Habilis

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***Homo habilis*** ("handy man") is a species of archaic human from the Early Pleistocene of East and South Africa about 2.3–1.65 million years ago (mya). Upon species description in 1964, *H. habilis* was highly contested, with many researchers recommending it be synonymised with *Australopithecus africanus*, the only other early hominin known at the time, but *H. habilis* received more recognition as time went on and more relevant discoveries were made. By the 1980s, *H. habilis* was proposed to have been a human ancestor, directly evolving into *Homo erectus* which directly led to modern humans. This viewpoint is now debated. Several specimens with insecure species identification were assigned to *H. habilis*, leading to arguments for splitting, namely into "*H. rudolfensis*" and "*H. gautengensis*" of which only the former has received wide support.

Like contemporary *Homo*, *H. habilis* brain size generally varied from 500–900 cm3 (31–55 cu in). The body proportions of *H. habilis* are only known from two highly fragmentary skeletons, and is based largely on assuming a similar anatomy to the earlier australopithecines. Because of this, it has also been proposed *H. habilis* be moved to the genus *Australopithecus* as ***Australopithecus habilis***. However, the interpretation of *H. habilis* as a small-statured human with inefficient long distance travel capabilities has been challenged. The presumed female specimen OH 62 is traditionally interpreted as having been 100–120 cm (3 ft 3 in–3 ft 11 in) in height and 20–37 kg (44–82 lb) in weight assuming australopithecine-like proportions, but assuming humanlike proportions she would have been about 148 cm (4 ft 10 in) and 35 kg (77 lb). Nonetheless, *H. habilis* may have been at least partially arboreal like what is postulated for australopithecines. Early hominins are typically reconstructed as having thick hair and marked sexual dimorphism with males much larger than females, though relative male and female size is not definitively known.

*H. habilis* manufactured the Oldowan stone tool industry and mainly used tools in butchering. Early *Homo*, compared to australopithecines, are generally thought to have consumed high quantities of meat and, in the case of *H. habilis*, scavenged meat. Typically, early hominins are interpreted as having lived in polygynous societies, though this is highly speculative. Assuming *H. habilis* society was similar to that of modern savanna chimps and baboons, groups may have numbered 70–85 members, with multiple males to defend against open savanna predators, such as big cats, hyenas and crocodiles. *H. habilis* coexisted with *H. rudolfensis*, *H. ergaster* / *H. erectus* and *Paranthropus boisei*.

## Taxonomy

### Research history

The first recognised remains—OH 7, partial juvenile skull, hand, and foot bones dating to 1.75 million years ago (mya)—were discovered in Olduvai Gorge, Tanzania, in 1960 by Jonathan Leakey. However, the actual first remains—OH 4, a molar—were discovered by the senior assistant of Louis and Mary Leakey (Jonathan's parents), Heselon Mukiri, in 1959, but this was not realised at the time. By this time, Louis and Mary had spent 29 years excavating in Olduvai Gorge for early hominin remains, but had instead recovered mainly other animal remains as well as the Oldowan stone tool industry. The industry had been ascribed to *Paranthropus boisei* (at the time "*Zinjanthropus*") in 1959 as it was the first and only hominin recovered in the area, but this was revised upon OH 7's discovery. In 1964, Louis, South African palaeoanthropologist Phillip V. Tobias, and British primatologist John R. Napier officially assigned the remains into the genus *Homo* as, on recommendation by Australian anthropologist Raymond Dart, *H. habilis*, the specific name meaning "able, handy, mentally skillful, vigorous" in Latin. The specimen's association with the Oldowan (then considered evidence of advanced cognitive ability) was also used as justification for classifying it into *Homo*. OH 7 was designated the holotype specimen.

