It's nonsense that human beings have not changed at all in the last 40,000-50,000 years, as Stephen J Gould had said. Maybe the changes are not obvious, but the evolution of the brain and the corresponding evolution of consciousness was been extraordinary.
Last-Minute Changes
Scientific orthodoxy says that human evolution stopped a long time ago. Did it?
By CHRISTOPHER F. CHABRIS
The debate over the validity of evolutionary theory may be real enough when it comes to religious belief and cultural outlook. But it has nothing to do with science. No evidence seriously contradicts the idea that the plant and animal species found on Earth today are descended from common ancestors that existed long ago. Indeed, the evidence for natural selection is infinitely stronger than it was when Charles Darwin proposed it 150 years ago, mainly because later discoveries in the field of genetics supplied the biological mechanisms to explain the patterns that Darwin and his contemporaries were observing.
But scientists do disagree over the pace and time-span of human evolution. Gregory Cochran and Henry Harpending begin "The 10,000 Year Explosion" with a remark from the paleontologist Stephen J. Gould, who said that "there's been no biological change in humans for 40,000 or 50,000 years." They also cite the evolutionist Ernst Mayr, who agrees that "man's evolution towards manness suddenly came to a halt" in the same epoch. Such claims capture the consensus in anthropology, too, which dates the emergence of "behaviorally modern humans" -- beings who acted much more like us than like their predecessors -- to about 45,000 years ago.
But is the timeline right? Did human evolution really stop? If not, our sense of who we are -- and how we got this way -- may be radically altered. Messrs. Cochran and Harpending, both scientists themselves, dismiss the standard view. Far from ending, they say, evolution has accelerated since humans left Africa 40,000 years ago and headed for Europe and Asia.
Evolution proceeds by changing the frequency of genetic variants, known as "alleles." In the case of natural selection, alleles that enable their bearers to leave behind more offspring will become more common in the next generation. Messrs. Cochran and Harpending claim that the rate of change in the human genome has been increasing in recent millennia, to the point of turmoil. Literally hundreds or thousands of alleles, they say, are under selection, meaning that our social and physical environments are favoring them over other -- usually older -- alleles. These "new" variants are sweeping the globe and becoming more common.
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The 10,000 Year Explosion
By Gregory Cochran and Henry Harpending
(Basic, 288 pages, $27)But genomes don't just speed up their evolution willy-nilly. So what happened, the authors ask, to keep human evolution going in the "recent" past? Two crucial events, they contend, had to do with food production. As humans learned the techniques of agriculture, they abandoned their diffuse hunter-gatherer ways and established cities and governments. The resulting population density made humans ripe for infectious diseases like smallpox and malaria. Alleles that helped protect against disease proved useful and won out.
The domestication of cattle for milk production also led to genetic change. Among people of northern European descent, lactose intolerance -- the inability to digest milk in adulthood -- is unusual today. But it was universal before a genetic mutation arose about 8,000 years ago that made lactose tolerance continue beyond childhood. Since you can get milk over and over from a cow, but can get meat from it only once, you can harvest a lot more calories over time for the same effort if you are lactose tolerant. Humans who had this attribute would have displaced those who didn't, all else being equal. (If your opponent has guns and you don't, drinking milk won't save you.)
To make their case for evolution having continued longer than is usually claimed, Messrs. Cochran and Harpending remind us that dramatic changes in human culture appeared about 40,000 years ago, resulting in painting, sculpture, and better tools and weapons. A sudden change in the human genome, they suggest, made for more creative, inventive brains. But how could such a change come about? The authors propose that the humans of 40,000 years ago occasionally mated with Neanderthals living in Europe, before the Neanderthals became extinct. The result was an "introgression" of Neanderthal alleles into the human lineage. Some of those alleles may have improved brain function enough to give their bearers an advantage in the struggle for survival, thus becoming common.
In their final chapter, Messrs. Cochran and Harpending venture into recorded history by observing two interesting facts about Ashkenazi Jews (those who lived in Europe after leaving the Middle East): They are disproportionately found among intellectual high-achievers -- Nobel Prize winners, world chess champions, people who score well on IQ tests -- and they are victims of rare genetic diseases, like Gaucher's and Tay-Sachs. The authors hypothesize that these two facts are connected by natural selection.
Just as sickle-cell anemia results from having two copies of an allele that protects you against malaria if you have just one, perhaps each Ashkenazi disease occurs when you have two copies of an allele that brings about something useful when you have just one. That useful thing, according to Messrs. Cochran and Harpending, is higher cognitive ability. They argue that the rare diseases are unfortunate side-effects of natural selection for intelligence, which Messrs. Cochran and Harpending think happened during the Middle Ages in Europe, when Jews rarely intermarried with other Europeans.
"The 10,000 Year Explosion" is important and fascinating but not without flaw. Messrs. Cochran and Harpending do not stop often enough to acknowledge and rebut the critics of their ideas. And though the authors cite historical sources and scientific articles in support of their thesis, they too often write in a speculative voice, qualifying claims with "possible," "likely," "might" and "probably." This voice is inevitable in any discussion of events tens of thousands of years ago. But it leads to another problem: The authors don't say enough about the developments in genetic science that allow them to make inferences about humanity's distant past. Readers will wonder, for instance, exactly how it is possible to recognize ancient Neanderthal DNA in our modern genomes. Despite all this, the provocative ideas in "The 10,000 Year Explosion" must be taken seriously by anyone who wants to understand human origins and humanity's future.
Mr. Chabris is a psychology professor at Union College in Schenectady, N.Y.
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