For Humans, Evolution Ain't What It Used To Be

AMONG LARGE LAND animals, there's one species that eats the others for breakfast, and you know which it is. The rise of human beings to the top of the food chain was so astonishingly swift and decisive that it's natural to view the evolution of intelligent mammals-us, in other words-as preordained, the working out of biology's deepest principles. And natural, also, to assume that the same trends will continue, filling the future with ever more perfect humans. If you like Homo sapiens (so cute, the way those thumbs work! so clever, they never run out of stuff to say!) you'll love what evolves out of them.

Unfortunately, evolutionary biology holds out no such promises. There's no strong reason to think human beings will be getting noticeably smarter in the future, because there's no evidence that they've been getting smarter up until now. Biologists calculate intelligence indirectly, from the ratio of brain to body size. That figure, which expanded rapidly beginning about 500,000 years ago, appears to have reached a peak and leveled off within the last $0,000 years. In exchange for the benefits of bipedalism, humans paid a price in limiting the size of the female pelvis and consequently the diameter of a babfs head that can go through it. "We've maxed out on head size," says Dean Falk, a professor of anthropology at the State University of New York at Albany. "I just saw my granddaughter delivered, and boy, it's true. The organ's size is fixed."

And contrary to what many people assume, human bodies aren't getting bigger, either, at least not as a result of evolution. It is true that successive generations of Americans have been getting taller and heavier, but that's because, until very recently, most people didn't get enough to eat. Biologists believe that in most developed countries, where malnutrition is now the exception, that trend has pretty much run its course--disregarding the possibility that people will keep the same stature and build but just get fatter. In fact, says Christopher Ruff, a biological anthropologist at Johns Hopkins, there's been a trend toward smaller, less robust bodies over the last 50,000 years, a decline he puts at about 10 percent. "I would interpret this, in a very broad sense, to mean that we're using our bodies less and less as we use tools more and more," Ruff says. If there's no competitive advantage to being big, then evolution will tend to produce smaller bodies, which need less food and are more likely to survive in times of famine.

The same principle, that physical attributes are lost when they no longer serve an adaptive function, is at work everywhere in the human physique. ',We don't need teeth any longer at all," says Loring Brace, a professor of anthropology at the University of Michigan. "Tooth size began declining as soon as we figured out how to stick a piece of meat in a pot, and. it's been decreasing at the rate of 1 percent per thousand years." A whole set of changes was set in motion by the shift away from hunting and gathering, and they're nothing to be proud of: an increase in vision problems, decreased thickness of the long bones, a higher incidence of baldness. "Hair is a cushion," Brace says. "Get whacked on the head while hunting, go all woozy, you've lost your meal."

If Brace is right, all of these trends will continue--assuming, of course, that hunting and gathering doesn't make a comeback after the next stock-market crash. As Stephen Jay Gould has argued, natural selection is blind to "progress"; creatures today aren't necessarily better than, say, the dinosaurs, in the same way that the push-button, phone is clearly an improvement over those with dials. It's safe to predict that telephones will be better in the future, along with a great many other things. Except, probably, us.