After description, it was hotly debated if *H. habilis* should be reclassified into *Australopithecus africanus* (the only other early hominin known at the time), in part because the remains were so old and at the time *Homo* was presumed to have evolved in Asia (with the australopithecines having no living descendants). Also, the brain size was smaller than what Wilfrid Le Gros Clark proposed in 1955 when considering *Homo*. The classification *H. habilis* began to receive wider acceptance as more fossil elements and species were unearthed. In 1983, Tobias proposed that *A. africanus* was a direct ancestor of *Paranthropus* and *Homo* (the two were sister taxa), and that *A. africanus* evolved into *H. habilis* which evolved into *H. erectus* which evolved into modern humans (by a process of cladogenesis). He further said that there was a major evolutionary leap between *A. africanus* and *H. habilis*, and thereupon human evolution progressed gradually because *H. habilis* brain size had nearly doubled compared to australopithecine predecessors. However, OH 24, at the time the oldest *H. habilis* specimen, was similar to the younger OH 13, which showed there was no evolutionary progression through the lineage.

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Many had accepted Tobias' model and assigned Late Pliocene to Early Pleistocene hominin remains outside the range of *Paranthropus* and *H. erectus* into *H. habilis*. For non-skull elements, this was done on the basis of size as there was a lack of clear diagnostic characteristics. Because of these practices, the range of variation for the species became quite wide, and the terms *H. habilis* sensu stricto ("in the strict sense") and *H. habilis* sensu lato ("in the broad sense") were in use to include and exclude, respectively, more discrepant morphs. To address this, in 1985, English palaeoanthropologist Bernard Wood proposed that the comparatively massive skull KNM-ER 1470 from Lake Turkana, Kenya, discovered in 1972 and assigned to *H. habilis*, actually represented a different species, now referred to as *Homo rudolfensis*. It is also argued that instead it represents a male specimen whereas other *H. habilis* specimens are female. It has been suggested that early *Homo* from South Africa can be variously assigned to *H. habilis* or *H. ergaster* / *H. erectus*, but in 2010, Australian archaeologist Darren Curoe proposed splitting off South African early *Homo* into a new species, *Homo gautengensis*.

In 1986, OH 62, a fragmentary skeleton, was discovered by American anthropologist Tim D. White in association with *H. habilis* skull fragments, definitively establishing aspects of *H. habilis* skeletal anatomy for the first time, and revealing more *Australopithecus*-like than *Homo*-like features. Because of this, as well as similarities in dental adaptations, Wood and biological anthropologist Mark Collard suggested moving the species to *Australopithecus* in 1999. However, reevaluation of OH 62 to a more humanlike physiology, if correct, would cast doubt on this. The discovery of the 1.8 Ma Georgian Dmanisi skulls in the early 2000s, which exhibit several similarities with early *Homo*, has led to suggestions that all contemporary groups of early *Homo* in Africa, including *H. habilis* and *H. rudolfensis*, are the same species and should be assigned to *H. erectus*.

### Classification

There is still no wide consensus as to whether or not *H. habilis* is ancestral to *H. ergaster* / *H. erectus* or is an offshoot of the human line, and whether or not all specimens assigned to *H. habilis* are correctly assigned or the species is an assemblage of different *Australopithecus* and *Homo* species. Nonetheless, *H. habilis* and *H. rudolfensis* generally are recognised members of the genus at the base of the family tree, with arguments for synonymisation or removal from the genus not widely adopted.

Though it is now largely agreed upon that *Homo* evolved from *Australopithecus*, the timing and placement of this split has been much debated, with many *Australopithecus* species having been proposed as the ancestor. The discovery of LD 350-1, the oldest *Homo* specimen, dating to 2.8 mya, in the Afar Region of Ethiopia may indicate that the genus evolved from *A. afarensis* around this time. The species LD 350-1 belongs to could be the ancestor of *H. habilis*, but this is unclear. The oldest *H. habilis* specimen, A.L. 666-1, dates to 2.3 mya, but is anatomically more derived (has less ancestral, or basal, traits) than the younger OH 7, suggesting derived and basal morphs lived concurrently, and that the *H. habilis* lineage began before 2.3 mya. Based on 2.1 million year old stone tools from Shangchen, China, *H. habilis* or an ancestral species may have dispersed across Asia. The youngest *H. habilis* specimen, OH 13, dates to about 1.65 mya.

