forager and chimpanzee foods according
to our three acquisition categories.
Chimpanzees obtain an average ofabout 95% of their diet from collectedfoods, whereas the foragers in our
sample obtain an average of 8% of
their food energy from collected resources. On the other hand, foragers
obtain about 60% of their food energy
from hunted resources and about 32%
from extracted resources, whereas
chimpanzees obtain about 2% of theirfood energy from hunted foods andabout 3% from extracted resources.
While these categories may be somewhat rough, it is clear that humans
are much more dependent on resources that can be obtained only bycomplicated techniques. Thus, the dietary data are consistent with our theoretical model. Humans appear to be
more dependent on resources that require skill and learning to acquire.

The Age Profile of Acquisition
for Collected and Extracted
Resources
The proposition that difficulty of acquisition predicts the age profile offood production can be tested in twoways, by looking at the daily amountof different resource types produced
by individuals of different age and byobservational and experimental measures of hourly rates of acquisition ofdifferent resource types by individualsof different ages. The daily data are
determined by both time allocation
and rates of return per unit of timespent on a resource, whereas thehourly data are based exclusively on
rates of return. Both sources of data
support the proposition that juveniles
cannot easily obtain extracted andhunted foods.
Beginning with the daily data, Figures 5 and 6 show, respectively, thedaily caloric contribution of various
food types acquired by Ache and Hiwi
males and females as a function ofage. The upper panels represent the

168 Evolutionary Anthropology

ARTICLE

Figure 5 and 6. Age-sex specific daily energy acquisition is calculated as described for Figure 2 and Tables 2 and 3. Mean daily acquisition
for each resource class was calculated by summing all the calories acquired for individuals of that age-sex category and that resourceclass, then dividing by all person days sampled in the relevant age category. Y values are plotted at the mean age for each class analyzed.

calorically less important foods (thosefor which daily production is less than
200 kcal/day); the lower panels represent the more important foods (⬎200
kcal/day). Among young Ache, bothmales and females acquire only fruits.
Both sexes reach their peak rates ofdaily fruit acquisition in their mid- tolate teens. Extracting palm hearts requires strength and some skill (about
three minutes of chopping in the rightspot); daily palm heart acquisition is
asymptotic for both sexes by the ageof 20 years. More skill and learning
are required to extract honey or palm
starch (knowing how to open a “window” to the resource and then extractit). The daily acquisition of these resources does not peak until individuals are in their late 20s. Daily returns
from hunting do not peak until individuals are about 35 years old.
Hiwi foragers show similar production patterns with age, except thatdaily fruit acquisition becomes as-

ymptotic at later ages. Hiwi fruit collection is more complicated because
many trips entail walking through thenight to distant groves, followed by areturn trip of 10 to 20 km with a heavy
load of fruit. Daily honey extraction
rate also reaches its peak level whenthese foragers are in their early 30s(these are very small nests of nativebees). Female root-production rates
increase four-fold from age 20 to age
40, but men’s meat production does
not peak until they are about age 35.
The fact that forager children, likechimpanzees, primarily acquire ripe
fruits is supported by additional data.
Among the Ache, children acquire fivetimes as many calories per day during
the fruit season as they do duringother seasons of the year.24 Amongthe Hadza, adolescent girls acquire
1,650 calories per day during the wet
season, when fruits were available,
and only 610 calories per day during
the dry season, when fruits are notavailable. If we weight the data for the
wet and dry season equally, teenage
Hadza girls acquire 53% of their calories from fruits, compared to 37%
and 19%, respectively, for reproductive-aged women and postreproductive women (all calculated from
Hawkes and coworkers.6) Hadza boys,
like Ache and Hiwi boys, switch fromeasier tasks, such as fruit collection,
shallow tuber extraction, and baobab
processing to honey extraction andhunting in their mid- to late teens.13,14
Chimpanzee juveniles also focus on
more easily acquired resources thando adult chimpanzees. Juvenile chimpanzees practice difficult extraction
activities such as collecting termites,
fishing for ants, or nut cracking lessthan adults do.20,26 Hunting is strictly
an adult or subadult activity.20,21,27
Hourly return rates provide further
evidence that important human foodresources require long periods oflearning and skill development. Ob-

Evolutionary Anthropology 169

ARTICLEcut across the palm leaf stalks. Girlstake longer to extract the starchy fiberfrom the trunk of a downed palm because they do not know how to cut
open a window nor how most efficiently to pound the fiber away fromthe hard outer trunk wood. Whenthese components activities of palm
extraction are combined, Ache women
do not reach peak return rates until
their early 20s (Fig. 9).
Supporting data are also available
from other groups. !Kung (Ju/’hoansi)
children crack mongongo nuts at amuch slower rate than adults do.28
Bock29 has shown that nut-cracking
rates among the neighboring Hambukushu do not peak until about age

Figure 7. All Ache data, including timed counts of fruits acquired per minute, were analyzed
for each age category. The mean plotted is the average from each independent monitored count for a given age class of acquirers. Fruits other than those that are simply
collected from the ground or low branches were not included in the analyses. Y values areplotted at the mean age for each class analyzed.

servations of Ache fruit collection
show that foragers generally acquire
the maximum observed rate by about
age 20 (Fig. 7). Some fruits (for example, pretylla) that are simply pickedfrom the ground are collected by children as young as one-and-a-half tothree years at 30% of the adult maximum rate. For fruits such as vijulla,which must be picked off branches,children do not reach 50% of the adultmaximum rate until age 15 (Fig. 7). As
mentioned earlier, fruit collection by
the Hiwi (Fig. 8), unlike that done others, is labor-intensive and requires
travel to distant food sites. Both males
and females reach maximum return
rates by about age 25.
Hadza data also show competentfruit collection by children. The
Kongoro berry collection rate ofyoung married girls is equal to theadult women’s rate.6 Baobab collecting and processing seems to reach
50% of the adult rate by about age
12.14 Baobabs are an interestingfood resource because they are bothcollected and extracted. While theycan easily be picked off the ground,much more food energy is obtained
when the pith is extracted with pound-ing and water. Young children do notpractice these activities, whereas older
children do.14
In contrast to the hourly acquisition
rate of fruits, that of extracted resources often increases through early
adulthood as foragers acquire necessary skills. Data on Hiwi women showthat their root-acquisition rates do not
become asymptotic until the womenare about age 35 to 45 years (Fig. 8).
The root-acquisition rate of 10-yearold girls is only 15% of the adult maximum. For Hiwi males, the honey-extraction rates peak at about age 25.
Again, the extraction rate of 10-yearolds is less than 10% of the adult maximum. Experiments done with Achewomen and girls clearly show that the
young adults are not capable of extracting palm products at the rate obtained by older Ache women (Fig. 9).
Girls take longer than women to cut
palms because they lack strength andbecause they cannot judge whether apalm will fall to the ground or getstuck in nearby tree branches. Girlstake longer than women to extract thegrowing shoot from the palm after itis on the ground because this task requires strength and knowing where to

Hourly return rates
provide further evidence
that important humanfood resources require
long periods of learningand skill development.
Observations of Achefruit collection show that
foragers generally
acquire the maximum
observed rate by about
age 20.

35. Hadza women, however, appear to
obtain maximum root-digging rates
by early adulthood,6 perhaps because
they obtain a good deal of practicethroughout childhood13,14 and thus
require only adult strength in order toproduce at adult rates.
Casual ethnographic observation
supports the generalization that fruit
collection is easily learned, extraction
skills require more time to develop,
and hunting is the most difficult foraging behavior. Anthropologists working with modern foragers often participate in fruit collection and canrapidly achieve aboriginal return rates.
Some types of extraction can also be170 Evolutionary Anthropology

ARTICLE

Hunting and the Role of Menin Human Reproduction
Male behavior plays a distinctiverole in human life histories in two
ways. First, hunting, the primarysubsistence activity of adult men, isthe most learning-intensive foraging
strategy practiced by humans. Second, unlike most higher primates,
men play a major role in the energetics of human reproduction.

