Saturday, January 05, 2008

On Human Evolution

For a long time we have believed two things about the genetics and evolution of our species. 1) We have believed ourselves to be virtual clones of each other, and 2) we have believed that human evolution, for all intents and purposes, stopped 40,000 years ago or so.

The Science (21 Dec. 2007) Breakthrough of the Year, Human Genetic Variation, and recent work by John Hawks et al in the Proceedings of the National Academy of Science on "Recent Acceleration of Human Adaptive Evolution" together show a very different picture.

It is now estimated that of the 3 billion DNA bases of the human genome, there are "15 million places along our genomes where one base can differ from one person or population to the next" (Science, 1842). That's a o.5% difference -- still smaller than the variation within most other species, including chimpanzees, but still it's more than we thought. Of course, those raw numbers tell us little, since change of one base can have catastrophic effects. But "Genomes can differ in many other ways. Bits of DNA ranging from a few to many thousands, even millions, of bases can get lost, added, or turned around in an individual's genome. Such revisions can change the number of copies of a gene or piece of regulatory DNA or jam two genes together, changing the genes' products or shutting them down" (1843). Something like an inversion can even result in isolation of breeding populations, leading to speciation. So that can be a very significant alteration. It has also been discovered that there are "Quite a few overlapped genes, including some implicated in our individuality -- blood type, smell, hearing, taste, and metabolism, for example. Individual genomes differed in size by as many as 9 million bases" (1843). Now we're talking about much larger kinds of variation, when the genomes can differ in size by as much as 0.3%. As the author of the article, Elizabeth Pennisi, notes, "These differences matter," especially when "in some populations almost 20% of differences in gene activity are due to copy-number variants" (1843) so that, for example, "People with high-starch diets -- such as in Japan -- have extra copies of a gene for a starch-digesting protein compared with members of hunting-gathering societies" (1843). The two variations mentioned give rise to a variation of about 1%, which is approaching a more significant number.

The Hawks et al article puts this into perspective. It seems positive selection "Has accelerated greatly during the last 40,000 years" (1). Thus, the above mentioned variation is of recent origin. Both explanations given make a great deal of sense: 1) "Larger populations generate more new selected mutations," and 2) they believe "human demographic growth to be linked with past changes in human cultures and ecologies (1).

The first observation should be obvious. If the mutation rate stays the same, but the population grows, there will be more mutations. Population genetics shows these mutations will spread rapidly through the population. Thus, mutations -- especially adaptive mutations -- will accumulate. The combination of larger population size and the dynamics of population genetics would result in an acceleration in accumulation of mutations.

The second observation is one championed by E.O. Wilson in the late 70's and early 80's, that culture drives human evolution. Our culture is our environment, so we should not be surprised if it should exhibit selective pressures. This would account for the variations the authors found between subsaharan Africans and those of European descent. We would expect this in two populations from different physical and cultural environments.

Now, the first thing to note is that Africans have more genetic as a group than do Europeans. This, too, should not be surprising as the African population is older than the European population, so they would have had time to develop more variation. Also, Africa has been much more tribal for much longer than Europe, meaning more separation of groups, which also drives variation. However, the Hawks, et al article observes that evolution seems to be accelerating overall, in both groups. No doubt much of these differences arose from differences in culture, including cultural complexity. A culture containing 300 million people (as the U.S. does) is necessarily more complex than one of 150 people (the maximum size of a tribe), as we learn from complex systems theory. There are necessarily different selective pressures on these two groups, as each lives in a very different cultural environment.

Now, of course, being humans, we want to know what this all means. When people ask this, they are typically not interested in answers like personalized medicine, nutrition, and exercise. No, they typically are interested in race. The tribalist in them hopes it means they are superior, the moralist in them (for more complex thinkers) prays it means nothing of the sort, and wishes (in order to silence their inner tribalist) people wouldn't even do this kind of research. Ashamed of their own racist conclusions, they accuse the researchers of racism, thus assuaging their shame. So let me answer by asking this question: which is superior, the grasshopper mouse or the field mouse? A ridiculous question? Of course it is.
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