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## Anatomy

### Skull

It has generally been thought that brain size increased along the human line especially rapidly at the transition between species, with *H. habilis* brain size smaller than that of *H. ergaster* / *H. erectus*, jumping from about 600–650 cc (37–40 cu in) in *H. habilis* to about 900–1,000 cc (55–61 cu in) in *H. ergaster* and *H. erectus*. However, a 2015 study showed that the brain sizes of *H. habilis*, *H. rudolfensis*, and *H. ergaster* generally ranged between 500–900 cc (31–55 cu in) after reappraising the brain volume of OH 7 from 647–687 cc (39.5–41.9 cu in) to 729–824 cc (44.5–50.3 cu in). This does, nonetheless, indicate a jump from australopithecine brain size which generally ranged from 400–500 cc (24–31 cu in).

The brain anatomy of all *Homo* features an expanded cerebrum in comparison to australopithecines. The pattern of striations on the teeth of OH 65 slanting right, which may have been accidentally self-inflicted when the individual was pulling a piece of meat with its teeth and the left hand while trying to cut it with a stone tool using the right hand. If correct, this could indicate right handedness, and handedness is associated with major reorganisation of the brain and the lateralisation of brain function between the left and right hemispheres. This scenario has also been hypothesised for some Neanderthal specimens. Lateralisation could be implicated in tool use. In modern humans, lateralisation is weakly associated with language.

The tooth rows of *H. habilis* were V-shaped as opposed to U-shaped in later *Homo*, and the mouth jutted outwards (was prognathic), though the face was flat from the nose up.

### Build

Based on the fragmentary skeletons OH 62 (presumed female) and KNM-ER 3735 (presumed male), *H. habilis* body anatomy has generally been considered to have been more apelike than even that of the earlier *A. afarensis* and consistent with an at least partially arboreal lifestyle in the trees as is assumed in australopithecines. Based on OH 62 and assuming comparable body dimensions to australopithecines, *H. habilis* has generally been interpreted as having been small-bodied like australopithecines, with OH 62 generally estimated at about 100–120 cm (3 ft 3 in–3 ft 11 in) in height and 20–37 kg (44–82 lb) in weight. However, assuming longer, modern humanlike legs, OH 62 would have been about 148 cm (4 ft 10 in) and 35 kg (77 lb), and KNM-ER 3735 about the same size. For comparison, modern human men and women in the year 1900 averaged 163 cm (5 ft 4 in) and 152.7 cm (5 ft) respectively. It is generally assumed that pre-*H. ergaster* hominins, including *H. habilis*, exhibited notable sexual dimorphism with males markedly bigger than females. However, relative female body mass is unknown in this species.

Early hominins, including *H. habilis*, are thought to have had thick body hair coverage like modern non-human apes because they appear to have inhabited cooler regions and are thought to have had a less active lifestyle than (presumed hairless) post-*ergaster* species. Consequently, they probably required thick body hair to stay warm. Based on dental development rates, *H. habilis* is assumed to have had an accelerated growth rate compared to modern humans, more like that of modern non-human apes.

### Limbs

The arms of *H. habilis* and australopithecines have generally been considered to have been proportionally long and so adapted for climbing and swinging. In 2004, anthropologists Martin Haeusler and Henry McHenry argued that, because the humerus to femur ratio of OH 62 is within the range of variation for modern humans, and KNM-ER 3735 is close to the modern human average, it is unsafe to assume apelike proportions. Nonetheless, the humerus of OH 62 measured 258–270 mm (10.2–10.6 in) long and the ulna (forearm) 245–255 mm (9.6–10.0 in), which is closer to the proportion seen in chimps. The hand bones of OH 7 suggest precision gripping, important in dexterity, as well as adaptations for climbing. In regard to the femur, traditionally comparisons with the *A. afarensis* specimen AL 288-1 have been used to reconstruct stout legs for *H. habilis*, but Haeusler and McHenry suggested the more gracile OH 24 femur (either belonging to *H. ergaster* / *H. erectus* or *P. boisei*) may be a more apt comparison. In this instance, *H. habilis* would have had longer, humanlike legs and have been effective long-distance travellers as is assumed to have been the case in *H. ergaster*. However, estimating the unpreserved length of a fossil is highly problematic. The thickness of the limb bones in OH 62 is more similar to chimps than *H. ergaster* / *H. erectus* and modern humans, which may indicate different load bearing capabilities more suitable for arboreality in *H. habilis*. The strong fibula of OH 35 (though this may belong to *P. boisei*) is more like that of non-human apes, and consistent with arboreality and vertical climbing.