Figure 8. Because Hiwi foragers target specific resources when they leave their central
camp, we recorded all time dedicated to foraging for different resource types over the
sample period. Total energy production for each resource type and age-sex class was
divided by the total number of hours reported to be out-of-camp foraging for that resource
type to obtain the hourly return rate from foraging for that resource type. The time foraging
included in-patch pursuit of resources as well as walking time to and from the patch. Y
values are plotted at the mean age for each class analyzed.

mastered with practice. One of us(KH) chops down palms as fast as
Ache women do and can extract theheart at a slightly slower rate. However, despite some practice, KH has
not achieved the rate of palm-fiber extraction of Ache women. We know ofno ethnographers who successfully
dig roots or crack nuts at the rate ofmembers of their study populations.

Indeed, the same patterns are seenamong captive animals released into
the wild. Animals that have grown toadulthood in captivity can be successfully released into the wild if theirfeeding niche is simple (for example,
if they are herbivores). However, thesuccess rate for carnivores and apes
with complex feeding strategies ismuch lower.

We know of no
ethnographers who
successfully dig roots or
crack nuts at the rate ofmembers of their study
populations. Indeed, thesame patterns are seenamong captive animals
released into the wild.
Animals that havegrown to adulthood in
captivity can be
successfully released
into the wild if theirfeeding niche is simple(for example, if they are
herbivores). However,the success rate forcarnivores and apes
with complex feeding
strategies is much lower.

Although a detailed quantitative
analysis of the learning process involved in human hunting has not yet
been conducted, it is clear that humanhunting differs qualitatively fromhunting by other animals. Unlike
most animals, which either sit andwait to ambush prey or use stealthand pursuit techniques, human hunters use a wealth of information to
make context-specific decisions, bothduring the search phase of hunting

Evolutionary Anthropology 171

ARTICLE

muddy bottoms by systematicallypoking the bottom of a pond, andspeared underwater by random
thrusts in drying lakes. The widely
varied kill techniques are tailored to awide variety of prey under a wide variety of conditions. Although allgroups probably specialize in themost abundant and vulnerable prey intheir area, the total array of species
taken is impressive, and probably ismuch larger than that of most, if notall, other vertebrate predators. For example, from 1980 to 1996 our sample
of weighed prey taken by the Ache

Figure 9. Data from experiments in which Ache females of different ages were asked tochop down a palm tree, extract its growing shoot (heart), and extract palm starch from the1palm for at least hour. Y values are plotted at the mean age for each class analyzed.
2Age-specific hunting returns are based on measured kg of live weight of game acquired byAche males between 1980 and 1984 and measured out-of-camp foraging time for males
during the same time period. Y values are plotted at the mean age for each class analyzed.
Differences between age categories may be due to differences in encounter rates withgame or success in pursuits of game after encounter.

and then after prey is encountered.
Specifically, information on ecology,
seasonality, current weather, expectedanimal behavior, and fresh animal
signs are all integrated to form multivariate mental models of encounter
probabilities that guide the search
and are continually updated as conditions change.30 Various alternative
courses of action are constantly compared and referenced to spatial andtemporal mental maps of resource
availability.30 This information is collected, memorized, and processedover much larger spatial areas thanchimpanzees ever cover. For example,
interviews with Ache men show thatfully adult men (those over the age of 35
years) had hunted in nearly 12,000 km2
of tropical forest during their lives. Almost all foragers surveyed use morethan 200 km2 in a single year, and many
cover more than 1,000 km2 in a year
(Table 4.1 in Kelly31). Male chimpanzees, on the other hand, cover only
about 10 km2 in a lifetime.32,33
After potential prey are encountered, humans employ a wide variety
of techniques to obtain them, using
astounding creativity. Here are justsome examples that Hill, Hurtado and
Kaplan have seen among the Ache,Hiwi, Machiguenga, and Yora: Arboreal animals have been shot with arrows from the ground or a tree, driven
by climbing, shaken down frombranches, frightened into jumping to
the ground, brought down by felling atree with an axe, lured by imitatedcalls, lured by making captured infants emit distress calls, captured by
the spreading of sticky resin on
branches to trap them, and captured
by scaffolding constructed from tree
branches and vines. Ground-dwellingprey are shot with arrows, driven toother hunters or capture devices, run
down upon encounter, slammed todeath against the ground, strangled at
the neck, or suffocated by stepping onthem while they are trapped in a tightspot. Burrowing prey are dug out,
chopped out of tree trunks, stabbed
through the ground with spears,
frightened to the point at which theybolt from the burrow, smoked out,
and captured by introducing a lasso
through a small hole. Aquatic prey areshot on the surface or below it, driven
into traps, poisoned, discovered on. . .it is clear that humanhunting differs
qualitatively fromhunting by otheranimals. Unlike mostanimals, which either sit
and wait to ambush
prey or use stealth andpursuit techniques,human hunters use a
wealth of information to
make context-specificdecisions, both during
the search phase of
hunting and then after
prey is encountered.

included a minimum of 78 different
mammal species, at least 21 species ofreptiles and amphibians, probably
more than 150 species of birds (morethan we have been able to identify)
and more than 14 species of fish.
Moreover, human hunters tend to select prey that is in prime condition
from the perspective of human nutritional needs rather than prey madevulnerable by youth, old age, or disease, as do many carnivorous animals.34,35
The skill-intensive nature of humanhunting and the long learning process172 Evolutionary Anthropology

involved are demonstrated dramatically by data on hunting return rates
by age. Hunting return rates among
the Hiwi do not peak until the men
reach the age of 30 to 35 (Fig. 8). The
acquisition rates of 10-year-old and20-year-old boys reach, respectively,
only 16% and 50% of the adult maximum. In the 1980s, the hourly return
rate for Ache men peaked when they
were in their mid-30s (Fig. 9). The return rate of 10-year-old boys is now
about 1% of the adult maximum, andthat of 20-year-olds is still only 25% of
the adult maximum.
It is not surprising that no ethnographer has ever described being able
to hunt at a rate equivalent to that ofstudy subjects. Indeed, most who hunt
make kills only after being led to thegame animal by a competent hunter,and then generally with a firearm (asis the case with recreational big-gamehunters). These patterns also mirror
the effects of acculturation in mostgroups. Foragers who grow up in settled communities can often collectfruits and other plant resources at
rates equivalent to those obtained byolder individuals who grew up in thebush (see, for example, Blurton Jonesand colleagues28 on the !Kung). But
most young Ache men who havegrown up on reservation settlements
cannot hunt using traditional technology nearly as successfully as older
men who grew up in the forest (unpublished data).
Chimpanzees too, appear to require
many years to learn successful hunting techniques. Older males are more
likely than younger adult males to ambush prey during a group hunt andperform more complicated maneuvers during the hunt.20 (But see Stanford.21)
Although the learning process islong, investments in hunting ability byhuman males allow them to be highly
productive as adults. The comparative
analysis of diets and productivity
among foragers shows that men play amajor role in the energetics of humanreproduction. For example, among
the Ache the total expected net caloric
production (food produced minus
food consumed) from age 18 to deathis ⫹21,638,000 calories for males and⫺924,000 calories for females. The
corresponding figures for the Hiwi are

ARTICLE⫹11,151,000 for males and ⫺3,096,000
for females. Even among the Hadza,
where women play a much greaterrole in subsistence production, males
provide as least as much support for
reproduction as females do, if not
more. While the estimates may require revision when researchers complete their analysis of age-specific production among the Hadza, our initialapproximation, based on publisheddata, is that over the entire expected
life course (including the probabilityof survival to each age), net production for Hadza males is ⫹16,671,000
calories, while that of females is only
⫹3,352,000 calories. Table 2 shows

