'Out of Africa' theory confirmed
2008-02-21 08:00
Washington - Two big genetic
studies confirm theories that modern humans evolved in Africa
and then migrated through Europe and Asia to reach the Pacific
and Americas.
The two studies also show that Africans have the most
diverse DNA, and the fewest potentially harmful genetic
mutations.
One of the studies shows European-Americans have more small
mutations, while the others show Native Americans, Polynesians
and others who populated Australia and Oceania have more big
genetic changes.
The studies, published in the journal Nature on Wednesday,
paint a picture of a population of humans migrating off the
African continent, and then shrinking at some point because of
unknown adversity.
Later populations grew and spread from this smaller genetic
pool of founder ancestors - a phenomenon known as a
bottleneck.
Populations that remained in Africa kept their genetic
diversity - something seen in many other studies.
"The one thing that I think we cannot say from this study
is that any one person's genome is any healthier or
evolutionarily fit than another person's genome," said Carlos
Bustamante of Cornell University in New York, who worked on one
study.
"You have to think of this at the population level,"
Bustamante said.
Bustamante's team has been looking at the DNA sequences of
15 African-Americans and 20 European-Americans, examining tiny
one-letter changes in the DNA code called single-nucleotide
polymorphisms or SNPs (pronounced "snips").
Fit or extinct
They tested these changes to qualify them as benign, or
potentially affecting genes, amino acids and eventually
proteins in a way that could damage health or make people less
"fit" - in evolutionary terms, less likely to survive and reproduce.
"Like every other study ... the African-American panel as a
whole showed more variation than the European-American panel," Bustamante said.
Then his team did a computer simulation of a bottleneck,
and found it predicted this pattern.
Bustamante said it is possible some of the SNPs are
beneficial, and he said his team and others should compare the
genetic changes they found to known genetic changes linked with
diseases.
"I wish we had done that (already)," he admitted.
In the other study, Noah Rosenberg and colleagues at the
University of Michigan and the National Institute on Ageing
analysed DNA from 485 people around the world.
They looked for three types of genetic variation, including
SNPs and larger changes that involve duplications, deletions
and repetitions of large segments of DNA.
The patterns they found produced what they call the
highest-resolution map yet of human genetic variation.
They also reinforce the idea that humans originated in
Africa, then spread into the Middle East, followed by Europe
and Asia, the Pacific Islands and finally to the Americas.
"Diversity has been eroded through the migration process,"
Rosenberg said.
People of African descent are the most genetically diverse,
followed by people from the Middle East, and then Asians and
Europeans. Native Americans resemble one another the most on a
DNA level.
The study also found it is sometimes possible to trace a
person's ancestry to a small group within a geographic region.