December 22, 2005

Human evolution and genetic diversification has been speeding up since we came Out of Africa

One of the dogmas of Evolutionary Psychology™ is that human beings stopped evolving after the "Era of Evolutionary Adaptation," which presumably was before modern humans left Africa, which was probably 50,000 or more years ago. Bob Moyzis, Eric Wang et al now have a paper finally out entitled "Global landscape of recent inferred Darwinian selection for Homo sapiens" [pdf] that Greg Cochran has been telling me about for months. It punches a big hole in the Evolutionary Psychologists' assumption about the genetic identicality of human racial groups.

Anthropologist John Hawks writes:

It would be hard for me to overstate how important this paper is. Even if it weren't central to my own current research (about which you will just have to wait for more), it brings home the vast importance of adaptive change during the most recent parts of human evolution.

When you stop and think about it, there are lots of reasons why evolutionary diversification would speed up after some humans left Africa, but I'll avoid mentioning a new one that Cochran is working on.

It's always controversial to argue that genetic differences cause cultural differences, but something that has largely been ignored is that cultural differences will also cause genetic differences. The canonical example is how the cultural trait of milking domesticated animals increased lactose tolerance over the last ten thousand or so years from close to zero to something like 95% in Northern Europe, which, in turn, had big impacts on that region's cultural traits such as population density, economy, lifestyle, and cuisine.

Now, it turns out that lactose tolerance has lots of company in the ranks of recent genetic diversification of humanity.

Bob Holmes writes in the New Scientist:

Darwin’s fingerprints can be found all over the human genome. A detailed look at human DNA has shown that a significant percentage of our genes have been shaped by natural selection in the past 50,000 years, probably in response to aspects of modern human culture such as the emergence of agriculture and the shift towards living in densely populated settlements.

... Robert Moyzis and his colleagues at the University of California, Irvine, US, searched for instances of linkage disequilibrium in a collection of 1.6 million SNPs scattered across all the human chromosomes. They then looked carefully at the instances they found to distinguish the consequences of natural selection from other phenomena, such as random inversions of chunks of DNA, which can disrupt normal genetic reshuffling.

This analysis suggested that around 1800 genes, or roughly 7% of the total in the human genome, have changed under the influence of natural selection within the past 50,000 years. A second analysis using a second SNP database gave similar results. That is roughly the same proportion of genes that were altered in maize when humans domesticated it from its wild ancestors.

Moyzis speculates that we may have similarly “domesticated” ourselves with the emergence of modern civilisation.

“One of the major things that has happened in the last 50,000 years is the development of culture,” he says. “By so radically and rapidly changing our environment through our culture, we’ve put new kinds of selection [pressures] on ourselves.”

Genes that aid protein metabolism – perhaps related to a change in diet with the dawn of agriculture – turn up unusually often in Moyzis’s list of recently selected genes. So do genes involved in resisting infections, which would be important in a species settling into more densely populated villages where diseases would spread more easily. Other selected genes include those involved in brain function, which could be important in the development of culture.

According to Michael Balter in ScienceNow Daily News:

Harnessing data from two existing databases of human diversity, the team found some 1800 genes that appeared to have been under selection during the last 10,000 to 50,000 years. According to team leader and genome researcher Robert Moyzis, this is between 10 and 100 times greater than the number found in previous studies (Science, 8 July, p. 234).

The genes belong to several biologically important categories, including genes important in defense against disease, controlling the cell cycle, protein metabolism, and nervous system functioning, the researchers report online this week in Proceedings of the National Academy of Sciences. One of the newly spotted genes, ASPM, was recently linked to brain evolution by a team from the University of Chicago (Science, 9 September, p. 1662).

The paper ends:

In conclusion, we have introduced a simple probabilistic method to detect unusual genetic architectures associated with recent selection that does not require haplotype information. It is, therefore, suitable for large chromosomal scans with large population samples. Homo sapiens have undoubtedly undergone strong recent selection for many different phenotypes, including but certainly not limited to the general categories we have defined in this work (Fig. 5). Such inferred selective events are not rare (Fig. 3). The numbers obtained, however, are similar to estimated numbers obtained for artificial selection (by humans) on the maize genome (45). Given that most of these selective events likely occurred in the last 10,000 - 40,000 years, a time of major population expansion out of Africa followed by regional shifts from hunter-gatherer to agrarian societies, it is tempting to speculate that gene - culture interactions directly or indirectly shaped our genomic architecture (46, 47). As such, we suggest that such recently selected alleles may provide useful "markers" for investigating the evolutionary migrations of our species, as an adjunct to studies using neutral markers. We also propose that many of these alleles, because of their high prevalence and recent selection, should be considered likely "functional candidates" for association with human variability and the common disorders afflicting humankind.

[Thanks to Dienekes and GNXP]

My published articles are archived at -- Steve Sailer

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