Although the learning
process is long,
investments in hunting
ability by human males
allow them to be highly
productive as adults.
The comparative
analysis of diets andproductivity among
foragers shows that men
play a major role in theenergetics of humanreproduction.

that men provide more food energy
per day than women do in all but one
or two of the ten foraging societies forwhich there is quantitative data. Men
also provide the vast majority of protein in the diet. This is critical, sincehigher daily protein intake increases
weight gain,36,37 immune function response,38 and survival.39,40
The fact that men produce more
food than women do in most low-latitude foraging societies is not conventional wisdom. This is a result of theinfluence of Richard Lee’s41,42 pioneering study of the !Kung (Ju/’hoansi), which showed that women provide more food than men do. There

has been a tendency to generalize
those results to all foraging societies.
Table 2 shows that those results are
not general; it is also shows the possibility that weaknesses in Lee’s study
have been misleading for the !Kung as
well. Lee’s sample covered only 28
days of one month of 1964, and on
two of those days he took women out
collecting mongongo nuts in his vehicle, thus inflating the collected portion of the diet and female return
rates. During much of the year, mongongo nuts are not abundant, nor are
they as abundant at other !Kung study
sites as at the Dobe site. Another study
of !Kung food production showsmuch higher hunting success than
Lee reported. Yellen43 provided data
showing that !Kung men acquired
twice as much meat per day when
they were in bush camps than they didin the permanent dry-season waterhole settlement where Herero raised
cattle, which probably had a depressive effect on game densities. Yellen’ssample of person consumption days
was larger than Lee’s and covered allmonths of the year. The daily meat
consumption in Yellen’s sample wasabout 1,600 calories per capita, which
would represent 68% of all calories if
total food consumption was the sameas Lee reported. In addition, both Lee
and Draper (p. 262 in Lee42) foundthat !Kung men spent more hours per
week on food acquisition than did!Kung women.
The fact that humans were successful in colonizing high-latitude ecologies where plant foods are not abundant and are available only for shortperiods also demonstrates the important role that men play in the energetics of reproduction. It is also interesting that many Neanderthal and early
Homo sapiens are found at high latitudes where plant consumption was
minimal and, in some cases, where
even fuel and residential construction
were provided by animal products(see Hoffecker44 for a review of east
European sites). Moreover, some archeological sites beginning with Homoergaster (for example, Boxgrove, England45) contain super-abundant animal remains and evidence of spears
and other hunting tools, but no evidence of plant consumption. More recent low-latitude archeological as-Evolutionary Anthropology 173

ARTICLEsemblages also often have extremely
dense bone scatter, which suggests
high meat consumption. At Kutikina,
Tasmania, for example, bone fragments comprise nearly 20% of theweight per cubic meter of some archeological strata.46 On the other hand,complete assessment of plant pollen
at some assemblages, among them
Tamar Hat on the north coast of Africa,47 suggests no edible plant species,
despite dense faunal scatters. Such evidence suggests that adult males haveoften been the main, and sometimes
the only food providers in foraging
societies.
In another vein, the Hadza research
team15,48,49 has argued that men do
not play a major role in the energetics
of human reproduction because thevagaries of hunting luck render meatan unreliable and indefensible resource. Two separate issues must be
distinguished in evaluating this argument. The first concerns the sourcesof the caloric and nutritional subsidization of human reproduction. The
second issue is whether or not the proceeds from hunting preferentially support the spouse and children of thehunter.
With respect to the first issue, thedata we present clearly demonstratethat hunted foods provide a substantial proportion of the energy and essential nutrients consumed by womenand children. The fact that humans insome places depend on hunting foralmost 100% of their energy needssuggests that this can be a reliablemeans of subsistence because they
have developed cultural solutions to
the variability problem. The main solutions are food sharing and, to alesser extent, food storage. The second
issue is more complex and, as yet, unresolved. Hawkes and associates49
claim that prey items are not controlled or owned because they are impossible to defend. Thus, males do notprovision their families, but simply
provide equally for everyone in a bandby hunting. There are, however, nodata to support this interpretation,other than the observation of sharing
itself. In fact, there is considerable evidence that carcasses can be defended
when conditions do not favor foodsharing. The Ache of Paraguay, who
supplied the initial data for the inde-

fensibility view of sharing share gameresources widely when on long foraging treks, yet withhold even large
game, sharing mainly with preferredpartners when on reservation settlements.50 Thus, the same resources areshared differently in the forest and in
the reservation. The same is true inAfrica, where the large game items
taken by the Hadza are treated as private property by other ethnic groups
who trade or sell them in the bushmeat market.

In our view, human pairbonding and maleparental investment is
the result ofcomplementarity
between males andfemales. The
commitment to caring
for and carrying
vulnerable young,common to primate
females in general,
together with the long
period required to learn
human hunting
strategies, renders
hunting unprofitable forwomen.

The indefensibility hypothesis also
asserts that shares given up by successful hunters are never repaid in any
useful currency (meat, other goods,
or services) and that shares are given
to everyone equally, regardless ofwhether they have done or will doanything to pay them back. However,recent research shows that contingent
giving is typical in all societies where
it has been examined. Ache and Hiwi
foragers share more with those whoshare with them and, when they aresick or injured, receive help fromthose with whom they have sharedand in relation to how generously they
have shared.50,51 Yanomamo gardeners work more hours in the gardens of
non-kin who work more hours in theirgardens.52 We believe that this typeof reciprocity, often in different currencies, is the basis for all humaneconomies, divisions of labor, andspecialization, and that its critical development in the hominid line distinguishes us from our ape relatives.
Taken together, currently available
evidence suggests that men generally
provide a considerable portion of theenergy consumed by juveniles and reproductive-aged women. This does
not mean that men contribute more
to society or reproduction than dowomen. Women process food (part ofproviding nutrients), care for vulnerable children, and do a variety of otherimportant tasks in all societies. It is
the partnership of men and womenthat allows long-term juvenile dependence and learning and high rates ofsurvival. Indeed, analyses show thatamong both the Ache and the Hiwi,
individual women produce less food iftheir husband is a high producer.53
Divorce or paternal death leads tohigher child mortality among theAche,10,54 the Hiwi,55 and the !Kung,56
but not the Hadza.57
In our view, human pair bonding
and male parental investment is theresult of complementarity between
males and females. The commitment
to caring for and carrying vulnerable
young, common to primate females ingeneral, together with the long periodrequired to learn human hunting
strategies, renders hunting unprofitable for women. The fact that humanmales can acquire very large packagesof nutrient-dense food means thatthey can make a great difference tofemale reproductive success. That is
not true of most other primates (theexception being callithricid males,
who provide other necessary assistance). This difference creates is a major discontinuity between humans
and apes, and results in a partnership
between men and women. That partnership is ecologically variable, in thatthe roles of men, women, and children
vary with the availability of food resources in the environment and therisks posed to children.28,55,58 – 60
When plants are abundant and174 Evolutionary Anthropology

ARTICLEtern typical of many living organisms.68 However, the minimum mortality rate of foragers is about 1% per
year, whereas the minimum rate forchimpanzees is about 3.5% per year
(Fig. 10). We propose that the character of food resources taken by humans
ultimately lowers the mortality rate ofjuveniles and young adults in three
ways. First, the hunted and extracted
foods taken by humans generally
come in large packages. Large, valuable packages favor food sharing,which reduces fluctuations in daily
food intake, allows sick and injured

Figure 10. Yearly mortality rates for the Ache are from Hill and Hurtado10 and for chimpanzees from Hill and coworkers8 as described for Figure 1. Raw mortality data were smoothedusing a double running average. Ages 1–5, 3 pt; ages 5–10 5 pt; ages ⬎10, 9 pt withtruncation at the end of the life table.

game is scarce (as in the Gwi environment), men specialize in providing therarest nutrients in the environment,
protein and lipid, because these nutrients are critical for growth and good
health,61,62 while women provide
more food energy. When plant foods
are scarce, men’s hunting provides thebulk of energy and women concentrate on the processing of food andother raw materials, and on childcare. There is much to be learned
about the determinants of male andfemale roles in foraging societies, but
the primary activity of adult males ishunting to provide nutrients for others.
In sum, hunting is the human activity that requires the longest period
before maximum return rates areachieved, but it also provides the highest overall return rate once maximum
skill levels are reached. As a result,hunting provides the greatest energy

component of human diets in many
foraging societies (Table 3) and is afundamental feature of the human
life-history adaptation. Resolution of
the debate about whether hunting is
primarily direct parental investment,
as we contend, or mating effort, as
proposed by Hawkes and coworkers,63 awaits further data and sophisticated tests. Nevertheless, the nutritional support of reproduction byhuman males is a fundamental feature of our species.

