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.
- THE HUMAN JOURNEY
- 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.
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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 populations—the Andamanese, the Onge and Great Andamanese—on 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."
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 ago—and 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.
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European Settlement
It has been widely assumed that modern humans—Homo sapiens—first traveled out of Africa and settled in central and Western Europe before heading to Eastern Europe. That may not be the case. Recent finds from a site in Russia about 250 miles south of Moscow suggest that the first humans in Europe were Eastern European.The discoveries include bone and carved ivory artifacts. Researchers calculated the date they were made by determining that the ash surrounding the artifacts came from an eruption that occurred 40,000 years ago in southern Italy.
"Some of the artifacts found right under the ash were almost certainly made by modern humans," says John Hoffecker, a University of Colorado archaeologist working at the site. Among them is an ivory piece that appears to be the head of a partially complete human figurine. Radiocarbon and other dating methods also helped establish that the artifacts—stone blade fragments, scrapers, shell ornaments, a bone awl, and various digging and carving tools—are an astonishing 42,000 to 45,000 years old.
The artifacts are most likely remnants from H. sapiens, not Neanderthals. "The shells came from the Mediterranean basin, a minimum of 300 miles away," Hoffecker says. "One of the things you see when modern humans show up is a big leap in the distances over which materials move." The carved piece of mammoth ivory is further proof of modern humans at work. "Carving ivory is confined to modern humans," Hoffecker says. "So whether it’s a human figurine or not doesn’t matter. A modern human carved it, and that human is older by a considerable amount than any previously found in Eastern Europe."
Humans Migrated Out of Africa, Then Some Went Back, A New Study Says
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.
Single Dispersal
The new study builds on the theory, laid out in the first part above. 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 group—one typical among Europeans—was 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.
Changing Climate
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."
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Are We All Asians?
One of the best-known theories about human evolution—that the ancestors of Homo sapiens originated in Africa before populating the rest of the world 2 million years ago—is coming under fire.
In a challenge to conventional wisdom, Robin Dennell of the University of Sheffield in England and Wil Roebroeks of Leiden University in the Netherlands argue that the "out of Africa" interpretation is built on shaky evidence. Maybe, they say, it is time to look to Asia instead.
Roebroeks and Dennell point out that recent fossil finds in the nation of Georgia suggest an Asian origin as much as an African one. "We know so little about Asia—and, for that matter, Africa—that we should be very careful not to turn a hypothesis into a stone-carved truth simply by repeating it too often," Roebroeks says. "We need comparable data sets from both continents."
Anthropologist Spencer Wells, whose genetic research supports a single African origin, welcomes this questioning of the status quo. "That Homo erectus could have origins in Asia would be potentially shocking," he says, "but I think that what Roebroeks and Dennell are saying reflects the state of the field. We certainly don't have enough fossils. Perhaps we are never going to be able to test this hypothesis."
Meanwhile, population geneticist Alan Templeton of Washington University in St. Louis is overturning ideas about human origins from another angle. He has analyzed genetic relationships among diverse groups of people and finds that today's humans show evidence of interbreeding among Homo erectus, Homo sapiens, and other early hominids over a wide span of time, from as far back as 1.5 million years ago until the last hypothesized global migration, around 80,000 years ago.
Templeton concludes that the humans who departed from Africa probably interbred with other early humans in Europe and Asia, contradicting the widely held notion that the Africans wiped out existing populations as they moved.
"We don't have a tree of human populations with branches for Europeans and Asians and Arabs," Templeton says. "It's more like a trellis: Things are intertwined."
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