may the force be with you

ADAPTATION The Yanomamo, left, and Ashkenazi Jews, center, may be examples of a society's genetic response to circumstances. Rice farming, practiced by East Asians for centuries, may have spurred evolutionary changes in physical and psychological traits

EAST ASIAN and European cultures have long been very different, Richard E. Nisbett argued in his recent book "The Geography of Thought." East Asians tend to be more interdependent than the individualists of the West, which he attributed to the social constraints and central control handed down as part of the rice-farming techniques Asians have practiced for thousands of years.

A separate explanation for such long-lasting character traits may be emerging from the human genome. Humans have continued to evolve throughout prehistory and perhaps to the present day, according to a new analysis of the genome reported last week by Jonathan Pritchard, a population geneticist at the University of Chicago. So human nature may have evolved as well.

If so, scientists and historians say, a fresh look at history may be in order. Evolutionary changes in the genome could help explain cultural traits that last over many generations as societies adapted to different local pressures.

Trying to explain cultural traits is, of course, a sensitive issue. The descriptions of national character common in the works of 19th-century historians were based on little more than prejudice. Together with unfounded notions of racial superiority they lent support to disastrous policies.

But like phrenology, a wrong idea that held a basic truth (the brain's functions are indeed localized), the concept of national character could turn out to be not entirely baseless, at least when applied to societies shaped by specific evolutionary pressures.

In a study of East Asians, Europeans and Africans, Dr. Pritchard and his colleagues found 700 regions of the genome where genes appear to have been reshaped by natural selection in recent times. In East Asians, the average date of these selection events is 6,600 years ago.

Many of the reshaped genes are involved in taste, smell or digestion, suggesting that East Asians experienced some wrenching change in diet. Since the genetic changes occurred around the time that rice farming took hold, they may mark people's adaptation to a historical event, the beginning of the Neolithic revolution as societies switched from wild to cultivated foods.

Some of the genes are active in the brain and, although their role is not known, may have affected behavior. So perhaps the brain gene changes seen by Dr. Pritchard in East Asians have some connection with the psychological traits described by Dr. Nisbett.

Some geneticists believe the variations they are seeing in the human genome are so recent that they may help explain historical processes. "Since it looks like there has been significant evolutionary change over historical time, we're going to have to rewrite every history book ever written," said Gregory Cochran, a population geneticist at the University of Utah. "The distribution of genes influencing relevant psychological traits must have been different in Rome than it is today," he added. "The past is not just another country but an entirely different kind of people."

John McNeill, a historian at Georgetown University, said that "it should be no surprise to anyone that human nature is not a constant" and that selective pressures have probably been stronger in the last 10,000 years than at any other epoch in human evolution. Genetic information could therefore have a lot to contribute, although only a minority of historians might make use of it, he said.

The political scientist Francis Fukuyama has distinguished between high-trust and low-trust societies, arguing that trust is a basis for prosperity. Since his 1995 book on the subject, researchers have found that oxytocin, a chemical active in the brain, increases the level of trust, at least in psychological experiments. Oxytocin levels are known to be under genetic control in other mammals like voles.

It is easy to imagine that in societies where trust pays off, generation after generation, the more trusting individuals would have more progeny and the oxytocin-promoting genes would become more common in the population. If conditions should then change, and the society be engulfed by strife and civil warfare for generations, oxytocin levels might fall as the paranoid produced more progeny.

Napoleon Chagnon for many decades studied the Yanomamo, a warlike people who live in the forests of Brazil and Venezuela. He found that men who had killed in battle had three times as many children as those who had not. Since personality is heritable, this would be a mechanism for Yanomamo nature to evolve and become fiercer than usual.

Since the agricultural revolution, humans have to a large extent created their own environment. But that does not mean the genome has ceased to evolve. The genome can respond to cultural practices as well as to any other kind of change. Northern Europeans, for instance, are known to have responded genetically to the drinking of cow's milk, a practice that began in the Funnel Beaker Culture which thrived 6,000 to 5,000 years ago. They developed lactose tolerance, the unusual ability to digest lactose in adulthood. The gene, which shows up in Dr. Pritchard's test, is almost universal among people of Holland and Sweden who live in the region of the former Funnel Beaker culture.

The most recent example of a society's possible genetic response to its circumstances is one advanced by Dr. Cochran and Henry Harpending, an anthropologist at the University of Utah. In an article last year they argued that the unusual pattern of genetic diseases found among Ashkenazi Jews (those of Central and Eastern Europe) was a response to the demands for increased intelligence imposed when Jews were largely confined to the intellectually demanding professions of money lending and tax farming. Though this period lasted only from 900 A.D. to about 1700, it was long enough, the two scientists argue, for natural selection to favor any variant gene that enhanced cognitive ability.

One theme in their argument is that the variant genes perform related roles, which is unlikely to happen by chance since mutations hit the genome randomly. A set of related mutations is often the mark of an evolutionary quick fix against some sudden threat, like malaria. But the variant genes common among the Ashkenazi do not protect against any known disease. In the Cochran and Harpending thesis, the genes were a response to the demanding social niche into which Ashkenazi Jews were forced and the nimbleness required to be useful to their unpredictable hosts.

No one has yet tested the Cochran-Harpending thesis, which remains just an interesting though well worked out conjecture. But one of its predictions is that the same genes should be targets of selection in any other population where there is a demand for greater cognitive skills. That demand might have well have arisen among the first settled societies where people had to deal with the quite novel concepts of surpluses, property, value and quantification. And indeed Dr. Pritchard's team detected strong selection among East Asians in the region of the gene that causes Gaucher's disease, one of the variant genes common among Ashkenazim.

source:http://www.nytimes.com/2006/03/12/weekinreview/12wade.html?ex=1299819600&en=4ffa8520bf6f7229&ei=5088&partner=rssnyt&emc=rss