Reduction in Juvenile and
Early Adult Mortality Rates
Human foragers have longer maximum life spans than chimpanzees do,suggesting that they may have lower
mortality rates over much of the life
span.64 – 67 Both groups experience
minimum mortality rates in the latejuvenile and early adult period, a pat-

. . .hunting is the humanactivity that requires thelongest period before
maximum return rates
are achieved, but it also
provides the highest
overall return rate once
maximum skill levels arereached. As a result,hunting provides thegreatest energy
component of humandiets in many foraging
societies and is afundamental feature of
the human life-history
adaptation.

individuals to recover at higher rates,
and facilitates provisioning of children. Second, the food types takenfavor larger foraging parties and residential groups, which offer protection, particularly for juveniles who associate closely with adults. Third, thefood niche has led to the developmentof tools and an understanding of animal behavior, which can be used effectively to repel predators.
Large package size has been positively associated with the probabilityof food transfers or the degree of foodtransfer in virtually every study inEvolutionary Anthropology 175

ARTICLE

which it has been examined.50,63,69 –75
Among the Ache and Hiwi, the percentage of foods not eaten by the acquirer’s nuclear family is directly related to the mean package size ofdifferent food categories72 or package
size within food categories.50 Daily
variability in the acquisition of different food categories also positively correlates with the percent shared in bothsocieties. Importantly, meat comes inlarger packages and is more variable
than extracted resources, which, inturn, come in larger, more variable
packages than do fruits. Meat is transferred most between non-kin, followed by extracted products, thenfruits.72 Chimpanzees also share meat
more than they do any other food resource.76,77 Thus, we propose that as
humans moved into a hunting-extraction feeding niche, levels of food sharing increased dramatically over thatseen among chimpanzees.
Among the Ache, individuals whoare sick or injured are frequently fed
by other individuals, often ones who
are not kin. Those who share a greater
percentage of their production areprovisioned by more individuals when
they are sick or injured.51 Illness andinjury are common among those inforaging societies, as we might imagine, given their frequent exposure todangers, parasites, and pathogens,and the lack of modern medical care.
A systematic health survey in one reservation Ache community in 1997
showed that adults required care at
the community clinic on 6% of all person days, whereas children requiredcare on 3.5% of all person days. Sugiyama and Chacon78 have calculated
that Yora men of Peru were unable tohunt on about 10% of all person days
monitored. Bailey79 reported that Efe
men came to ask for medical treatment on 21% of all man days and often had problems that precluded foraging. Most relevant to our hypothesis
is how often men, women, or children
are sick or injured for many days in a
way that would preclude food acquisition. No data are yet available on
this topic. However, we have seen avariety of serious medical problems
including snakebites, injuries sustained in piranha and jaguar attacks,
broken bones, large punctures, andanimal bites, as well as, arrow

wounds, massive infections, and occasional illness that have kept people
from foraging for more than a week at
a time. Indeed, we researchers haveall experienced medical problems thatlasted for a week or more and prevented us from working in the field. If
people were not food-subsidized during such periods, mortality rates
would undoubtedly be higher.
Large, widely dispersed food patches
may promote grouping among animals.80,81 Human women and children generally spend the day in parties that contain many individuals of
the foraging band, whereas chimpanzee females and juveniles are often
dispersed into parties of one or two
individuals.76,82 This appears to have
two important consequences. First,

Large package size has
been positively
associated with theprobability of foodtransfers or the degreeof food transfer invirtually every study inwhich it has been
examined.

human males experience higher mortality from predators than do females.
Eight of the nine Ache who died fromjaguar attacks in the twentieth century were men, as were 14 of the 18
who died from snakebite.10 Second,
human juveniles are rarely killed by
predators. In recent memory and historical mythology, no Ache child has
been killed by a jaguar. The only
!Kung reportedly killed by a predator
in Howell’s83 demographic study wasan older man. Nevertheless, there isgood evidence that chimpanzee juveniles are killed by predators20,76 andthat adult females are probably killed
more often than adult males.
Human tool use probably provides
a good deal of protection from predators. Fire appears to frighten many

predators and may provide a gooddeal of nocturnal protection. Hunting
tools can inflict serious injury or
death on predators, thus providing
protection for women and children,
particularly when multiple males arepresent. The knowledge and analysisof animal behavior, fundamental tohuman hunting, can also be employed
to fend off predators. Ache often know
when the group is being stalked by ajaguar by analyzing its footprints andmovement patterns. When they become aware of a predatory jaguar,
they build brush walls around thecamp and take turns acting as sentinels through the night.
These quantitative and qualitative
data about food sharing, tools, andknowledge provide suggestive evidence that the shift to large-package,high-quality foods indirectly acted tolower mortality rates. Research on
causes of death and risks of predationand illness among both humans andchimpanzees is necessary to determine the factors responsible for the
more than three-fold differences in juvenile and adult mortality rates.

DISCUSSION
Primate Life-History Evolution
The life-history traits and large
brains of fully modern Homo sapiens
may be seen as the extreme manifestation of a process that defines theprimate order as a whole. Our theoryorganizes the major evolutionary
events in the primate order and thespecific changes that occurred in thehominid line.
The early evolution of the primate
order (60 mya to 35 mya) was characterized by small increases in encephalization. Relatively little is known
about early life-history evolution except that it appears that even the more“primitive” prosimian primates werelong-lived and delayed in reaching reproductive maturity as compared tomammals of similar body size. Austad
and Fischer84,85 have related this evolutionary trend in the primates to thesafety provided by the arboreal habitat. They compare primates to birds
and bats, which are also slow in developing and long-lived for their bodysizes. Thus, the first major grade shiftthat separated the primate order from176 Evolutionary Anthropology

other mammalian orders was achange to a lowered mortality rate
and the subsequent evolution ofslower senescence rates.
The second major grade shift occurred with the evolution of the anthropoids, the lineage containingmonkeys, apes, and humans, beginning about 35 mya. This was characterized by an increasing emphasis onplant foods as opposed to insects, andby more rapid increases in brain size
relative to body size.80 The major defining characteristic of the evolutionof the anthropoids was reorganizationof the sensory system from one inwhich olfaction and hearing were relatively dominant to one completely
dominated by binocular color vision.80 This grade shift is almost certainly tied to a dietary shift toward adiverse array of plant parts, particularly fruits and leaves.
The diet of the anthropoids has
been characterized as both “broadand selective.”86,87 The diet is broad
precisely because it is so selective. Anthropoid primates tend to select foods
on the basis of the ripeness, fiber content, nutrients, and toxicity of foods
consumed early in the day.88 –90 This
selectivity requires the allocation ofincreased brain tissue to visual processing.91,92 It also requires that manydifferent species of plants and animals
be included in the diet, increasing thedemands for memory and learning.93,94
This grade shift is reflected in brain
size. Regressions of log brain size onlog body size show that the intercept
is significantly lower for strepsirhine
primates (including most prosimians)
than for the haplorhine primates (including all anthropoids and a few prosimians).
The third major grade shift in primates occurred with the evolution of
the hominoid lineage, the branchleading to apes and humans. This shift
included further encephalization. Theintercept in a regression of log brain
size on log body size is significantly
higher for apes than monkeys, andapes clearly perform better on mosttasks reflecting higher intelligence.93,95
Evidence on teeth and tooth wear
among early hominoids suggests adiet composed mainly of soft, ripe
fruits.96 The frugivorous emphasis of

