Early Humans Settled Globe Gradually, Gene Study Says

Brian Handwerk
for National Geographic News
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.

Ramachandran and her colleagues studied the genes of 53 indigenous populations around the world.

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