"Recent acceleration of human adaptive evolution"

Here's the abstract and last paragraph of the big paper. (Click the title in blue for the 8 page PDF of the full paper.)

Recent acceleration of human adaptive evolution

John Hawks, Eric T. Wang, Gregory Cochran, Henry C. Harpending, and Robert K. Moyzis

Genomic surveys in humans identify a large amount of recent positive selection. Using the 3.9-million 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 (i) site heterozygosity at least 10-fold lower than is observed in humans, (ii) a strong relationship of heterozygosity and local recombination rate, which is not observed in humans, (iii) an implausibly high number of adaptive substitutions between humans and chimpanzees, and (iv) nearly 100 times the observed number of high-frequency linkage disequilibrium 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.

The last paragraph of the discussion:

It is sometimes claimed that the pace of human evolution should have slowed as cultural adaptation supplanted genetic adaptation. The high empirical number of recent adaptive variants would seem sufficient to refute this claim. It is important to note that the peak ages of new selected variants in our data do not reflect the highest intensity of selection, but merely our ability to detect selection. Due to the recent acceleration, many more new adaptive mutations should exist than have yet been ascertained, occurring at a faster and faster rate during historic times. Adaptive alleles with frequencies under 22% should then greatly outnumber those at higher frequencies. To the extent that new adaptive alleles continued to reflect demographic growth, the Neolithic and later periods would have experienced a rate of adaptive evolution more than 100 times higher than characterized most of human evolution. Cultural changes have reduced mortality rates, but variance in reproduction has continued to fuel genetic change. In our view, the rapid cultural evolution during the Late Pleistocene created vastly more opportunities for further genetic change, not fewer, as new avenues emerged for communication, social interactions, and creativity.

Linda Seebach has a column about it:

The old story was that around the time agriculture started to replace hunting and gathering as a way of life for human beings, biological evolution faded into insignificance because it was so much slower. There’s hardly been enough time – only about 10,000 years or so – for human biology to have changed enough so’s you’d notice.

Not so, as studies of the human genome are demonstrating. When people began domesticating animals, planting crops, and living in settled communities, they created environments for themselves quite different from any environments the human species had occupied before, and natural selection proceeded, as it always does, to favor survival and reproductive success for individuals who were, by chance, better adapted to those environments.

The marquee example is retaining the ability to digest milk into adulthood, which is nearly universal among people of European descent, and quite rare elsewhere.

But the shift to agriculture was also important, Cochran said, because agriculture can support a larger population. Any individual might be the next one to draw a winning ticket in the genetic lottery, and the human species was suddenly buying a lot more lottery tickets.

Think of the genes as a crew of thousands of extras, swarming around the sets and the studio lots, auditioning for jobs. They play more than one role, in lots of different movies, for different directors, as circumstances permit.

One day on the set where they’re filming a swords-and-circuses epic, the director spots a spear-carrier who has given an especially elegant performance, impaling a charioteer.

“You there, with the spear? Could you do that again? Yeah?! I’m gonna make you a star!” And – for a quick glimpse into how biology and culture drive each other – if the spear-carrier proves to be bankable, he’ll get more roles, the director will get more movies, the studio will tilt toward making more epics and the extras with good spear-carrying traits will be more likely to succeed in that environment.

Success, in the gene world, is reproductive success – leaving descendants – but given the role of the casting couch in Hollywood, maybe that’s not pushing the metaphor too far.

On the next set over, the director is casting a chick flick. He says to his assistant, “see that looker over there, the one with the purple eyes? Ask her whether she can ride a horse.”

Now the aspiring starlets competing with Elizabeth Taylor for a role in “National Velvet” can take riding lessons. But they can’t do anything to give themselves violet eyes.

The hunter-gatherers competing with pastoralists for food resources could certainly have learned to keep domesticated animals. But if they and their children could not digest milk much past the average age of weaning, it wouldn’t do them any good. In hard times they’d starve while the meek drinkers of milk inherit the earth.

Or at least a broad swath of it from Iceland to South India.

Accelerated biological evolution in humans doesn’t mean that we’re turning into aliens, but there is evidence that hundreds of human genes are under selection pressure, having to do with such things as diet, vitamin metabolism, the functioning of the central nervous system, disease resistance, hair, skin and eye color, the shape of the skeleton and behavioral traits better suited to living in large groups. “We’re tamer,” Cochran said.

I asked him why we’re not developing floppy ears like the silver foxes bred for tameness. [MORE]

Co-author John Hawks is promising an FAQ about it later on Monday evening.

The indefatigable Jason Malloy of GNXP.com offers this summary of initial media reaction:

First wave of media articles:

[AP] Researchers: Human evolution speeding up

"Science fiction writers have suggested a future Earth populated by a blend of all races into a common human form. In real life, the reverse seems to be happening.

People are evolving more rapidly than in the distant past, with residents of various continents becoming increasingly different from one another, researchers say."

http://www.usatoday.com/tech/science/discoveries/2007-12-10-evolution-speeding-up_N.htm

[Reuters] Rapid acceleration in human evolution described

"Genes have evolved relatively quickly in Africa, Asia and Europe but almost all of the changes have been unique to their corner of the world."

http://africa.reuters.com/wire/news/usnN10432286.html

[Eureka] Genome study places modern humans in the evolutionary fast lane

"Adds Hawks: "We are more different genetically from people living 5,000 years ago than they were different from Neanderthals.""

http://www.eurekalert.org/pub_releases/2007-12/uow-gsp120507.php

[Wired News] Humans Evolving More Rapidly Than Ever, Say Scientists

"Asked about James Watson's controversial claims that intelligence evolved less effectively in people of African descent, Harpending said the study wasn't designed to test such characteristics. He also cautioned against interpreting the findings as suggesting that people are becoming fundamentally better...

"Evolution is a double-edged sword," he said. "What evolution cares about is that I have more offspring. If you can do it by charming and manipulating, and I'm a hardworking farmer that's going to feed the kids ten years down the road, then you're going to win. Hit-and-run, irresponsible males are reproducing more. That isn't good for anyone except those males, but that's evolution.""

http://blog.wired.com/wiredscience/2007/12/humans-evolving.html

[Times UK] Why the human race is growing apart

"Armand Leroi, Reader in Evolutionary Biology at Imperial College, London, said: "In principle, this could have led to speciation if it had continued, but in practice, it has got to be the case that that cannot happen now. The reason is that this study has looked at largely separated populations in the past, but everything about human history since the industrial revolution weighs overwhelmingly against separation and thus against speciation too. Huge increases in gene flow are going to wipe this trend out.""

http://www.timesonline.co.uk/tol/news/uk/science/article3031104.ece

My published articles are archived at iSteve.com -- Steve Sailer