ARTICLEthe hominoid lineage is evident inlater hominoid species as well.96
Wrangham and colleagues97,98 also
show that the chimpanzee diet isbased on a much greater percentage
of ripe fruits than is the diet of othersympatric primate frugivores. (This isprobably true of orangutans as well).
The cognitive demands of a diet thatemphasizes ripe fruits are likely to bemuch greater than simple frugivory.
For one thing, there are greater perceptual demands in detecting the stateof fruit from visual cues against abackground.91 For another, the lower
abundance of ripe fruits and the short

Although, as comparedto humans,
chimpanzees engage in
relatively little extractive
foraging and hunting,they do much morethan monkeys. In thissense, their superior
intelligence and greater
encephalization thanoccurs in monkeys
illustrates the same
evolutionary forces that
separate humans fromapes.

time in which they are available (ripe,
but not yet eaten by competitors) is
likely to impose greater demands withrespect to monitoring the environment, remembering the state of individual trees, and predicting when the
fruits will become ripe on the basis of
their current state.
In addition to eating ripe fruits,
chimpanzees and gorillas also use
complex techniques for extracting
foods from protected substrates, suchas nut-cracking, termite and ant fishing, and removal of bark to get atpith.93 Gibson99,100 identified extractive foraging in primates as an impor-tant selective force in primate intelligence and presented evidence insupport of this idea. Furthermore,these behaviors vary between groups
as social traditions. In a comprehensive review of chimpanzee cultures using 151 years of chimpanzee observations from seven long-term studies, 39
behavior patterns were found to becustomary or habitual in some communities but absent from others
where ecological explanations could
be discounted.101 Of these, 19 were
patterns of extractive foraging. An additional 14 extractive foraging behaviors were identified but failed toachieve habitual status in any one
community. Furthermore, chimpanzee males are avid hunters. Boesch
and Boesch20 present data indicatingthat older chimpanzee males are capable of predicting escape patterns of
their prey and of predicting how prey
will respond to the behaviors of otherchimpanzees. This appears to require
even greater levels of cognitive processing than are required for extractive foraging. Although, as comparedto humans, chimpanzees engage in
relatively little extractive foraging andhunting, they do much more thanmonkeys. In this sense, their superior
intelligence and greater encephalization than occurs in monkeys illustrates the same evolutionary forcesthat separate humans from apes. (See
Lancaster and associates102 for a review of chimpanzee behavior and cognition.)
There is some debate, however,about the relative importance of dietversus group living in the evolution ofprimate intelligence and brain size.
According to one view, the increase inbrain size was largely driven by thecomplexities of the primate diet. Jerison103,104 suggested that brain tissue
evolves in response to two kinds ofdemands: One depends on body size,
based on monitoring and supportingan animal’s body tissue and particularly its surface area; the other is theability to assimilate, integrate, and remember environmental information.
He therefore predicted that differences in brain size, after controllingfor body mass, would be associatedwith an animal’s ecological niche andits demands for information processing. Jerison103 hypothesized that theEvolutionary Anthropology 177

ARTICLE

need to process information in a complex three-dimensional environment
was the cause of the large brain ofprimates relative to the brains of othermammals.
Clutton-Brock and Harvey105 testeda version of this hypothesis with intergenera comparisons within families ofprimates. They reasoned that frugivores need to assimilate and retain
more environmental information thanfolivores do because fruits are more
scarcely distributed than leaves, requiring more specific locational memory. They found that, after controllingfor body size, both dietary emphasis(leaves versus fruits and insects) andthe size of the home range predicted
brain size. Milton23,89,106 extended
their work, focusing on gut specialization and brain size as alternativeroutes to energetic efficiency. Leaves,
while abundant, tend to contain highamounts of fiber and often toxins as
well. The ability to extract nutrientsfrom leaves depends on the size of thegut and other specializations designed
to facilitate fermentation for nutrient
extraction. Fruits, on the other hand,are ephemeral resources, patchily distributed but offering a higher densityof easily processed energy. Milton
showed in paired interspecific comparisons that gut size and brain size
were inversely correlated, corresponding to the dietary emphasis on fruits
versus leaves.
Another view is that brain-size evolution was driven primarily by thecomplexities of social life in primategroups.107,108 Many species of primates exhibit complex dominance hierarchies that are mediated by political alliances and relations among
relatives in genetic lineages.109 –112 It
is not clear why higher primates havesuch complex social relations, but it
appears that group living is at least
partially a response to predation thatsignificantly lowers predation. Among
species that tend to eat higher-quality
foods that are easy to monopolize, social relationships also mediate accessto foods within groups.113
The most recently published analyses with the largest samples show thatboth group living and diet are associated with brain size and the size of the
neocortex in primates.91,92 In a separate set of analyses, Allman and col-leagues66 have shown that group living and diet are positively associatedwith maximum life span in primates
after controlling for body size. They
also have shown a high positive correlation between brain size and life
span, again controlling for body size.
Smith114 showed that after controllingfor body size, brain size in primates ispositively associated with the age offirst molar eruption, an indicator of
the age at which individuals begin toconsume adult diets, as well as withage of first reproduction and longevity. The relationship is strongest for

It seems likely that thecognitive ability
associated with foraging
in a complex threedimensional
environment is an
important preadaptation for social
intelligence andcomplex social
relations. Of particularinterest here are thespecializations inherent
in the primate visualsystem . . .

age of first molar eruption, probably
because brain growth and postcranial
morphological development compete.115
Kaplan and colleagues116 have recently conducted multivariate analyses specifically designed to test thepresent theory. They found that whenbrain size is regressed on body size,
age at first reproduction, maximum
life span, percent of fruit in the diet,range size, and group size in a multivariate model, all but group size weresignificant predictors of brain size. In
addition, the frugivory and range size
variables, meant to capture the cognitive demands of the diet, also predictthe life-history variables (age of first

reproduction and maximum lifespan)
after controlling for body weight.
Regardless of the importance ofgroup size in determining brain size
in monkeys, the grade shift in brain
size between monkeys and apes is almost surely related to diet and not togroup size.95 Apes show dietary specialization, but do not live in particularly large groups. In fact, gibbons,orangutans, and to a lesser extent, gorillas, live in relatively small groups
while chimpanzees and bonobos form
groups of the same size as do baboons
and vervets.
It seems likely that the cognitive
ability associated with foraging in acomplex three-dimensional environment is an important pre-adaptation
for social intelligence and complex social relations. Of particular interest
here are the specializations inherent
in the primate visual system, including binocular vision, high visual acuity, and associated increases in thesize of the lateral geniculate nucleus.64,65,91 Barton91 has shown that thesize of the parvocellular system involved in the processing of visual stimuli increases with increasing group size
among nonhuman primates. Thus, it
may be that selection favoring social
intelligence increases once the necessary cognitive pre-adaptations exist.
This may be one reason why primates
exhibit such high levels of social complexity relative to other group-living
mammals, such as many herbivores.
Adept social manipulation that leads
to a higher position in a dominancehierarchy may be favored by natural
selection, for dominance has correlated positively with measures of fitness in a multitude of primate studies.109 –113,117 Yet this would probably
be true for other social mammals as
well. Many of the same abilities tostore and analyze information may be
employed to solve dietary and social
problems.94 Once social intelligenceand social complexity evolve, the cognitive adaptations may be maintainedeven when dietary complexity is secondarily reduced in some species, asis the case with the derived simplicityof the diet of colobine monkeys.105
Apes show remarkably sophisticatedsocial intelligence94,112,118,119 andstand out in comparison to monkeys.
Although this cannot be a result of178 Evolutionary Anthropology

