Climate-Driven Evolution

There are several rather distinct traditions in the contemporary study of human evolution. One is the detailed study of anatomical differences among fossils. A second emphasizes classical ecology with detailed attention to ancient climates and environments and comparative biogeography. A third has a focus on behavioral ecology (i.e., what used to be called sociobiology). It appears to this outsider that these are not so much competing traditions as they are nearly independent and seemingly unaware of each other. They each have much to contribute, they are each prominent in particular current disagreements in the field, and they each have glaring weaknesses. Anatomical details are central to the literature on Australopithecines and their relatives, primates in Africa on the human side of the ancient chimp–human split from five to ten million years ago to about two and half million years ago when we start calling the fossils Homo rather than Australopithecus. The anatomists are also in the thick of debates these days about the hobbit, the small hominid remains found on the island of Flores. Details of the wrist are apparently critically important in distinguishing whether these creatures are derived from Australopithecines, from Homo erectus (perhaps having undergone island dwarfism), or are simply pathological dwarves. A weakness of the anatomical tradition is the treatment of anatomical details as if they were selectively neutral, mere traits that can be tabulated to generate answers by consensus. There is not much awareness of quantitative genetics. The behavioral ecology tradition emphasizes mating competition and intergroup struggles as the important drivers of human evolution. With roots in economics, comparative animal behavior, and the new population genetics of interaction started by W.D. Hamilton in the 1960s, social interactions themselves are the prime drivers of human evolution. For decades there has been a kind of faith that there was some ‘‘environment of evolutionary adaptedness’’ (EEA) in the past to which humans are well adapted and that if we could know about this EEA then we could understand human nature. The ecologists look to ancient environments, climate, and climate change to explain changes in the distribution of humans and related species in time and space. They see humans responding to the physical and biotic factors like geography, climatic shifts, the availability of exploitive technologies, or the availability of domesticates. In this literature, humans are members of communities, and these communities are like organisms evolving to become better and better at provisioning their members. This group or species view of evolution contrasts sharply with the implicit model of the behavioral ecologists in which reproductive competition between individuals within groups is the important dynamic. The Humans Who Went Extinct is firmly in the third, that is, classical ecological tradition. This admirable book is a summary and interpretation of human evolution from the first bipeds several million years ago to the earliest settled agricultural populations after the end of the Pleistocene. The title does not do justice to the scope of the book: there is only a weak focus on the extinction of the Neanderthals. It is an excellent summary of the classical ecological approach to human evolution with a dazzling portrayal of the relevant paleoclimatology, paleoecology, and biogeography. These traditions ought to be complementary; instead they are rather isolated from each other. The discipline would be enriched if we acknowledged that the narratives that each tells are models and that models should be evaluated and falsified with data. There are numerous situations where models from the ecological and behavioral traditions compete directly, and such situations ought to be exploited for direct tests of the competing models. Why did settled farming appear after the Last Glacial Maximum in several areas of the world, but never in the prior interglacial about 120,000 years before? The ecologists explain this in terms of geography and climate, the availability of domesticates, increasing demand from a larger human population, and a lot of chance. Some in the behavioral ecology tradition see settled agriculture as a consequence of the formation of regional polities with militias that suppress local raiding and warfare, leading to population growth, land scarcity, and increasing labor inputs into subsistence. These models could both be true or both false, but they ought to be tested as competing hypotheses. Finlayson C (2009) The Humans Who Went Extinct: Why Neanderthals died out and we survived. New York: Oxford University Press. pp. XI+273. ISBN (cloth): 978-0-19-923918-4. US$29.95. doi:10.1371/journal.pbio.1000383.g001