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Early Humans Settled
Globe Gradually:
Gene Study Says
National Geographic News: Brian
Handwerk:October 18, 2005:
If all modern humans originated in Africa and only later
migrated around the globe, as theory holds, the paths of our ancestors'
wanderings may still be visible in our genes.
A new genetic study supports just such a scenario and suggests that early
Africans colonized the planet gradually through a series of small migratory
steps.
Results of the worldwide genetic sampling project show a strong correlation
between genetic diversity and geographic distance. The closer modern people
live to one another, as measured along the ancient migration routes that led
humans out of Africa, the more similar is their DNA.
"Geographic distance is very good at predicting genetic distance. The
correlation between the two is very high," said Sohini Ramachandran, an
evolutionary biology doctoral candidate at Stanford University.
Ramachandran is the lead author of the study, published in today's issue of
the journal Proceedings of the National Academy of Sciences.
Genes and Geography
Geneticists have long known that genetic differences between populations
increase with physical distance. Groups that live close to one another
interact, interbreed, and become more genetically related.
The new study adds a more detailed perspective to the concept.
It's not simply distance that has helped shape the modern genome, the
authors suggest, but the way in which humans migrated over those distances.
The new data show that genetic diversity decreases as one traces ancient
migration routes out of Africa.
"There's a very linear decrease of [genetic diversity] as you leave Africa,
and it's a bit surprising that it would fit the pattern so well,"
Ramachandran said.
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"We tried to figure out what kind of picture of human evolutionary history
would explain the high [genetic] correlation that we observed," said Noah
Rosenberg, a University of Michigan geneticist and study co-author.
Out of Africa
The team says their research kept pointing back to a single place of human
origin—Africa.
"When we searched over 4,000 points around the world, we found that no point
outside of Africa had as high a fit as any point inside of Africa,"
Rosenberg said. "So this seems to support an 'Out of Africa' historical
model for human evolution."
Genetic diversity is highest, and thus oldest, in Africa. This fact has led
many geneticists to point to the continent as the birthplace of humankind.
Genetic data suggest what scientists call a serial founder effect. The
theory holds that each group of migrating humans begat a later, smaller
subgroup that subsequently continued humankind's journey around the globe.
Each time a subset migrated onward, genetic diversity narrowed. As a result,
naturally occurring random genetic variations—also known as genetic
drift—increasingly influenced the genetic makeup of gradually more
homogenous populations.
Genetic diversity was found to be lowest in the Americas, which are widely
believed to be the last continents settled by humans.
The team concludes that perhaps 75 percent of humankind's modern genetic
variation is the result of random genetic drift.
The researchers suggest that only 25 percent of our genetic diversity stems
from the evolutionary process of natural selection—though such a number is
still significant.
"Undoubtedly natural selection has played an important role in altering our
genome during this migration out of Africa," Ramachandran said. "But it is
kind of new to think that genetic drift might have been responsible for this
much of human genetic variation." |
