How we're evolving
Posted: Monday, October 19, 2009 7:20 PM by Alan Boyle
UW-Madison |
Anthropologist John Hawks makes a study of skulls.
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Our skulls and our genes show that we're still evolving, but not always in the ways you might expect.
For example, the typical human head has actually been getting smaller over the past few thousand years, reversing the earlier evolutionary trend. Meanwhile, East Asians are becoming lighter-skinned - and appear to have more sensitive hearing than their ancestors did 10,000 years ago.
John Hawks, an anthropologist and blogger at the University of Wisconsin at Madison, points to such trends as evidence that "recent evolution is real."
Hawks delved into a few of his favorite scientific tales over the weekend in Austin, Texas, at the annual CASW New Horizons in Science meeting.
You've no doubt heard some of those tales already. There's the one about the genetic mutation about 7,500 years ago that enhanced Europeans' ability to digest milk in adulthood - which in turn encouraged the rise of dairy farming. And then there's the still-debated claim that early humans' skin became lighter as they migrated northward because the need for vitamin D absorption outweighed the risk of skin cancer.
Other researchers have found that several genetic strategies for fighting off malaria have arisen among populations in sub-Saharan Africa, including a mutation that can also lead to sickle-cell anemia.
Such findings have come about thanks to detailed studies of how genetic mutations are passed along - and how beneficial mutations tend to become more widespread, even if those benefits are accompanied by secondary risks. The fingerprint of such changes, Hawks said, is a phenomenon known as linkage disequilibrium, in which characteristic snippets of genetic code show up in combination among members of a population. The level of genetic linkage can indicate how much of a role natural selection is playing in particular genes.
Hawks said about 3,000 of the genes that distinguish humans from chimpanzees show signs of linkage disequilibrium - and that suggests that a quarter of the evolutionary divergences between the two genomes are continuing today.
It's not just genes that are revealing these changes. One of Hawks' specialties is measuring how the typical shape of human skulls has changed over the course of thousands of years. The current view, based on skull measurements as well as genetics, is that the modern head isn't as "long" as it was 10,000 years ago, with a resulting reduction in brain volume. "Brains are shrinking," Hawks said.
This isn't necessarily a bad thing: The brain is the human body's hungriest organ, consuming half of the glucose we take in. The modern brain may be packing more power into a smaller space and as a result cutting down on the biological energy requirements - with the help of external memory devices.
"What do we need these brains for? We've got iPods," Hawks joked.
But often we're too close to the situation to second-guess what natural selection is doing to us. "Efficiency demands that the brain should be smaller," Hawks said. "Maybe we got better with smaller brains, but I gotta tell you that maybe we're getting dumber. How can we know?"
That aura of uncertainty applies to other ongoing evolutionary changes as well. One of the genes under heavy selection in East Asian populations plays a role in the development of the inner ear's machinery. That suggests that more sensitive hearing may be conferring some sort of advantage on those populations, and Hawks speculates that it may have something to do with the tonal character of most Asian languages. That's only a guess, however.
The guesswork becomes even murkier when it comes to figuring out why genetic coding linked to redheadedness and lighter skin color is becoming more prevalent among Asians. "Our species is evolving like crazy in pigmentation in different ways in different populations, presumably because of the same underlying selection pressures," Hawks said.
Hawks doesn't think the vitamin D factor alone can explain why skin color is being affected by natural selection. Some theorists, including Charles Darwin himself, have suggested that sexual selection may be at work - that having lighter skin somehow improves an individual's reproductive prospects. But in this more evolved age, voicing that kind of view can make your typical researcher sound like a Neanderthal.
So what does Hawks think is behind the skin-color issue? "That's a box I don't want to open," he told me.
Further thoughts from John Hawks:
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Some genetic mutations confer clear benefits on the folks who have them but may not spread widely among populations because they don't enhance reproductive fitness, Hawks. Classic examples would be mutations that tend to extend longevity, such as the one that gives Italian villagers in Limone sul Garda
extra resistance to cardiovascular disease.
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The recent analysis of a 4.4 million-year-old
hominid fossil known as Ardi could lead to big changes in how we view our evolutionary family tree. "It's not a tree. It's not a bush. It's like a network where things reconnect," Hawks told me. The latest findings suggest that the common ancestor for chimps and humans was less chimplike than previously thought. In some areas - for example, the hands - humans may be considered more "primitive" than chimps, Hawks pointed out.
The New Horizons in Science seminar is presented annually by the Council for the Advancement of Science Writing. I've been on CASW's board for several years, and this year I'm serving as the organization's treasurer.
Join the Cosmic Log team by signing up as my Facebook friend or following b0yle on Twitter. And reserve your copy of my book, "The Case for Pluto," which is coming out this month.
