Thoughts on the EEA

For the past twenty years, a key concept in evolutionary psychology has been the ‘environment of evolutionary adaptedness’ (EEA). This is the ancestral environment within which our species first evolved and whose selection pressures shaped our current psychology and behavior. Usually, writers situate this environment in the Pleistocene before Homo sapiens began to spread out of Africa some 50,000 years ago.

The EEA concept has increasingly come under fire in recent years, especially with the recent Hawks et al. study. It now appears that human genetic evolution did not stop 50,000 years ago. Nor has it since slowed down. In fact, it has accelerated by as much as a 100-fold. In light of these findings, can the EEA concept be salvaged? Should it?

Interestingly, its earliest proponents, John Tooby and Leda Cosmides, have always been reluctant to narrow it down to a specific place and time:

Although the hominid line is thought to have originated on edges of the African savannahs, the EEA is not a particular place or time. The EEA for a given adaptation is the statistical composite of the enduring selection pressures or cause-and-effect relationships that pushed the alleles underlying an adaptation systematically upward in frequency until they became species-typical or reached a frequency-dependent equilibrium (most adaptations are species-typical; see Hagen, Chapter 5, this volume). Because the coordinated fixation of alleles at different loci takes time, complex adaptations reflect enduring features of the ancestral world. (Tooby & Cosmides, 2005, p. 22)

According to Tooby and Cosmides, there are potentially as many EEAs as there are human adaptations. Therefore, some human characteristics may have originated in very old EEAs and others in more recent ones.

How recent? For Tooby and Cosmides, the limiting factor is complexity. The more complex the adaptation, the more genes it will involve, and the longer the evolutionary time to coordinate all those genes. Therefore, recent human evolution has probably only involved simple traits, certainly nothing as complex as behavior.

The problem with this argument is that complex traits do not arise ex nihilo. They arise from changes to existing traits that may be just slightly less complex. A point mutation can greatly alter the functioning of a trait that involves thousands upon thousands of genes. Keep in mind that genes vary considerably in their effects. At one extreme, a single ‘structural’ gene may code for one protein. At the other, a single ‘regulatory’ gene may control the output of numerous structural genes … or even numerous regulatory genes like itself. 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. A border collie's herding is recognizably derived from wolf behaviors, as is a terrier's aggressiveness, but this hardly means that collies, wolves, and terriers are all the same. Paternal investment may be particularly fragile and easily lost in mammals, because parental investment via internal gestation and lactation is engineered into females but not males.

In all fairness, when the EEA concept was first developed, few people were arguing that natural selection has modified human behavior over the last 50,000 years. In fact, the dominant view was the opposite: that natural selection has not shaped any specific human behavioral traits, not now, not over the past fifty thousand years, and not over the past fifty million. Not ever. The mind was a tabula rasa. Even sociobiologists, often castigated as biological determinists, commonly thought that people were simply predisposed to learn adaptively: “natural selection has produced in humans a general motivation to maximize one’s inclusive fitness—i.e., a domain-general psychological mechanism” (Buss, 1991, p. 463).

The EEA was part of a new paradigm, now called evolutionary psychology, to move away from the domain-general approach of sociobiology and to search for specific innate mechanisms within the human mind. Its earliest proponents saw the EEA not as a dogma, but as a guide—as a way of making people look at human nature from a broader evolutionary perspective, and not from the narrower one of modern industrial life.

The EEA concept has served us well. But it is now time to move on.

References

Buss, D.M. (1991). Evolutionary personality psychology. Annual Review of Psychology, 42, 459-491.

Harpending, H. and G. Cochran. 2002. "In our genes", Proceedings of the National Academy of Sciences 99(1):10-12.

Tooby, J. and L. Cosmides. (2005). Conceptual Foundations of Evolutionary Psychology. In David M. Buss (Ed.) The Handbook of Evolutionary Psychology. (pp. 5-67), Hoboken, NJ: Wiley.