OH 8, a foot, is better suited for terrestrial movement than the foot of *A. afarensis*, though still retains many apelike features consistent with climbing. However, the foot has projected toe bone and compacted mid-foot joint structures, which restrict rotation between the foot and ankle as well as at the front foot. Foot stability enhances the efficiency of force transfer between the leg and the foot and vice versa, and is implicated in the plantar arch elastic spring mechanism which generates energy while running (but not walking). This could possibly indicate *H. habilis* was capable of some degree of endurance running, which is typically thought to have evolved later in *H. ergaster* / *H. erectus*.

## Culture

### Society

Typically, *H. ergaster* / *H. erectus* is considered to have been the first human to have lived in a monogamous society, and all preceding hominins were polygynous. However, it is highly difficult to speculate with any confidence the group dynamics of early hominins. The degree of sexual dimorphism and the size disparity between males and females is often used to correlate between polygyny with high disparity and monogamy with low disparity based on general trends (though not without exceptions) seen in modern primates. Rates of sexual dimorphism are difficult to determine as early hominin anatomy is poorly known, and are largely based on few specimens. In some cases, sex is arbitrarily determined in large part based on perceived size and apparent robustness in the absence of more reliable elements in sex identification (namely the pelvis). Mating systems are also based on dental anatomy, but early hominins possess a mosaic anatomy of different traits not seen together in modern primates; the enlarged cheek teeth would suggest marked size-related dimorphism and thus intense male–male conflict over mates and a polygynous society, but the small canines should indicate the opposite. Other selective pressures, including diet, can also dramatically impact dental anatomy. The spatial distribution of tools and processed animal bones at the FLK Zinj and PTK sites in Olduvai Gorge indicate the inhabitants used this area as a communal butchering and eating grounds, as opposed to the nuclear family system of modern hunter gatherers where the group is subdivided into smaller units each with their own butchering and eating grounds.

The behaviour of early *Homo*, including *H. habilis*, is sometimes modelled on that of savanna chimps and baboons. These communities consist of several males (as opposed to a harem society) in order to defend the group on the dangerous and exposed habitat, sometimes engaging in a group display of throwing sticks and stones against enemies and predators. The left foot OH 8 seems to have been bitten off by a crocodile, possibly *Crocodylus anthropophagus*, and the leg OH 35, which either belongs to *P. boisei* or *H. habilis*, shows evidence of leopard predation. *H. habilis* and contemporary hominins were likely predated upon by other large carnivores of the time, such as (in Olduvai Gorge) the hunting hyena *Chasmaporthetes nitidula*, and the saber-toothed cats *Dinofelis* and *Megantereon*. In 1993, American palaeoanthropologist Leslie C. Aiello and British evolutionary psychologist Robin Dunbar estimated that *H. habilis* group size ranged from 70–85 members—on the upper end of chimp and baboon group size—based on trends seen in neocortex size and group size in modern non-human primates.

*H. habilis* coexisted with *H. rudolfensis*, *H. ergaster* / *H. erectus*, and *P. boisei*. It is unclear how all of these species interacted. To explain why *P. boisei* was associated with Olduwan tools despite not being the knapper (the one who made the tools), Leakey and colleagues, when describing *H. habilis*, suggested that one possibility was *P. boisei* was killed by *H. habilis*, perhaps as food. However, when describing *P. boisei* five years earlier, Louis Leakey said, "There is no reason whatever, in this case, to believe that the skull represents the victim of a cannibalistic feast by some hypothetical more advanced type of man."

### Diet

It is thought *H. habilis* derived meat from scavenging rather than hunting (scavenger hypothesis), acting as a confrontational scavenger and stealing kills from smaller predators such as jackals or cheetahs. Fruit was likely also an important dietary component, indicated by dental erosion consistent with repetitive exposure to acidity. Based on dental microwear-texture analysis, *H. habilis* (like other early *Homo*) likely did not regularly consume tough foods. Microwear-texture complexity is, on average, somewhere between that of tough-food eaters and leaf eaters (folivores), and points to an increasingly generalised and omnivorous diet.

