By comparing the skulls and DNA of human species from around the world—including these Australian and African skulls—researchers assert that modern-day humans do indeed have a single origin in Africa. The roots and dispersal of modern humans have sustained debate for decades, with some researchers arguing that Homo sapiens arose independently in other parts of the world. Photographs courtesy Tsunehiko Hanihara
Modern Humans Came Out of Africa, "Definitive" Study Says
for National Geographic News
July 18, 2007
We are solely children of Africa—with no Neandertals or island-dwelling "hobbits" in our family tree, according to a new study.
Scientists who compared the skulls and DNA of human remains from around the world say their results point to modern humans (Homo sapiens) having a single origin in Africa.
The study didn't find any evidence to suggest that human species living elsewhere in the world contributed to our direct ancestors' make-up.
A team led by Andrea Manica at the University of Cambridge, England, combined analysis of global genetic variations with comparisons of more than 6,000 skulls from more than a hundred ancient human populations.
The team found that loss of genetic diversity was very closely mirrored by reduced physical variation the farther away people lived from Africa.
Only Out of Africa
The new data support the single origin, or "out of Africa" theory for anatomically modern humans, which says that these early humans colonized the planet after spreading out of the continent some 50,000 years ago.
In the past, experts have also argued a "multiregional" theory, which held that Homo sapiens arose from different human populations in different areas of the world.
"The origin of anatomically modern humans has been the focus of much-heated debate," lead author Manica said.
"We have combined our genetic data with new measurements of a large sample of skulls to show definitively that modern humans originated from a single area."
Previous studies have found that genetic differences in human populations can be explained by distance from Africa.
The new study also looked at 37 measurements from male and female skulls from around the world. The chosen skulls were all less than 2,000 years old, making them better preserved and more likely to give accurate measurements than older skulls.
Many skull features were determined by the different environments where the humans had lived.
But distance from Africa was still found to account for up to 25 percent of variation in the features.
The researchers made sure that the DNA analysis used the same framework as the analysis for the skulls—so the two could be fully compared, Manica said.
"I would argue we had two independent shots at getting the same answer, and remarkably, the answer is exactly the same," he added.
The lowest amount of variation was found in ancient populations from South America and Australia, the two main inhabited regions most remote from Africa.
The study team, writing in the latest issue of the journal Nature, argues that this low variation in remote regions relative to Africa would be expected if Homo sapiens arose solely in Africa.
That's because populations built up genetic and physical diversity for some 150,000 years before the fossil record suggests the first pioneers started spreading elsewhere.
But it wasn't until between about 20,000 and 30,000 years ago that modern humans reached South America and Australia, the team noted.
"The more you move away from that center of diversity where you started, the less diversity you have," Manica said.
This pattern was remarkably consistent globally, the researchers found.
The study places the original roots of modern humans in south-central Africa. In the middle of this region lies the Great Rift Valley—often referred to as the "cradle of humanity."
Some researchers believe that modern humans are at least in part the product of non-African species descended from Homo habilis, which left Africa at least 1.5 million years ago. (Related: "China's Earliest Modern Human Found" [April 3, 2007].)
Such groups include the Neandertals of Europe and western Asia, archaic human types in eastern Asia and Australia, and perhaps even the controversial hobbit humans from the Indonesian island of Flores. (Related: "Hobbit-Like Human Ancestor Found in Asia" [October 27, 2004].)
'No Other Source'
"What we can confidently say is that there has not been a wave [of anatomically modern humans] starting from somewhere else, because then you'd find a second area with more variability," Manica said.
What Manica can't say is "that matings with the Neandertals never ever happened, but if it did happen, none of the descendants stayed around." Effectively, any mating had no contribution whatsoever to modern humans, he added.
Anthropologist Erik Trinkhaus of Washington University in St. Louis, Missouri, has found fossil evidence suggesting that Homo sapiens and Neandertals did interbreed. Trinkhaus is critical of the latest findings. (Related: "Neandertals, Modern Humans May Have Interbred, Skull Study Suggests" [January 16, 2007].)
Certain genetic and anatomical traits "cannot be explained as a simple and complete expansion of modern humans out of Africa," he said.
"The idea that humans get more uniform further from Africa is simply ludicrous," he added, noting that modern-day Chinese and Australian Aborigines look no more similar to each other than do Africans and Europeans.
Fred Smith, an anthropologist at Loyola University of Chicago who is unaffiliated with the research, agrees that the findings confirm there is an African origin for modern humans.
Smith nevertheless argues that the study is not at odds with the idea he first proposed in 1989 that there was "some low-level assimilation of archaic peoples into these modern populations."
And Charles Roseman, of the University of Illinois at Urbana-Champaign, said: "It could very well be that there was a recent out-of-Africa expansion, coupled with some either small or large amount of genetic exchange with humans outside of Africa."
Out of Africa, The Sequel
By Michael Balter
ScienceNOW Daily News
18 July 2007
The fight over modern human origins is heating up. A new study of thousands of human skulls claims to confirm genetic evidence that our species arose in Africa and then spread over the globe. But some researchers say that an alternative scenario has not been ruled out.
Researchers have long debated two opposing hypotheses for modern human origins. According to the Out of Africa hypothesis, our ancestors appeared in Africa about 200,000 years ago and then replaced all other human species, including Homo erectus and the Neandertals, with little or no interbreeding. The multiregional hypothesis holds that modern humans emerged from populations of "archaic" hominids in Africa, Europe, and Asia that evolved locally but also exchanged genes. Numerous genetic studies support the single-origin model, finding that the genetic diversity of today's human populations is greatest in Africa and decreases steadily with distance from that continent. The idea is that diversity declined because each group of migrants founded a new population, creating genetic bottlenecks. But some researchers see traces of mixing between moderns and archaics in the genetic data.
A team led by population biologist Andrea Manica of the University of Cambridge in the U.K. set out to test the two hypotheses with skulls rather than genes. The researchers analyzed 4666 male skulls from 105 worldwide populations. Based on 37 measurements--ranging from the length of the cranium to the height of the eye sockets--the team reports this week in Nature that the worldwide pattern of skull shapes closely matches the genetic data: The diversity of cranial shape within a population falls off the farther it is from Africa. Similar results came from a second study of 1579 female skulls. The researchers could find no evidence for multiple centers of diversity outside Africa, as might be predicted by the multiregional model. They concluded that their results strongly support the Out of Africa model.
"This is an important piece of work because it compares results from large sets of genetic and cranial data using similar analytical approaches," says paleoanthropologist Chris Stringer of the Natural History Museum in London, a primary advocate of the Out of Africa model. Yet Stringer cautions that the study cannot rule out the possibility of gene flow between Homo sapiens and other humans such as Neandertals. That exchange might not show up in the skulls because the authors used only crania that were no more than 2000 years old.
Charles Roseman, an anthropologist at the University of Illinois, Urbana-Champaign, says that he is not convinced that the Nature authors have adequately tested the Out of Africa model versus its multiregional rival. The researchers assumed that the multiregional model requires that modern humans arose more than once. "Proponents of the multiregional model have been very clear for some time that their models do not posit multiple origins, as suggested in the paper," Roseman says.