Skip to comments.Humans Migrated Out Of Africa, Then Some Went Back, Study Says
Posted on 12/29/2006 3:48:38 PM PST by blam
Humans Migrated Out of Africa, Then Some Went Back, Study Says
for National Geographic News
December 14, 2006
Humans first moved out of Africa about 70,000 years ago, but 30,000 years later some of them moved back.
That's according to a new study based on DNA evidence from ancient human remains found in Africa.
The study shows that a small group of early humans returned to Africa after migrating to the Middle East.
In addition, the research suggests that the humans' return occurred around the same time that another group of humans left the Middle East and moved into Europe.
"We were rather surprised by the age of the migration back to Africa," said Antonio Torroni, a geneticist at the University of Pavia in Italy.
"We did not really expect that it was 40,000 to 45,000 years old."
"But the age and the fact that the migration had originated in the Levant [a geographical term referring to a large part of the Middle East] led us to link the migration to Africa to that occurring at the same time toward Europe from the same region," added Torroni, who led the research team.
The findings are reported in tomorrow's issue of the journal Science.
The new study builds on the theory, laid out in two separate studies published in Science last year, that humans migrated from Africa in a single dispersal about 70,000 years ago.
That theory suggests that modern humans left East Africa by crossing the Red Sea, then journeyed south, following a coastal route along the Arabian Peninsula and on to India, Malaysia, and Australia (see a map of human migration). Other models have suggested that humans left Africa in multiple waves of migration via northern and southern routes.
The single "out of Africa" dispersal is believed to have given rise to all modern non-African populations.
However, scientists have been puzzled by two genetic populations found only in northern and eastern Africa, whose ancestors appear to have been Asian.
In the new study, scientists sequenced the mitochondrial DNA, which is passed from mother to daughter, from 81 individuals in both of these genetic groups.
They found that the two populations must have arisen in southwestern Asia and returned to Africa about 40,000 to 45,000 years ago.
The groups did not, however, follow the same southern coastal route back that was used in the single dispersal out of Africa.
Instead, the study suggests, they arrived from the Middle East, the same area from which another genetic groupone typical among Europeanswas at the same time moving toward Europe.
"It's a finding that supports the view that the first [Late Stone Age] cultures in North Africa and Europe had a common homeland in the Levant," Torroni said.
Vincent Macaulay, the lead author on one of the two single-dispersion studies published in Science last year, agrees with the findings.
"These results make perfect sense and wrap up some loose ends," said Macaulay, a genetic statistician at the University of Glasgow in Scotland.
The authors of the new study believe that before reaching the Levant, migrating humans may have paused at the Persian Gulf for some time because of a hostile climate.
Environmental evidence suggests that migrating north from southwestern Asia would have been impossible earlier than 50,000 years ago because of a vast desert that extended from northern Africa to central Asia.
"When weather conditions improved, the desert was fragmented and reduced in size," said Anna Olivieri, a geneticist in Torroni's lab and a co-author of the study.
"The human groups living in the coastal regions of southwestern Asia were able to move inland."
"Some of them colonized first the Levant and from there all surrounding regions including Europe and North Africa," she said.
"Consider also that the Sahara desert in North Africa was reducing its size. Thus, that region became interesting from a human colonization perspective."
Neither group was thrilled with the Middle East, it appears.
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.
Migrations from those areas may have been occurring even before the melting of the glacial ice and the raising of the sea levels.
Some level of trade and travel may have existed..
The end of the Ice Age may have simply completed population shifts that had already begun...
Was there a substantial sea crossing??
What about ice ??
Nine or Ten Years of frozen ice over the Australian straits probably wouldn't even show up in a geological record..
But it would be ample time for humans to simply walk to Australia..
Ice age sea levels combined with periodically frozen southern oceans would provide the means and opportunity to migrate to Australia..
Likewise, ice fishers accidentally marooned on ice floes could have found themselves brought to the continent by ocean currents..
The Toba eruption could very well have driven humans south in to Australia.. ( 6 year "nuclear" winter, subsequent 1000 year ice age provides adequate ice-overs in the southern hemisphere for desperate humans to attempt escaping disaster )
It may be found that the first migrations were even earlier than presently thought..
Good link.. (bookmarked for future reference)
The Out of Eden senarios are looking even more likely.
more contrived feelgood therapy for the afrocentrists
Yes, but much shorter than today.
"What about ice ??
No ice. The equator runs right through Indonesia and just north of New Guinea (Sundaland). An ideal place for humans to thrive during the Ice Age.
The equator runs through South America too and I suspect we have some big ancient suprises waiting down there.
The studies of Neandertal mtDNA do not show that Neandertals did not or could not interbreed with modern humans. However, the lack of diversity in Neandertal mtDNA sequences, combined with the large differences between Neandertal and modern human mtDNA, strongly suggest that Neandertals and modern humans developed separately, and did not form part of a single large interbreeding population. The Neandertal mtDNA studies will strengthen the arguments of those scientists who claim that Neandertals should be considered a separate species which did not significantly contribute to the modern gene pool.
I'm presently reading a book titled, Before The Dawn, by Nicholas Wade and he says that the migration of Modern Humans out of Africa was blocked for thousands of years by the Neanderthals in Europe and the Middle East.
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