HOW WE'RE EVOLVING
Posted: Monday, October 19, 2009 7:20 PM by Alan Boyle

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:

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.


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.

All humans are genetically equal. I refuse to believe this!

That's because you're the smartest human ever!

That's because you're the smartest human ever!

Did Neanderthals have sex with modern man?
Mystery may be solved when entire Neanderthal genome is reported

By Charles Q. Choi

updated 11:36 a.m. ET, Fri., Nov . 6, 2009
We are currently the only human species alive, but as recently as maybe 24,000 years ago another one walked the earth — the Neanderthals.

These extinct humans were the closest relatives we had, and tantalizing new hints from researchers suggest that we might have been intimately close indeed. The mystery of whether Neanderthals and us had sex might possibly get solved if the entire Neanderthal genome is reported soon as expected. The matter of why they died and we succeeded, however, remains an open question.

First recognized in the Neander Valley in Germany in 1856, Neanderthals revealed that modern humans possess a rich and complex family tree that includes now-extinct relatives

Neanderthals — also called Neandertals, due to changes in German spelling over the years — had robust skeletons that gave them wide bodies and short limbs compared to us. This made them more like wrestlers, while modern humans in comparison are more like long-distance runners.

They were probably less brutish and more like modern humans than commonly portrayed. Their brains were at least as large as ours. They controlled fire, expertly made stone tools, were proficient hunters, lived complex social groups and buried their dead. The discovery of the remains of an adult male Neanderthal with severely deformed arm bones, suggesting a major disability perhaps since childhood, hints they may have taken care of their sick. Genetic research even suggests they might have shared basic language capabilities with modern humans.

"They were a lot more closely related to us than anything alive today," said paleoanthropologist Katerina Harvati at the University of Tübingen in Germany.

Why did Neanderthals go extinct?
Roughly 30,000 years ago, the Neanderthals disappeared, although pockets might have survived until as recently as 24,000 years ago. Since they vanished just as modern humans were emerging there, scientists have long speculated that we might have driven their extinction.

"I think we did away with our competition," asserted paleoanthropologist Ian Tattersall at the American Museum of Natural History in New York. "We either did it indirectly by out-competing them over resources or directly by conflict. Homo sapiens is completely different from any other hominid that ever existed — we process information about the world in a different way."

Other scientists have suggested that Neanderthals weren't destroyed so much as absorbed by modern humans. "Maybe they were wiped out by disease or by conquerors, but maybe they did leave important genes into our gene pool," said paleoanthropologist Milford Wolpoff at the University of Michigan.

Paleoanthropologist Tim Weaver at the University of California at Davis also noted that our species apparently could live at higher population densities than Neanderthals. As a result, this slight difference, with or without interbreeding, would have led to us replacing them.

"A lot of scenarios have been imagined, from peaceful 'flower child' behavior to violent interactions to even cannibalism," Harvati said. "I think a lot of these scenarios happened. I think we probably tried interbreeding and maybe it worked, maybe it didn't. I'm sure there was violence at times. I think in some places they went extinct before modern humans even arrived. But you don't even need any of those if there is even a slight advantage in how many offspring modern humans produce successfully as opposed to Neanderthals."

However, if Neanderthals did coincide with modern humans until 24,000 years ago, then we might not have had anything to do with their disappearance. Instead, evolutionary biologist Clive Finlayson at the Gibraltar Museum in Spain speculates the Neanderthals fell victim to a cooling of the climate that deteriorated their environment too rapidly for them to adapt.

Did Neanderthals have sex with modern humans?
Did hybrids occur between humans and Neanderthals? In a surprisingly bold statement, the leader of the international consortium of researchers sequencing the Neanderthal genome, Svante Pääbo, recently said he was "sure that they had sex."

"Would they have recognized each other as possible mates?" Harvati asked. "We know when closely related primate species meet, they sometimes interbreed in nature, not just in zoos, and this is something we see not just in primates, but with other closely related species among mammals."

Past research had shown that Neanderthal genomes and ours were 99.5 percent identical, based on DNA extracted from three Croatian fossils. At an October conference in Cold Spring Harbor Laboratory in New York, Pääbo — a geneticist of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany — said the two species had sex, but it remained an open question as to whether children resulted and left a legacy in our genomes.

"It's a good, valid idea, and it needs to be examined," Harvati said. "Uncovering this could be vital to understanding our own origins."