Humans Migrated Out Of Africa, Then Some Went Back, 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.

"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 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 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.

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."

Early Humans May Have Crossed Sea to Leave Africa

Hillary Mayell
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 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."

"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 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.

European Settlement

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.

The problems with these studies were so bad that Henry Gee, a member of the editorial staff for the journal, Nature, harshly described the studies as "garbage." After considering the number of sequences involved (136 mtDNA sequences), Gee calculated that the total number of potentially correct parsimonious trees is somewhere in excess of one billion.25

Geneticist Alan Templeton (Washington University) suggests that low-level mixing among early human populations may have scrambled the DNA sequences sufficiently so that the question of the origin of modern humans and a date for "Eve" can never be settled by mtDNA.22

In a letter to Science, Mark Stoneking (one of the original researchers) acknowledged that the theory of an "African Eve" has been invalidated.23

Another interesting aspect of the "molecular clock" theory is the way in which the mutation rate itself was determined.

Contrary to what many might think, the mutation rate was not initially determined by any sort of direct analysis, but by supposed phylogenic evolutionary relationships between humans and chimps

In other words, the mutation rate was calculated based on the assumption that the theory in question was already true. This is a rather circular assumption and as such all results that are based on this assumption will be consistent with this assumption - like a self-fulfilling prophecy.

Since the rate was calculated based on previous assumptions of evolutionary time, then the results will automatically "confirm" the previous assumptions.

If one truly wishes independent confirmation of a theory, then one cannot calibrate the confirmation test by the theory, or any part of the theory, that is being tested. And yet, this is exactly what was done by scientists such as Sarich, one of the pioneers of the molecular-clock idea.

Sarich began by calculating the mutation rates of various species "...whose divergence could be reliably dated from fossils." He then applied that calibration to the chimpanzee-human split, dating that split at from five to seven million years ago.

Using Sarich's mutation calibrations, Wilson and Cann applied them to their mtDNA studies, comparing "...the ratio of mitochondrial DNA divergence among humans to that between humans and chimpanzees."24 By this method, they calculated that the common ancestor of all modern humans, the "African Eve", lived about 200,000 years ago.

Obviously then, these dates, calculated from the mtDNA analysis, must match the presupposed evolutionary time scale since the calculation is based on this presupposition.

The circularity of this method is inconsistent with good scientific method and is worthless as far as independent predictive value is concerned.

The "mitochondrial clock" theory was and is basically a theory within a theory in that it has no independent predictive power outside of the theory of evolution.

It is surprising then that scientists did not catch this inherent flaw earlier. Interestingly enough though, this flaw in reasoning was not detected for many years and perhaps would have remained undetected for much longer if a more direct mutation-rate analysis had not been done

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