greater group size per se, it is again
possible that selection on social applications of intelligence increased when
the cognitive adaptations to feeding
acted as a pre-adaptation.
It is useful to think of primate evolution as both a branching and a directional process. It is branching inthe sense that ecological variation, intraniche competition, and segregation
generate variable selection pressuresthat lead to the evolution of multiple
species with different traits. Some are
selected to rely on more easily acquired foods and travel less, to invest
more in gut physiology and less inbrains, and to mature more rapidly
and live shorter lives. Others are selected to rely on more complex feeding strategies and exhibit their respective, correlated life histories. It is adirectional process in the sense thatcognitive and life-history adaptations
that evolved previously act as pre-adaptations for further selection in thesame direction in response to niche
competition and ecological change.
Homo sapiens is an extreme in one ofthose directions.

The Hominid LineThe fourth major grade shift in primate evolution occurred with divergence of the hominid line, particularly
the evolution of genus Homo. The
brain and life span of modern humans
are clearly “outliers” compared tothose of other mammals, and even ascompared to the relatively largebrained, slow-living primates. The
evolution of these extreme adaptations in the hominid line is built on ahominoid base that already showed asignificant tendency toward large
brains, long lives, and exploitation ofhigh-quality foods. Available evidence
on Australopithecines suggests thatbipedalism preceded changes in brain
size and life history.114,115
Although the record is still incomplete, it appears that brain enlargement and life-history shifts co-occurred. Early Homo ergaster shows
both significant brain expansion andan elongated developmental period,120
but much less so than modern
humans. Neanderthals display bothbrain sizes and dental developmentthat are in the same range as those ofmodern humans. Bipedalism can be

ARTICLEthought of as a pre-adaptation thatevolved to facilitate terrestrial locomotion, allowing for long day ranges
through energetic efficiency.121 Achemen walk an average of 15 km perday, about three times the distancecovered by chimpanzee males,32,33
even though both inhabit tropical forests. Bipedality also had the secondary effect of freeing the hands for specialization in manipulative activities.
Environmental conditions in the early
Pleistocene interacted with bipedality
to favor an increased emphasis on extractive foraging, hunting, learning,
prolonged development, and long
lives for at least one line of the hominid family. It may be that this uniquesuite of conditions is responsible forthe extreme differences between humans and other primates in intelligence, development, longevity, and resource flows across generations. The

Although the record isstill incomplete, itappears that brain
enlargement and lifehistory shifts
co-occurred.

complexities of a highly variable climate also would have favored a trend
toward cognitive solutions that led to
new foraging opportunities. Hominids with such solutions were probably
better able to withstand rapid climate
and habitat change than those who
were more inflexible.122
The shift toward a high-quality dietbased on learned foraging techniques
also implies another co-evolutionary
process. Just as the juvenile period
and the life span co-evolved, so too didbrain and gut sizes co-evolve with lifehistory traits and with each other.23,123 Many other physiologicaltraits are probably related to thisevolved complex. For example, it islikely that hidden estrus among humans is related to the male energy
subsidization of adult female reproduction,62 and that the postreproduc-tive life span of both sexes is related tohigh food production by older individuals. Almost certainly, the reason thathuman females have shorter interbirth intervals than apes do124 (andprobably different physiological sensitivity to nursing stimulation) is because
reproductive-aged human women areable to decrease rather than increase
their food production during lactation
due to subsidies by other age and sex
classes.102 This commitment to foodsharing is evident in human physiology and behavior. Women obtain
more of the extra energy required for
pregnancy and lactation by increases
in energy intake125 and reductions inenergy expenditure12,126 rather thanby an increase in fat mobilization or
metabolic economy.125 Reductions inbasal metabolic rate during earlypregnancy127 are not sufficient tomake up for the increased energy requirements. It is likely that this response evolved in the context of provisioning.
Figure 11 illustrates our historicalhypothesis regarding the hominid dietand its impact on both pay-offs tolearning and development and on
mortality rates. The figure begins with
two important exogenous changes.
The first is a change in the distribution of foods with the emergence ofAfrican savannas in the Pleistocene,
which increased the abundance ofhigh-quality but protected plant foods(nuts and tubers, in particular) andanimal foods. The second change isthe pre-adaptation of bipedality,
emerging in the Australopithecines asa locomotor adaptation. This adaptation frees the hands for tool use,which allows more efficient extraction
and hunting and frees the hands forcarrying large packages of food suitable for sharing.128,129 It also results inefficient terrestrial locomotion andhigher daily mobility that would increase the daily encounter rate withrare but energetically rich resources.
These changes led to an increased emphasis on large, high-quality, but difficult-to-acquire foods. Our hypothesis is that this feeding niche had
multiple effects: It increased the premium on learning and intelligence,delaying growth and maturation; increased nutritional status and decreased mortality rates through foodEvolutionary Anthropology 179

ARTICLE

Hadza women produce more foodthan younger women do, postreproductive women produce less than
Hadza men (Fig. 2). Furthermore, it isunlikely that the Hadza pattern ofhigh food production by postreproductive women is common among
other foragers. Ache and Hiwi postreproductive women do not acquire
even half the daily food energy acquired by adult men. !Kung data alsosuggest that older women produce
very little, even leading the Hadza research team to ask “Why don’t elderly!Kung women work harder to feed
their grandchildren?” (Blurton Jones

Figure 11. Ecology and co-evolutionary process in the Hominid line.

sharing (predicted by the provisioningof young sick or injured individuals);
and released selection against larger
group size, which lowers predationmortality.

The Grandmother Hypothesis
The present theory shares some features with a model of menopause andhuman life history recently proposed
by Hawkes and colleagues,130 often referred to as the grandmother hypothesis. This hypothesis proposes thathumans have a long life span relative
to that of the other primates becauseof the assistance that older postreproductive women contribute to descendant kin through the provision of difficult-to-acquire plant foods. Women,
therefore, are selected to invest in
maintaining their bodies longer thanchimpanzee females do. Thus, boththeories focus on the exceptionallylong human life span. Rather than regarding the cessation of reproductionin the fifth decade of life as the criticaladaptation (a feature shared withapes), both theories attempt to explain the extension of the human life
span beyond menopause.24,130 Thisimplies that some fertility earlier inlife is given up in order to prolong theadult life span.24 The theories differ inimportant ways, however.
First, the grandmother theory focuses on the productivity of older
women, but not on investments inlearning and development. The
present theory specifically links highproductivity later in life to investments in a large brain and to theunique features of growth in humanchildren. The grandmother hypothesis is silent about the expansion of thecostly human brain. It also offers no
explanation of why human children
grow so slowly and take so long tomature, except for the fact that adultmortality rates are low. In fact,
Hawkes and colleagues130 suggestthat learning is a secondary effect of along juvenile period (determined by
the long lifespan) rather than thecause of the long juvenile period. Second, the grandmother theory fails toaccount for why men live to about thesame age as women. If the benefits ofliving longer derive, for women, fromprovisioning descendant kin and, formen from direct reproduction, there
is no reason why their life spansshould be so similar. Third, the grandmother model fails to capture the important role that human males play insupporting women’s reproduction. It
also fails to explain the age profile ofproduction among males and why
men take so long to reach their productive peaks.
The evidentiary basis of the grandmother hypothesis is also very weak.
There is no direct evidence that postreproductive women are the major
food providers in any society. While
Hadza data do suggest that older

