Time was, teachers used to tell their students that accelerating cultural evolution – the alphabet, the wheel, movable type, the steam engine, the computer, whatever – meant human biological evolution wasn’t important any more. It was too slow.
A blockbuster paper published online today by the Proceedings of the National Academy of Sciences turns that old story on its head.
Cultural evolution diversifies the physical environments where humans live, creating a multitude of places where useful mutations can thrive. And greatly increased population density means ever more bodies where mutations may be selected for usefulness.
The result, says Greg Cochran, one of the authors, is that human biological evolution has accelerated, to perhaps 100 times as fast as in prehistory. (And that’s before we had the genetic tools to modify it on purpose.)
I don’t intend to dismiss the other authors; it’s only that Cochran happens to be the one I talked with. The others are John Hawks, Eric T. Wang, Henry C. Harpending, and Robert K. Moyzis. I’ll add a link to the specific article at PNAS as soon as I have one.
UPDATE: Steve Sailer at http://isteve.blogspot.com/ links to the paper at
http://www.anthro.utah.edu/PDFs/accel.pnas.smallpdf.pdf
and also has links to early press coverage at the LA Times, Reuters and elsewhere
Also, lead author John Hawks has further comment about the study on his blog,
http://johnhawks.net/weblog/topics/evolution/selection/acceleration_embargo_ends_2007.html
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.
So where is that evidence coming from? In part, from an ambitious international effort called The HapMap Project (www.hapmap.org).
The human genome has about 3 billion base pairs, strung out in long chains in 23 pairs of chromosomes. Between any two people, most of the base pairs will be the same, but researchers estimate there may be some 10 million locations along the chromosomes where variant spellings of the genetic code appear. If you should happen to hear geneticists talking about “snips,” that’s what they mean; it’s short for “single nucleotide polymorphism.”
The HapMap project is intended to make that vast amount of information more manageable so medical researchers can more efficiently go looking for genes that may influence health. “Genetic variants that are near each other tend to be inherited together,” the HapMap site explains. “These regions of linked variants are known as haplotypes” (hence the name).
A relatively small number of “tag” SNPs is enough to uniquely identify a haplotype, and if researchers know what those are, they don’t need to look at every base pair.
Because chromosomes are typically cut apart into several pieces and reassembled during reproduction, it is approximately correct that the longer a haplotype is, the younger it is. So the HapMap serves as a genetic clock for the species.
The ability to digest lactose goes back roughly 8,000 years. Hey, useful mutations don’t always come along just when you want them. Malaria resistance begins to appear about 5,000 years ago.
Most haplotypes are old, and found in almost every human population, though not necessarily with the same frequency. But the clocks in different places started at different times and they’re not all running at the same speed. The HapMap project is studying three groups of 90 people each, one from the Yoruba tribe in Nigeria, one from East Asia (Tokyo and Beijing) and one of Americans of northern European descent. All the groups have numerous genes under selection, but not all the same ones. If the evolutionary price of resistance to malaria is sickle-cell anemia, it is far too high a price to pay for populations living where it’s too cold for malarial mosquitoes anyway. If the evolutionary price of light skin is melanoma, the price is too high in equatorial Africa.
You understand there is no agency or designer deciding these things. There’s just a small statistical advantage in each generation for those with a more favorable version of a gene, “favorable” meaning only that children in that environment who inherit it are slightly more likely to grow up and have children themselves than those who don’t. Researchers might not even know which gene in a haplotype is being selected for, or what it does.
They do know, though, that it’s happening faster than we used to think. Cochran said, according to a press release from the University of Utah, “History looks more and more like a science fiction novel in which mutants repeatedly arose and displaced normal humans – sometimes quietly, by surviving starvation and disease better, sometimes as a conquering horde. And we are those mutants.”