We do not fully know the nature of these recent genetic changes. John Hawks suggests they may reflect adaptations to new ecological and cultural settings, specifically to cold, to new diets (cereals, milk, etc.), to new epidemic diseases associated with the spread of agriculture (smallpox, malaria, yellow fever, typhus, cholera), and to new forms of “communication, social interactions, and creativity.”
There thus seem to have been multiple EEAs in relatively recent times, and not simply one situated in the Pleistocene. Some of them would correspond to the different physical environments that modern humans moved into as they spread out of Africa 40 to 50 thousand years. Most however, seem to have arisen in the past 10 thousand years and correspond to different cultural environments.
John Hawks is certainly not the first one to suggest that culture has been a key part of the human adaptive landscape. Usually referred to as ‘gene-culture co-evolution’, this paradigm has had many proponents, notably Pierre van den Berghe, Charles Lumsden, and E.O. Wilson. It has nonetheless remained marginal, even among evolutionary psychologists. This is partly because of the influence of John Tooby and Leda Cosmides, whose influence was critical during the early years of evolutionary psychology:
It is no more plausible to believe that whole new mental organs could evolve since the Pleistocene—i.e., over historical time—than it is to believe that whole new physical organs such as eyes would evolve over brief spans. It is easily imaginable that such things as the population mean retinal sensitivity might modestly shift over historical time, and similarly minor modifications might have been made in various psychological mechanisms. However, major and intricate changes in innately specified information-processing procedures presentover brief spans of historical time. (Tooby & Cosmides, 1989)
In a more recent article, they have backed away from this position: “Although the hominid line is thought to have originated on edges of the African savannahs, the EEA is not a particular place or time.” Each biological adaptation has its own EEA, which is simply a composite of whatever selection pressures brought it into being (Tooby & Cosmides, 2005). There are thus potentially as many EEAs as there are adaptations. It follows, then, that some EEAs may have existed later in time than others.
How much later? Tooby and Cosmides considered complexity to be one limiting factor. The more complex the adaptation, the more genes it would involve, and the longer the time needed to coordinate the evolution of all those genes. Therefore, recent biological evolution has probably only involved simple traits, certainly nothing as complex as mental ones. Such traits could have arisen only through a faster process, notably cultural evolution.
The problem with this argument is that complex traits do not arise ex nihilo. They arise through modifications, deletions, or additions to existing traits. And such changes can occur through a single point mutation at a regulatory gene. As Harpending and Cochran (2002) point out:
Even if 40 or 50 thousand years were too short a time for the evolutionary development of a truly new and highly complex mental adaptation, which is by no means certain, it is certainly long enough for some groups to lose such an adaptation, for some groups to develop a highly exaggerated version of an adaptation, or for changes in the triggers or timing of that adaptation to evolve. That is what we see in domesticated dogs, for example, who have entirely lost certain key behavioral adaptations of wolves such as paternal investment. Other wolf behaviors have been exaggerated or distorted.
Gene-culture co-evolution also presents difficulties that are inherent to the paradigm itself:
1. The linkages between genes and culture tend to be remote, indirect, multiple, and complex. There are some straightforward ones, such as between lactose intolerance and consumption of dairy products, but such linkages are probably unrepresentative of gene-culture co-evolution.
2. With only a few minor exceptions, gene-culture co-evolution is specific to humans. Cross-species comparisons, so common in other fields of evolutionary study, are thus of little help (van den Berghe & Frost, 1986).
These difficulties are not insuperable. To some degree, they reflect an unconscious desire to study human evolution with the same conceptual tools that have been used to study the evolution of other species. Other tools will have to be developed, or simply borrowed from the social sciences of psychology, sociology, and anthropology. Thus, there are no real barriers to renewed use of this paradigm, particularly as we move beyond the single-EEA model and investigate this 7% of the human genome that has apparently changed over the past 40,000 years.
References
Harpending, H. & G. Cochran. (2002). "In our genes", Proceedings of the National Academy of Sciences, 99(1), 10-12.
Hawks, J., E.T. Wang, G.M. Cochran, H.C. Harpending, & R.K. Moyzis. (2007). Recent acceleration of human adaptive evolution. Proceedings of the National Academy of Sciences (USA), 104, 20753-20758.
Tooby, J. & L. Cosmides. (2005). Conceptual foundations of evolutionary psychology, in: D. M. Buss (Ed.) The Handbook of Evolutionary Psychology, Hoboken, NJ: Wiley, pp. 5-67.
Tooby, J. & L. Cosmides. (1989). Evolutionary psychology and the generation of culture, Part I. Theoretical considerations, Ethology and Sociobiology, 10, 29-49.
van den Berghe, P.L., & Frost, P. (1986). Skin color preference, sexual dimorphism and sexual selection: A case of gene-culture co-evolution? Ethnic and Racial Studies, 9, 87-113.