It is typically thought that the diets of *H. habilis* and other early *Homo* had a greater proportion of meat than *Australopithecus*, and that this led to brain growth. The main hypotheses regarding this are: meat is energy- and nutrient-rich and put evolutionary pressure on developing enhanced cognitive skills to facilitate strategic scavenging and monopolise fresh carcasses, or meat allowed the large and calorie-expensive ape gut to decrease in size allowing this energy to be diverted to brain growth. Alternatively, it is also suggested that early *Homo*, in a drying climate with scarcer food options, relied primarily on underground storage organs (such as tubers) and food sharing, which facilitated social bonding among both male and female group members. However, unlike what is presumed for *H. ergaster* and later *Homo*, short-statured early *Homo* are generally considered to have been incapable of endurance running and hunting, and the long and *Australopithecus*-like forearm of *H. habilis* could indicate early *Homo* were still arboreal to a degree. Also, organised hunting and gathering is thought to have emerged in *H. ergaster*. Nonetheless, the proposed food-gathering models to explain large brain growth necessitate increased daily travel distance. It has also been argued that *H. habilis* instead had long, modern humanlike legs and was fully capable of effective long distance travel, while still remaining at least partially arboreal.

Large incisor size in *H. habilis* compared to *Australopithecus* predecessors implies this species relied on incisors more. The bodies of the mandibles of *H. habilis* and other early *Homo* are thicker than those of modern humans and all living apes, more comparable to *Australopithecus*. The mandibular body resists torsion from the bite force or chewing, meaning their jaws could produce unusually powerful stresses while eating. The greater molar cusp relief in *H. habilis* compared to *Australopithecus* suggests the former used tools to fracture tough foods (such as pliable plant parts or meat), otherwise the cusps would have been more worn down. Nonetheless, the jaw adaptations for processing mechanically challenging food indicates technological advancement did not greatly affect diet.

### Technology

*H. habilis* is associated with the Early Stone Age Oldowan stone tool industry. Individuals likely used these tools primarily to butcher and skin animals and crush bones, but also sometimes to saw and scrape wood and cut soft plants. Knappers appear to have carefully selected lithic cores and knew that certain rocks would break in a specific way when struck hard enough and on the right spot, and they produced several different types, including choppers, polyhedrons, and discoids. Nonetheless, specific shapes were likely not thought of in advance, and probably stem from a lack of standardisation in producing such tools as well as the types of raw materials at the knappers' disposal. For example, spheroids are common at Olduvai which features an abundance of large and soft quartz and quartzite pieces, whereas Koobi Fora lacks spheroids and provides predominantly hard basalt lava rocks. Unlike the later Acheulean culture invented by *H. ergaster* / *H. erectus*, Oldowan technology does not require planning and foresight to manufacture, and thus does not indicate high cognition in Oldowan knappers, though it does require a degree of coordination and some knowledge of mechanics. Oldowan tools infrequently exhibit retouching and were probably discarded immediately after use most of the time.

The Olduwan was first reported in 1934, but it was not until the 1960s that it become widely accepted as the earliest culture, dating to 1.8 mya, and as having been manufactured by *H. habilis*. Since then, more discoveries have placed the origins of material culture substantially backwards in time, with the Oldowan being discovered in Ledi-Geraru and Gona in Ethiopia dating to 2.6 mya, perhaps associated with the evolution of the genus. Australopithecines are also known to have manufactured tools, such as the 3.3 Ma Lomekwi stone tool industry, and some evidence of butchering from about 3.4 mya. Nonetheless, the comparatively sharp-edged Oldowan culture was a major innovation from australopithecine technology, and it would have allowed different feeding strategies and the ability to process a wider range of foods, which would have been advantageous in the changing climate of the time. It is unclear if the Oldowan was independently invented or if it was the result of hominin experimentation with rocks over hundreds of thousands of years across multiple species.

In 1962, a 12 ft × 14 ft × 1 ft (366 cm × 427 cm × 30 cm) circle made with volcanic rocks was discovered in Olduvai Gorge. At 2–2.5 ft (61–76 cm) intervals, rocks were piled up to 6–9 in (15–23 cm) high. Mary Leakey suggested the rock piles were used to support poles stuck into the ground, possibly to support a windbreak or a rough hut. Some modern day nomadic tribes build similar low-lying rock walls to build temporary shelters upon, bending upright branches as poles and using grasses or animal hide as a screen. Dating to 1.75 mya, it is attributed to some early *Homo*, and is the oldest claimed evidence of architecture.

