Friday, December 14, 2007

The rising curve

It was long thought that human genetic evolution pretty much ended with the advent of culture. As Paul Ehrlich (2000, p. 63) wrote:

The evolution of that body of extragenetic information—cultural evolution—has been centrally important in making us the unique beasts we are. Cultural evolution rests on a foundation of genetic (or biological) evolution—especially that of our brains and tongues—but can proceed at what by comparison is a lightning pace. … cultural evolution can vastly outpace genetic evolution because it’s not constrained by generation time. Our genes are passed only from one generation to relatives in succeeding generations. In contrast, the units of culture—ideas, basically—are passed among both relatives and nonrelatives not only between generations (in both
directions) but also within generations.

So did cultural evolution make genetic evolution obsolete? Paul Ehrlich seemed to draw this conclusion … only to pull himself back. “There are many ways in which culture can alter selection pressures,” he says, noting that genes have co-evolved with changes to diet, farming practices, and shelter (Ehrlich, 2000, p. 64). Indeed, the same properties that make cultural evolution so fast have also been diversifying the adaptive landscape at an unparalleled rate. Whenever our species came up with a cultural innovation—a new technology, domestication of a plant or animal, or the advent of agriculture itself—our environment changed as fundamentally as if we had moved to a new ecosystem.

So which factor has mattered most in determining the pace of human genetic evolution? Has cultural evolution been resolving more and more adaptive problems that were formerly resolved by genetic evolution? Or has genetic evolution been resolving more and more adaptive problems because human environments have been diversifying more and more?

The second factor, apparently. A recent study has concluded that genetic evolution has actually accelerated over the past 40,000 years and even more over the past 10,000-15,000. This is partly because there are many more of us and partly because we are spread over an increasingly diverse range of natural and man-made environments. At least 7% of the human genome appears to have changed since the advent of Homo sapiens. And the rate of change has increased a 100-fold since the advent of agriculture (Hawks et al., 2007).

These are high numbers. As one of the study’s authors observes:

Personally, I can't believe that nobody noticed how extreme these estimates of recent selection really are. I guess that folks doing genomics just weren't as primed in evolutionary theory to perceive how weird the human estimates looked compared to what is measured in the wild on other species, or even over the span of human evolution!

In the earliest studies, when people were finding that 3 or 4 percent of a sample of genes had signs of recent selection, those numbers were already extremely high. They got even higher, as more and more powerful methods of detecting selection came online. Our current estimate is the highest yet, but even this very high number is perfectly consistent with theoretical predictions coming from human population numbers.

These figures, if anything, err on the low side. They do not capture recent selective pressures that are just emerging above noise in the data. Nor do they capture older selective pressures that have already pushed many alleles to fixation. The real figures won’t become known until we’ve retrieved the human genome that existed 40,000 years ago—something that is certainly within the realm of possibility.

All this underlines a point I made in an earlier post: human evolution is not a straight line. It’s a logarithmic curve with most of the evolutionary change in the recent past. If we met a Homo erectus face to face, or even a Neanderthal (who was probably just an arctic-adapted Homo erectus), we wouldn’t consider it to be human. It would look to us like an overgrown ape. Nor would its behavior reassure us otherwise.


References

Ehrlich, P.R. (2000). Human Natures. Genes, Cultures, and the Human Prospect. Penguin: New York.

Hawks, J., E.T. Wang, G.M. Cochran, H.C. Harpending, and R.K. Moyzis. (2007). Recent acceleration of human adaptive evolution. Proceedings of the National Academy of Sciences (USA), 104(52), 20753-20758.

3 comments:

Anonymous said...

Professor Frost:

I don't know if you read the comments, but first, a compliment and a thank you. Second, here's a thought I've had since I first heard the 'evolution's stopped' argument close to 30 years ago as an undergrad: Absent any data showing the contrary (like the Hawks, et al, stuff), wouldn't that be (almost necessarily) the natural conclusion? What I mean is that from the perspective of an organism at any time, it would never appear to one that evolution was ongoing. It would always be one damn thing after the other--"oh look who just died". Now given that, wouldn't the rational default assumption have to be against such an hypothesis (the end of Evo) given that we know evolution would have seemed to have stopped even from the perspective of even dinosaurs? As a principle of rationality, if you know that all the evidence one has at the moment is perfectly consistent with the falsity of some theory (the end of Evo), and you know that your current epistemic position has been held by literally untold species in which case it actually did turn out to be false, then wouldn't the conservative, rational approach be, if anything (given the history), at least moderately disposed against it? I could never figure out why people, back then or even up until now, would say that evolution had stopped given this principle.

Peter Frost said...

I agree. We all like to think we're the "end of history." Unfortunately, history has a nasty habit of marching on ... with or without us.

MacGregor said...

Paul Ehrlich may not have been responsible for the title of his book, but it does give a good indication of his style of thinking.

"Human Natures", not Nature but "Natures" conveys his attitude. I doubt he would have let anyone call a publication of his Butterfly Geneticses.
He trys to nullify implications arising from the widespread knowledge of human variation across the earth - and its correlations.

Ehrlich (and an awful lot of others) were always wide awake to the conclusions to be drawn.


Peter Frost - para 6 quote- "estimates of recent election" reads like an typo error to me.