The present theory
specifically links highproductivity later in life
to investments in a large
brain and to the uniquefeatures of growth inhuman children. The
grandmother hypothesis
is silent about the
expansion of the costly
human brain. It alsooffers no explanation of
why human children
grow so slowly and takeso long to mature. . .

and coworkers,13 p. 388). And certainly none of the high-meat-consuming societies of Table 4 nor any of thesocieties in human history dwelling athigh latitudes could have been mostly
dependent on the food production of
postreproductive women. While weagree that the age profile of production is shifted toward older ages
among women as well as men, andthat postreproductive women provide
many important services in foraging
societies, among them child care, tool
making, food processing, and camp
maintenance, there is no evidence tosupport the hypothesis that they have180 Evolutionary Anthropology

ARTICLE

been the “breadwinners” in most societies during human history.
The present theory organizes all ofthese facts. Both males and females
exploit high-quality, difficult-to-acquire foods (females extracting plantfoods and males hunting animal
foods), sacrificing early productivity
for later productivity, with a life-history composed of an extended juvenile
period in which growth is slow, alarge brain is programmed, and a highinvestment is made in mortality reduction and maintenance to reap therewards of those investments.

co-evolutionary selection among lifehistory traits without requiring any
trait to be treated as extrinsic. Thisis more realistic than previous approaches, but makes for increasingly
demanding empirical analysis. There
are many possible pay-offs to timespent in development. The most general benefit of increased time spent indevelopment is increased body size
and its effects on energetic turnover,survival, and mating success. The coevolutionary selection measures posited here should apply to those effects,
but of primary interest in our theory
are the benefits associated with learn-

PREDICTIONS OF THE THEORYTO BE TESTED IN FUTURE
RESEARCH

ing and information storage. Varia-

Comparative Biology
The co-evolutionary selection pressures posited by our theory and supported by the quantitative theoreticalmodel should be generally applicable.
Holding pay-offs to time spent as ajuvenile constant, increased survival
rates during the juvenile period should
select for delayed reproduction, as
predicted by the models of Kozlowski
and Weigert5 and Charnov.1,9 Holding
constant extrinsic mortality hazards
and the pay-offs to investments inmortality reduction, increased payoffs to time spent in development as ajuvenile should select for increased allocations to survival and lower mortality rates. The latter is a novel prediction of our theory. The testing ofour predictions will require careful
comparative research within taxonomic groups at different levels ofanalysis (for example, among birds,
mammals, and reptiles, and amongorders and genera within those higher
taxonomic levels). It will be necessary
to distinguish pay-off functions andmortality hazards from the actual levels achieved, given the observed allocations. For example, it will be necessary to distinguish the risk of dying
from different causes, such as predation and infectious disease, from observed mortality rates due to thosecauses, because the observed mortality rate will be affected by allocations
to predator avoidance and feeding,and by allocations to immune function.
A principal innovation of the proposed theory is the incorporation of

A principal innovation of
the proposed theory is
the incorporation of coevolutionary selection
among life-history traits
without requiring any
trait to be treated as
extrinsic. This is more
realistic than previous
approaches, but makes
for increasingly
demanding empiricalanalysis.

tion within birds, carnivores, pinnipeds, and primates should beparticularly fertile ground for testing
predictions generated by the theory
with respect to learning, brain size,and life-history traits. The generalprediction is that within those taxonomic groups, species that rely on
more learning-intensive, complex
feeding strategies will have longer developmental periods, longer lifespans, and larger brains relative tobody size. With respect to brain size,
the correlations of brain size with lifehistory characteristics and feeding
strategies should be reflected in thoseparts of the brain associated withlearning and information storage, andnot with raw perceptual processing,such as sonar and visual acuity (unless, of course, those functions are
more developed in organisms thatlearn and store information). Suggestive evidence in support of the theory
is available for birds131 and primates,116 but rigorous testing of thosepredictions awaits further research
and analysis.

Hominid Evolution
Following this line of reasoning, thefirst major increases in brain size inthe hominid line should be accompanied by extensions of the juvenile
period, increased longevity, and increased complexity of learned foraging strategies. Smith’s analyses of
tooth eruption among early Homo ergaster provide suggestive evidence ofextension of the juvenile period beyond that of chimpanzees and Australopithecines, but to a lesser extent
than that of fully modern humans. So
far, the record is silent with respect tolongevity and mortality rates. Ourability to test the prediction that longevity increases with brain size duringhominid evolution may await new developments such as advances in theextraction of DNA from fossil remains
and in understanding the genetics ofaging. Nevertheless, this extension of
the expected life span with increased
emphasis on learning in development
and brain size is a firm prediction of
our theory.
Although the early evolution of thegenus Homo could have been accompanied by increased complexity of foraging strategies with respect to either
hunting or gathering, we strongly
suspect that future research will
demonstrate increases in both theimportance of hunted foods andcomplex extractive technologies forgathering.23,121,128,129,132,133 Recently,
O’Connell and associates49 have rejected hunting as being important inearly hominid evolution on two
grounds. First, some archeological assemblages that include large accumulations of animal bone, thought to illustrate the hunting life-style, havenow been reinterpreted as possibly resulting from natural processes. Second, recent data show that chimpanzees hunt considerably more than waspreviously thought. Therefore, hunting cannot be the cause of the changes

Evolutionary Anthropology 181

ARTICLEin hominid life histories and social
systems.
With respect to the first point, it ispremature to reject the “hunting hypothesis” simply because the causes ofbone accumulation at hominid sites
are not well understood and are open
to various interpretations. It has neverbeen demonstrated that early hominids did not rely heavily on hunted
foods. It is also the case that too muchemphasis has been placed on hunting
large game (Jones presents suggestive
evidence that bone accumulations at
Olduvai reflect reliance on small gamehunting134). Chimpanzees hunt small
game, as do many modern hunter
gatherers. For example, the Ache rely
heavily on small game: About 50% of
the animal food in the Ache diet issmall game acquired using handhunting techniques (no projectile
weapons). In fact, it may be that thelearning demands of a diet based on
small game may be greatest because it
can require killing many different species at regular intervals. Thus, muchencounter-specific and species-specific knowledge and creativity may
also be required.
With respect to the second point,
Tables 1 and 2 show that humans andchimpanzees exploit very different
food niches. While chimpanzees dohunt, the most successful chimpanzee
hunters obtain less than 10% of thedaily per capita energy intake frommeat reported for any human foraging group. To claim that humans andchimpanzees both must have thesame life history and social system because they both hunt is equivalent toasserting that chimpanzees must havethe same social system as black andwhite colobus monkeys because theyboth eat some ripe fruit. We predictthat early hominids will show a major
increase in the consumption of
hunted foods. Analyses of vitamins
and minerals in hominid bone remains may provide new avenues fortesting that hypothesis, in addition toarcheological evidence.135
It is clear that the process of hominization occurred over a long time.
Our theory is silent about the dates ofimportant events and evolutionarychanges in the hominid line. Whether
the process will look ratchet-like, inthat small increases in longevity will
precede small increases in brain size,
which, in turn, lead to further in-creases in longevity, is also an open
question. Whether the process is gradual or punctuated, our theory predicts
that changes in brain size, the dietaryimportance of meat and other difficult-to-acquire foods, gut size, and life
span will be seen to co-evolve in thearcheological and paleontologicalrecord. It is important to recognizethat many of those changes may bequantitative as well as qualitative. For
example, although chimpanzees huntmonkeys, they only pursue them in avery small proportion of their encounters with them, presumably because
most of those encounters do not occur
under conditions likely to result in a

To claim that humans
and chimpanzees bothmust have the same life
history and social
system because theyboth hunt is equivalent
to asserting thatchimpanzees must havethe same social systemas black and whitecolobus monkeys
because they both eatsome ripe fruit.

successful kill. Ache foragers only
rarely ignore an encounter with monkeys, even when they hear them at agreat distance, because most such encounters do result in successful kills,
presumably because of the Ache’s skill
and effective weaponry. The quantitative changes in brain size and lifehistory traits during hominid evolution should be accompanied byquantitative changes in diet and skillintensive foraging techniques.

