Human evolution kicks into high gear

By Kathleen McAuliffe

updated 2:21 p.m. ET Feb. 10, 2009

For decades the consensus view — among the public as well as the world’s preeminent biologists—has been that human evolution is over. Since modern Homo sapiens emerged 50,000 years ago, “natural selection has almost become irrelevant” to us, the influential Harvard paleontologist Stephen Jay Gould proclaimed. “There have been no biological changes. Everything we’ve called culture and civilization we’ve built with the same body and brain.”

This view has become so entrenched that it is practically doctrine. Even the founders of evolutionary psychology, Leda Cosmides and John Tooby, signed on to the notion that our brains were mostly sculpted during the long period when we were hunter-gatherers and have changed little since. “Our modern skulls house a Stone Age mind,” they wrote in a background piece on the Center for Evolutionary Psychology at the University of California at Santa Barbara.

So to suggest that humans have undergone an evolutionary makeover from Stone Age times to the present is nothing short of blasphemous. Yet a team of researchers has done just that.

They find an abundance of recent adaptive mutations etched in the human genome; even more shocking, these mutations seem to be piling up faster and ever faster, like an avalanche. Over the past 10,000 years, their data show, human evolution has occurred a hundred times more quickly than in any other period in our species’ history.

The new genetic adaptations, some 2,000 in total, are not limited to the well-recognized differences among ethnic groups in superficial traits such as skin and eye color. The mutations relate to the brain, the digestive system, life span, immunity to pathogens, sperm production and bones — in short, virtually every aspect of our functioning.

Many of these DNA variants are unique to their continent of origin, with provocative implications. “It is likely that human races are evolving away from each other,” says University of Utah anthropologist Henry Harpending, who co-wrote a major paper on recent human evolution. “We are getting less alike, not merging into a single mixed humanity.”

Harpending theorizes that the attitudes and customs that distinguish today’s humans from those of the past may be more than just cultural, as historians have widely assumed. “We aren’t the same as people even a thousand or two thousand years ago,” he says. “Almost every trait you look at is under strong genetic influence.”

Not surprisingly, the new findings have raised hackles. Some scientists are alarmed by claims of ethnic differences in temperament and intelligence, fearing that they will inflame racial sensitivities. Other researchers point to limitations in the data. Yet even skeptics now admit that some human traits, at least, are evolving rapidly, challenging yesterday’s hallowed beliefs.

A bone to pick
Bones don’t lie. John Hawks of the University of Wisconsin at Madison likes evidence he can put his hands on, so he takes me on a tour of the university’s bone laboratory. There, the energetic 36-year-old anthropologist unlocks a glass case and begins arranging human skulls and other skeletal artifacts — some genuine fossils, others high-quality reproductions — on a counter according to their age. Gesturing toward these relics, which span the past 35,000 years, Hawks says, “You don’t have to look hard to see that teeth are getting smaller, skull size is shrinking, stature is getting smaller.”

These overriding trends are similar in many parts of the world, but other changes, especially over the past 10,000 years, are distinct to specific ethnic groups. “These variations are well known to forensic anthropologists,” Hawks says as he points them out: In Europeans, the cheekbones slant backward, the eye sockets are shaped like aviator glasses, and the nose bridge is high. Asians have cheekbones facing more forward, very round orbits, and a very low nose bridge. Australians have thicker skulls and the biggest teeth, on average, of any population today.

“It beats me how leading biologists could look at the fossil record and conclude that human evolution came to a standstill 50,000 years ago,” Hawks says.

By his account, Hawks’ theory of accelerated human evolution owes its genesis to what he could see with his own eyes. But his radical view was also influenced by newly emerging genetic data. Thanks to stunning advances in sequencing and deciphering DNA in recent years, scientists had begun uncovering, one by one, genes that boost evolutionary fitness. These variants, which emerged after the Stone Age, seemed to help populations better combat infectious organisms, survive frigid temperatures, or otherwise adapt to local conditions. And they were popping up with surprising frequency.

What’s fueling human evolution?
Taken together, the skeletal and genetic evidence convinced Hawks that the ruling “static” view of recent human evolution was not only wrong but also quite possibly the opposite of the truth. He discussed his ideas with Harpending, his former postdoc adviser at the University of Utah, and Gregory Cochran, a physicist and adjunct professor of anthropology there. They both agreed with Hawks’ interpretation. But why, they wondered, might evolution be picking up speed? What could be fueling the trend?

Then one day, as Hawks and Cochran mulled over the matter in a phone conversation, inspiration struck. “At exactly the same moment, both of us realized, gee, there’s a lot more people on the planet in recent times,” Hawks recalls. “In a large population you don’t have to wait so long for the rare mutation that boosts brain function or does something else desirable.”

The three scientists reviewed the demographic data. Ten thousand years ago, there were fewer than 10 million people on earth. That figure soared to 200 million by the time of the Roman Empire. Since around 1500 the global population has been rising exponentially, with the total now surpassing 6.7 billion. Since mutations are the fodder on which natural selection acts, it stands to reason that evolution might happen more quickly as our numbers surge. “What we were proposing was nothing new to animal breeders of the 19th century,” Cochran notes. “Darwin himself emphasized the importance of maintaining a large herd for selecting favorable traits.”

The logic behind the notion was undeniably simple, but at first glance it seemed counterintuitive. The genomes of any two individuals on the planet are more than 99.5 percent the same. Put another way, less than 0.5 percent of our DNA varies across the globe. That is often taken to mean that we have not evolved much recently, Cochran says, “but keep in mind that the human and chimp genomes differ by only about 1 to 2 percent—and nobody would call that a minor difference. None of this conflicts with the idea that human evolution might be accelerating.”

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