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An Asian Origin for Human Ancestors?
4 June 2012 3:04 pm
Researchers agree that our immediate ancestors, the upright walking apes, arose in Africa. But the discovery of a new primate that lived about 37 million years ago in the ancient swamplands of Myanmar bolsters the idea that the deep primate family tree that gave rise to humans is rooted in Asia. If true, the discovery suggests that the ancestors of all monkeys, apes, and humans—known as the anthropoids—arose in Asia and made the arduous journey to the island continent of Africa almost 40 million years ago.
Until 18 years ago, fossils of every suspected early anthropoid were found in Egypt and dated to about 30 million years ago. Then, starting in the 1990s, researchers began discovering the remains of petite primates that lived 37 million to 45 million years ago in China, Myanmar, and other Asian nations. This suggested that anthropoids may have actually arisen in Asia and then migrated to Africa a few million years later. But paleontologists have lacked the fossils to show when and how these anthropoids trekked from Asia to Africa, says paleontologist K. Christopher Beard of the Carnegie Museum of Natural History in Pittsburgh, Pennsylvania.
In 2005, Beard and an international team of researchers sifting fossils of early fish, turtle, and ancestral hippo teeth from fossil beds near the village of Nyaungpinle in Myanmar found a molar the size of a kernel of popcorn. The tooth, dated to about 38 million years ago, belonged to a new species of ancient primate, which would have been the size of a small chipmunk. After several more years of arduous fieldwork, the team has collected just four molars of this primitive anthropoid, which they named Afrasia djijidae. "It's a difficult place to work; it took us 6 years to find four teeth," says Beard.
The four molars were enough to show Beard and team leader Jean-Jacques Jaeger of the University of Poitiers in France that Afrasia was closely related to another primitive anthropoid that lived at about the same time, but in Africa—Afrotarsius libycus from Libya. When the researchers examined the teeth from the two primates under a microscope, they were so similar in size, shape, and age that they could have belonged to the same species of primate, says Beard. Such close resemblance between an Asian and African fossil anthropoid has "never been demonstrated previously," the authors write online today in the Proceedings of the National Academy of Sciences.
On closer examination, however, the team noticed that the new molars from the Asian Afrasia were more primitive than those of Afrotarsius from Libya, particularly in the larger size of a tiny bulge at the back of its last lower molar. These primitive traits, as well as the greater diversity and age of early, or "stem," anthropoids in Asia rather than Africa suggest that this group arose in Asia and migrated to Africa 37 million to 39 million years ago. "Anthropoids didn't arrive in Africa until right before we find their fossils in Libya," says Jaeger.
The Out-of-Asia scenario may have been complex. The team proposes that more than one species of anthropoid migrated from Asia to Africa at about this time, because there are at least two other types of early anthropoids alive at about the same time as Afrotarsius in Libya, yet they are not closely related to Afrotarsius or Afrasia. This may be because once they got to Africa, they found ideal lush conditions with few carnivores and underwent a "starburst of evolution," says Beard, rapidly giving rise to a number of new species.
Others agree that if both the new species of primates from Myanmar and Libya are indeed early anthropoids, they would greatly strengthen the case for the Asian origins of anthropoids. "If proven, the biogeographical significance of these results is profound," says paleontologist Richard Kay of Duke University in Durham, North Carolina. It would show that there was a major migration of primates and probably other mammals between the two continents at a time when it was not easy to get across the ancient Tethys Sea that divided Africa from Asia. And for humans, it would suggest that our deepest primate roots were in Asia, not Africa.
Still, the similarity between the species rests on just four molars of Afrasia, Kay notes, although teeth are the most reliable way to measure relatedness. And some researchers have yet to be convinced that Afrotarsius in Libya is a stem anthropoid rather than an ancestor of tarsiers, primates that are not anthropoids and, thus, are more distant relatives. Kay, however, says the scales are tipping toward an Asian origin. "We've all heard about Out-of-Africa for human origins," adds Beard. "Now we think there was an Out-of-Asia migration into Africa first."