Modern Humans Versus
Chimpanzees
There are at least two major avenues of future research with extant

hunter-gatherers and chimpanzees
for testing the predictions of our theory. First, we have provided only suggestive evidence that food sharing andprovisioning lower both juvenile andadult mortality rates in humans andthat this is a major cause of the difference between human and chimpanzee
age-specific survival probabilities. We
need to know a great deal more about
the frequency of illness in both species
and the relationship between morbidity and mortality. What is the relationship between illness and food intake
rates in both species? What is the relationship between reduced food intake and mortality? Our theory predicts that weight loss accompanying
illness will be more frequent and severe, and that illness will more frequently result in mortality among
chimpanzees than humans. Our theory also predicts that chimpanzee juveniles will spend more time than human juveniles in contexts that expose
them to predation, and that this difference will be due to both group size
and time spent in the food quest. Our
theory also predicts absolutely greater
frequency of predation on chimpanzees than on humans.
A second avenue of research is on
the components of foraging success. A
principal difference between our theory and the grandmother hypothesisis that we propose that the human life
course and large brain are the resultsof a long learning process that is necessary for successful foraging, andthat both males and females engage inthis process. Recently, on the basis ofsome experiments among Hadza juveniles, Blurton Jones and Marlowe136
have suggested that human foraging is
not very difficult to learn. We proposethat hunting, as practiced by humans,
but not necessarily by other predators, is exceedingly difficult to learn
and requires many years of experience. Our observations of hunters insix different groups suggests to us that
it is not marksmanship, but the knowledge of prey behavior and remote
signs of that behavior such as tracks
and vocalizations that are the mostdifficult features of human hunting.30
This impression is testable. We
should find that marksmanship is acquired relatively early in the learning
process and that naive individuals
such as anthropology graduate students could be trained to be effective

182 Evolutionary Anthropology

marksmen with relatively little practice. In contrast, we should find thatknowledge of animal behavior andsigns of behavior are learned only
gradually over many years. We should
also find that encounter rates withprey are much more strongly age-dependent than are kill rates upon encounter. It should also be the case thatkills that rely on marksmanship areless age-dependent than kills that rely
on finesse and skill, such as luring animals out of holes. In addition, it
should be much more difficult to train
naive individuals to find prey than toshoot projectile weapons accurately.
Similarly, we should find that age effects on chimpanzee hunting should
be a result of differences in knowledgeof prey behavior. Boesch and Boesch20
provide suggestive evidence of such
effects, but more research is necessary
for a definitive test. An analogous setof predictions also could be tested
with extractive gathering techniques
and the specific knowledge required
for each (Bock29,137 presents a seriesof experiments with grain processing
as a model of how such research could
be conducted.

CONCLUSIONSThe human adaptation is broad andflexible in one sense, but narrow andspecialized in another sense. It is
broad in the sense that, as huntergatherers, humans have existed successfully in virtually all of the world’s
major habitats. This has entailed eating a very wide variety of foods, bothplant and animal, both within andamong environments. It also has entailed a great deal of flexibility in thecontributions of individuals of different ages and sex. The relative contributions of men and women to foodproduction appear to vary from groupto group. Even the contributions ofchildren and teens to food production
vary predictably with the abundanceof easy-to-acquire foods.
Our adaptation is narrow and specialized in that it is based on a dietcomposed of large, nutrient-dense,
difficult-to-acquire packages and a life
history with a long, slow development, a large commitment to learningand intelligence, and an age profile ofproduction shifted toward older individuals. We do not expect to find any
human population that subsists on

ARTICLE

leaves or other low-quality foods. Indeed, we expect humans to remain at
the very top of the food hierarchy inevery environment they live in (for example, humans often exterminate allother top predators in their habitat).
Humans ingest foods that are alreadyhigh in quality and do not require
much digestive work or detoxification.
And if a food contains toxins, they are
generally removed prior to ingestion
by processing techniques. This dietarycommitment is reflected in the extremely reduced size of the human

Finally, the effect of thecommitment to foodsharing is evident in the
reproductive physiologyof human women.
Provisioning permits
human women, in
contrast to other female
primates, to reduce
rather than increase
their rate of energy
production during theirreproductive years,
when they have bothinfant and juvenile
nutritional dependents
and a greatly reduced
spacing between births.

hindgut.23 Humans use their great intelligence to extract and hunt thosefoods. In order to achieve this diet,humans also engage in extensive foodsharing both within and among age
and sex classes of individuals. Finally,
the effect of the commitment to foodsharing is evident in the reproductive
physiology of human women. Provisioning permits human women, in
contrast to other female primates, toreduce rather than increase their rateof energy production during their reproductive years, when they have bothinfant and juvenile nutritional depen-

dents and a greatly reduced spacing
between births.
The model and the data we havepresented suggest that the human life
course is based on a complex set ofinterconnected, time-dependent processes and the co-evolution of physiology, psychology, and behavior.
There appears to be a tight linkage
among the ordering of major psychological milestones (language learning,understanding and mastering thephysical, biological, and social environment); the timing of brain growth;
growth rates during childhood andadolescence; developmental changes
in survivorship; behavioral, psychological, and physiological changes
with the transition to adulthood; profiles of risk with age; and rates of senescence and aging. It is very likelythat a species-typical life course
evolved in response to the demands ofa hunting and gathering lifestyle thatwas broad and flexible enough to allow successful exploitation of theworld’s environments, but specialized
toward the acquisition of learned
skills and knowledge to obtain very
high rates of productivity later in life.

ACKNOWLEDGEMENTS
We thank K.G. Anderson, Theodore
Bergstrom, John Bock, Robert Boyd,
Pat Draper, Peter Ellison, Michael
Gurven, Sarah Hrdy, Monique Borgerhoff Mulder, David Lam, and Peter
Richerson for helpful discussions ofthese issues as well as Anne Pusey andJanette Wallis for sharing data on
chimpanzee development and birthspacing. We especially thank Nicholas
Blurton Jones, Eric Charnov, Kristen
Hawkes, and James O’Connell whoseseminal work in this area both challenged us and helped stimulate ourthinking, and who took time to critique various versions of this manuscript. Arthur Robson deserves special
recognition for performing the mathematical analysis to generate the results outlined in Box 1 and writing theMathematical Appendix, which isavailable from the authors.
Field research on the Ache, Hiwi,
and Machiguenga has been supported
by grants to Kim Hill and A. M. Hurtado by the National Science Foundation (BNS-8613215, BNS-538228,
BNS-8309834, BNS-8121209, BNS9617692), the National Institutes ofEvolutionary Anthropology 183

ARTICLE

Health (RO1HD16221-01A2), Fundacion Gran Mariscal de Ayacucho, Caracas, Venezuela, and two grants fromthe L.S.B. Leakey Foundation. Further support has gone to H. Kaplan
for field research on the Machiguenga
and Yora by the National Science
Foundation (BNS-8718886) and the
L.S.B. Leakey Foundation, and for life
history theory and life course evolution by the National Institute of Aging(1R01AG15906-01).

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some explanations of symmetry in evolutionary
ecology. Oxford: Oxford University Press.
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November 15-19, 2000
American Anthropological Association
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