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See Inside June 2008

Need for Speed?

Studies challenge the idea that human evolution recently accelerated

Addis Ababa is the center of the genetic world. Not because of any special skill in genome analysis but because variation in the human genetic code diminishes as distance from the Ethiopian capital increases. After all, modern humans originated in East Africa.

Yet this genetic fact has led to some divergent conclusions. Some researchers have looked at the differing makeup of populations and argued that human evolution must be speeding up to explain all the variation. Others see that variation as yet more proof that a relatively small number of individuals migrated out of Africa and founded the current populations in other parts of the world.

Last December a group of anthropologists and geneticists concluded that human evolution must have accelerated in the past 40,000 years after examining the 3.9 million DNA sequences from 270 people from four populations known as the HapMap. “We found very many human genes undergoing selection,” says Gregory Cochran of the University of Utah, a member of the team. “We believe that this can be explained by an increase in the strength of selection as people became agriculturalists, a major ecological change, and a vast increase in the number of favorable mutations as agriculture led to increased population size.”

Such mutations include regionally different genes that have lightened skin tone and conferred the ability to digest milk in adulthood. In separate work, Lluís Quintana-Murci of the Pasteur Institute in Paris and his colleagues count about 55 of these mutations in total.

But other studies of changes to individual DNA amino acids (single-nucleotide polymorphisms), DNA sequences (haplotypes) or longer sections of the genetic code (copy number variants) all agree that the peoples of Africa have the most diversity in their genome. People in different regions—say, Africans and Europeans—may have a few different genes, but that minuscule difference is dwarfed by the amount of genetic code all humanity has in common. In fact, two Africans are likely to have greater genetic differences than an African and a European do. “We are a young species,” remarks geneticist Noah Rosenberg of the University of Michigan at Ann Arbor, who participated in a comprehensive study of genetic variation that appeared in Nature in February. “Different human populations have not been separated for long enough periods of time to develop their own new alleles.”

Many geneticists therefore express doubt that genes have evolved very much in the relatively short span of human existence in all parts of the planet. And most genetic instructions that do vary are more likely to be harmful than helpful, according to another analysis of DNA amino acid variation.

Instead of evolution working on a relatively small number of genes to actively promote functional adaptations—a process known as positive selection—“the alternative is a demographic factor, which is a bottleneck,” explains geneticist Marcus Feldman of Stanford University. A bottleneck describes the rise of a new population from a few individuals. Feldman participated in a study of DNA samples from 938 people in 51 different populations, finding evidence for the alternative explanation in declining DNA sequence (haplotype) variation with increasing distance from Africa. The work appeared in Science in February.

“A bottleneck followed by population growth may explain the slight increases in the proportion of European-American-specific amino acid variation predicted to adversely impact protein structure and stability,” says Carlos Bustamante of Cornell University, who computationally models the changes in genes. Other computational biologists, such as Itsik Pe’er of Columbia University, also point out that demographic changes are consistent with the overall distribution of human genes.

A new initiative, called the 1000 Genomes Project, could settle the question of whether human evolution has picked up. Scheduled for completion in 2011, it should provide a “cornucopia of sequence data from many more individuals from more populations,’’ notes Kirk Lohmueller, a graduate student in Bustamante’s lab.

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