Wednesday, December 12, 2007

Rapid human evolution

It is commonly accepted among evolutionary psychologists (at least in the Santa Barbara school*) that "our modern skulls house a stone age mind". As Tooby & Cosmides put it,
natural selection... takes a long time to design a circuit of any complexity. The time it takes to build circuits that are suited to a given environment is so slow it is hard to even imagine -- it's like a stone being sculpted by wind-blown sand. Even relatively simple changes can take tens of thousands of years. The environment that humans -- and, therefore, human minds -- evolved in was very different from our modern environment. Our ancestors spent well over 99% of our species' evolutionary history living in hunter-gatherer societies. That means that our forebearers lived in small, nomadic bands of a few dozen individuals who got all of their food each day by gathering plants or by hunting animals.
Similarly, Edward Hudgens explains,
Evolutionary psychologists downplay the possibility of significant cognitive evolution in the 10,000 or so years since the advent of agriculture (a period of time known as the Holocene) for reasons of both science and political correctness. Scientifically, 10,000 years (500 generations) is not much time for natural selection to act, and it certainly is not enough time to evolve new, complex adaptations—sophisticated mechanisms coded for by numerous genes.
New research just released in PNAS has the potential to undermine these claims fatally. John Hawks and his colleagues argue that human evolution accelerated very rapidly in the last 40,000 years. The abstract:
Genomic surveys in humans identify a large amount of recent positive selection. Using the 3.9M HapMap SNP dataset, we found that selection has accelerated greatly during the last 40,000 years. We tested the null hypothesis that the observed age distribution of recent positively selected linkage blocks is consistent with a constant rate of adaptive substitution during human evolution. We show that a constant rate high enough to explain the number of recently selected variants would predict (1) site heterozygosity at least tenfold lower than is observed in humans, (2) a strong relationship of heterozygosity and local recombination rate, which is not observed in humans, (3) an implausibly high number of adaptive substit utions between humans and chimpanzees, and(4) nearly 100 times the observed number of high-frequency LD blocks. Larger populations generate more new selected mutations, and we show the consistency of the observed data with the historical pattern of human population growth. We consider human demographic growth to be linked with past changes in human cultures and ecologies. Both processes have contributed to the extraordinarily rapid recent genetic evolution of our species.
(See also: John Hawks's two blog entries on his study, and Reuters' report)

*The Santa Barbara school of evolutionary psychology is the best known type of EP, its foremost exponents are Leda Cosmides, John Tooby, Donald Symons, David M. Buss, Steven Pinker, Margo Wilson and Martin Daly. (It's so called because Symons, Tooby & Cosmides are at UC Santa Barbara).

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