Early Humans May Have Crossed Sea to Leave Africa
for National Geographic News
|May 13, 2005|
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."
"Based on the mutations, we estimated that they must have migrated about 50,000 to 70,000 years ago, taking the southern sea route," he said.
Following the Coastline
Vincent Macaulay, a genetic statistician at the University of Glasgow in Scotland, led a separate genetic study. The results, which were also based on ecological and archaeological evidence, led Macaulay and his colleagues to conclude that modern humans left Africa via a southern migration route.
The researchers say evidence suggests modern humans could not have taken a northern route prior to 50,000 years ago, as one competing theory suggests. That's because the whole of North Africa, Arabia, and the Middle East into Central Asia was desert up until that time. The scientists also cite evidence of human settlement in Australia dating back to 63,000 years ago.
For modern humans to leave Africa via a southern route, as Macaulay and his colleagues argue, modern humans would have had to master ocean travel.
Macaulay said that crossing the Red Sea, which separates North Africa from the Arabian Peninsula, would not have been impossible. It was only a few kilometers across and modern humans "would have been able to see across to the other side. So [it was] perhaps not quite swimmable, but certainly floatable on a raft."
But how could modern humans have reached Australia, hundreds of nautical miles from the nearest landmass? On this question, Macaulay is more vague.
"The crossing at the other end, to Australia, is much more mysterious. That was a substantial sea crossing," he said. "But there's evidence for gene flow between Africa and Arabia post-60,000 years [ago], as well. So there were other sea crossings going on."
"Modern humans reached India by 66,000 years ago, Malaysia by 64,000 years agoand they reached Australia by about 63,000 years ago," Macauley said. "It's a rather rapid expansion. In 3,000 years they went something like 12,000 kilometers (7,500 miles)."
"If you do the math, it's about 4 kilometers (2.5 miles) a year, which is pretty speedy, maybe suggesting they were using boats to make longer excursions along the coast."
Four kilometers a year is comparable to estimates of the dispersal that settled the Americas, as humans moved from Asia across the ancient Bering land bridge into North American and, from there, into South America.
Macaulay's study postulates that Europe was populated as the result of an offshoot of migrants who traveled the southern route somewhere east of Africa. They moved up into Europe, beginning around 50,000 years ago as the climate improved.
The earliest known archaeological evidence for modern humans in Europe is on the order of 45,000 years old.
Macaulay's team suggests that the first groups of modern humans who left Africa to settle other continents likely numbered in the hundreds.
Their size relative to their impact on human history is astonishing, said Peter Forster, a geneticist at the McDonald Institute for Archaeological Research at the University of Cambridge in England. Forster wrote a Science commentary on the two new studies.
"At these early periods between 60,000 and 30,000 years ago, human population densities were low," he said. "We're talking on a scale of only a few hundred individuals that must have made it out of Africa 60,000 years ago.
"And these small founder groups, from which billions of people are descended, had the tremendous genetic impact that we see in the diversity today," Forster said.
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