for National Geographic News
Where did we come from, and how did we get here? Most scientists agree on the most basic answers to these questions, suggesting modern humans first evolved in Africa, probably around 150,000 years ago, and later colonized the globe.
But precisely when this migration started and the route it followed has been hotly debated. One theory holds that a wave of migration from Africa began about 50,000 years ago, with modern humans moving north through North Africa into the Middle East, then moving east and west into Asia and Europe.
Another model suggests that modern humans left Africa in multiple waves of migration that started perhaps as early as 80,000 years ago, with ancient settlers dispersing globally via northern and southern routes.
Two separate studies published in the current edition of the research journal Science support a third theory: that a single rapid dispersal occurred somewhere between 60,000 to 75,000 years ago.
The studies suggest that modern humans left East Africa by crossing the Red Sea, then journeyed south, following a coastal route along the Arabian Peninsula to India, Malaysia, and Australia.
One of the two new studies was led by Kumarasamy Thangaraj, a geneticist at the Centre for Cellular and Molecular Biology in Hyderabad, India. Thangaraj and his colleagues investigated populations on the Andaman and Nicobar Islands near the coast of Thailand.
The study focused on mitochondrial DNA, genetic material that is passed maternally and found in every human cell. All humans can be traced via this specialized DNA to a single ancestral female who lived about 150,000 to 200,000 years ago, many scientists say.
Thangaraj and colleagues used this genetic material as signposts to trace the deep ancestry of six isolated indigenous tribal populations on the islands. The tribes included the Nicobarese, Onge, Andamanese, and Great Andamanese.
Earlier studies had shown that the Nicobarese are of Southeast Asian origin and probably reached the islands relatively recently, between 15,000 and 18,000 years ago.
In the past scientists believed that three of the tribal populationsthe Andamanese, the Onge and Great Andamaneseon the islands were "closer to the Asians than Africans," Thangaraj said.
"But when we sequenced [their] complete mitochondrial genome[s], we found unique variations, which have not been found anywhere in the world, so far," he said.
The findings led Thangaraj and his colleagues to suggest that the tribes descend from "the very early migrants out of Africa."
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