## Aquatic ape

The **aquatic ape hypothesis** (**AAH**), also referred to as **aquatic ape theory** (**AAT**) and more recently the **waterside model**, is the idea that certain ancestors of modern humans were habitual waders, swimmers and divers. The hypothesis in its present form was proposed by the marine biologist Alister Hardy in 1960, who argued that a branch of apes was forced by competition over terrestrial habitats to hunt for food such as shellfish on the sea shore and sea bed leading to adaptations that explained distinctive characteristics of modern humans such as functional hairlessness and bipedalism.

This proposal was built upon by Elaine Morgan in her 1972 book *The Descent of Woman*.

### History

The German pathologist Max Westenhöfer (1871–1957) discussed in 1942 various human characteristics (hairlessness, subcutaneous fat, the regression of the olfactory organ, webbed fingers, direction of the body hair etc.) that could have derived from an aquatic past, quoting several other authors who had made similar speculations. He stated: "The postulation of an aquatic mode of life during an early stage of human evolution is a tenable hypothesis, for which further inquiry may produce additional supporting evidence."

### The Hardy/Morgan hypothesis

Independently of Westenhöfer's writings, the marine biologist Alister Hardy had since 1930 also hypothesized that humans may have had ancestors more aquatic than previously imagined. On the advice of his colleagues, Hardy delayed presenting the hypothesis for approximately thirty years. After he had become a respected academic and knighted for contributions to marine biology, Hardy voiced his thoughts in an article in *New Scientist* on 17 March 1960.

Hardy's hypothesis as outlined in the *New Scientist* was:

My thesis is that a branch of this primitive ape-stock was forced by competition from life in the trees to feed on the sea-shores and to hunt for food, shell fish, sea-urchins etc., in the shallow waters off the coast. I suppose that they were forced into the water just as we have seen happen in so many other groups of terrestrial animals. I am imagining this happening in the warmer parts of the world, in the tropical seas where Man could stand being in the water for relatively long periods, that is, several hours at a stretch.
Hardy argued a number of features of modern humans are characteristic of aquatic adaptations. He pointed to human's lack of body hair as being analogous to the same lack seen in certain other marine mammals, such as the whale and the hippopotamus, and noted the layer of subcutaneous fat humans have that Hardy believed other apes lacked, although it has been shown that captive apes with ample access to food have levels of subcutaneous fat similar to humans. Additional features cited by Hardy include the location of the trachea in the throat rather than the nasal cavity, the human propensity for front-facing copulation, tears and eccrine sweating, though these claimed pieces of evidence have alternative evolutionary adaptationist explanations that do not invoke an aquatic context. Hardy additionally posited that bipedalism evolved first as an aid to wading before becoming the usual means of human locomotion, and tool use evolved out of the use of rocks to crack open shellfish. These last arguments were cited by later proponents of AAH as an inspiration for their research programs.

#### Wading and bipedalism

AAH proponent Algis Kuliukas, performed experiments to measure the comparative energy used when lacking orthograde posture with using fully upright posture. Although it is harder to walk upright with bent knees on land, this difference gradually diminishes as the depth of water increases and is still practical in thigh-high water.

#### Diving behavior and performance

Aside from working as a professor, Erika Schagatay is also an experienced scuba and freediver whose research centers around human diving abilities. She suggests that such abilities are consistent with selective pressure for underwater foraging during human evolution, and discussed other anatomical traits speculated as diving adaptations by Hardy/Morgan. John Langdon suggested that such traits could be enabled by a human developmental plasticity.

#### Vernix caseosa

Tom Brenna, professor of pediatrics whose primary research focuses on fats, oils, and fatty acids, listened to the 2005 radio documentary *Scars of Evolution* where David Attenborough reported an observation that harbor seals were born with something that resembled human vernix caseosa. Intrigued, Brenna led a team that collaborated with Judy St Leger at San Diego Seaworld to compare the chemistry of human vernix and samples from California sea lion pups. They established that the molecular composition of both is similar, being rich in branched chain fatty acids and squalene.