Evidence Of Very Recent Human Adaptation: Up To 10 Percent Of Human Genome May H

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Evidence Of Very Recent Human Adaptation: Up To 10 Percent Of Human

Genome May Have Changed

http://www.sciencedaily.com/releases/2007/07/070711134400.htm

A Cornell study of genome sequences in African-Americans, European-

Americans and Chinese suggests that natural selection has caused as

much as 10 percent of the human genome to change in some populations

in the last 15,000 to 100,000 years, when people began migrating

from Africa.

The study, published in the June 1 issue of PLoS (Public Library of

Science) Genetics, looked for areas where most members of a

population showed the same genetic changes. For example, the

researchers found evidence of recent selection on skin pigmentation

genes, providing the genetic data to support theories proposed by

anthropologists for decades that as anatomically modern humans

migrated out of Africa and experienced different climates and

sunlight levels, their skin colors adapted to the new environments.

However, the study found no evidence of differences in genes that

control brain development among the various geographical groups, as

some researchers have proposed in the past.

" We undertook a very careful study of genetic differences within and

among major human groups, and aimed to explain why certain parts of

the genome differed, " said on, the study's lead author

and a Cornell assistant professor of biological statistics and

computational biology. " We aimed to eliminate as many possible

confounding variables as possible, and when all is said and done, we

find that as much as 10 percent of the genome may have been affected

by one of these bouts of recent selection. "

Previous studies at Cornell and elsewhere have searched for signs of

selection -- the divergence of genes from a common ancestor millions

of years ago -- by comparing an individual human to a chimpanzee or

mouse, for example, or by comparing genetic variation in protein

coding genes among humans to differences between humans and a

chimpanzee. But this study scanned genome sequences that compared

many humans to each other throughout the entire genome, with new

strict statistical methods that correct for many potential biases

that creep into this kind of analysis.

In the latest study, the researchers identified 101 regions of the

human genome with strong evidence of very recent selection. These

regions include genes that control proteins that help muscle cells

attach to surrounding cells (mutations of this gene lead to muscular

dystrophy), receptors that relate to hearing, genes involved in

nervous system function and development, immune system genes and

heat shock genes.

The gene scan method also detected selection in a gene involved in

digestion of lactose, an enzyme found in milk. Prior to animal

domestication, humans lost the ability to digest milk after infancy.

But, as humans migrated and domesticated animals, Europeans and

other populations developed a gene for tolerating lactose (and milk)

throughout their lives. This finding has been well established in

previous research, so arriving at similar results provided an

internal validation for the accuracy of the new method.

Overall, close to 10 percent of the Chinese and European-American

genomes and only 1 percent of the African-American genome were

linked to areas with evidence of recent selection. Since Africans

have the greatest genetic diversity and the statistical method

searched for areas where the majority of members within a population

group have the same genetic changes, signs of evolution were much

easier to detect in the less diverse European-American and Chinese

genomes.

" It is important to emphasize that the research does not state that

one group is more evolved or better adapted than another, " said co-

author Bustamante, a Cornell assistant professor of

biological statistics and computational biology. " Rather as humans

have populated the world, there has been strong selective pressure

at the genetic level for fortuitous mutations that allow digestion

of a new food source or tolerate infection by a pathogen that the

population may not have faced in a previous environment. "

Rasmus Nielsen, an adjunct professor of biological statistics and

computational biology at Cornell and now a professor at the

University of Copenhagen, Denmark, is the paper's senior author.