sábado, 8 de junio de 2013

Palm-Size Fossil Resets Primates’ Clock, Scientists Say

June 5, 2013

A nearly complete skeleton of a tiny, ancient primate — one that weighed no more than an ounce, had a tail longer than its body and would fit in the palm of your hand — is the earliest well-preserved fossil primate ever found, dating back some 55 million years and dialing back the fossil record for primates by an impressive eight million years, a research team declared on Wednesday.

An artist's interpretation of a tiny primate that is thought to be the earliest known ancestor of nocturnal primates living today in Southeast Asia. Xijun Ni/Chinese Academy of Sciences
The finding adds weight to the evidence that primates originated in Asia — not Africa — and that they emerged relatively soon after the extinction of the dinosaurs, which happened about 66 million years ago.

The older date brings scientists closer to pinpointing a pivotal event in primate and human evolution: the divergence between the lineage leading to anthropoids — which include modern monkeys, apes and humans — and the one leading to tarsiers.

In a report published in the journal Nature, an international team of paleontologists led by Xijun Ni of the Chinese Academy of Sciences in Beijing said that the skeleton, recovered from an ancient lake bed in Hubei Province in central China, set a new benchmark for the time that primates started roaming the planet.

The primate skeleton belongs to a species never seen before, one the researchers identified as the earliest known ancestor of tarsiers — a type of small, nocturnal primate living today in Southeast Asian forests. This unprepossessing early primate was even smaller than today’s smallest primate, the pygmy mouse lemur of Madagascar.

Dr. Ni said in a statement that the findings represent “the first time that we have a reasonably complete picture of a primate close to the divergence,” calling it “a big step forward in our efforts to chart the course of the earliest phases of primate and human evolution.

K. Christopher Beard, a paleontologist at the Carnegie Museum of Natural History in Pittsburgh and an author of the journal report, said: “We’ve heard of the ‘out of Africa’ theory of human evolution, but that’s recent history. So there may now be the ‘into Africa’ problem.

How and when did some primates finally make it to Africa, which was an island until as recently as 16 million years ago, to set in motion the emergence of the human species?

There is evidence that 38 million years ago, some primates had apparently crossed open water to colonize the African continent.

The fossil from Hubei does not answer the question of how that happened, but it does give paleontologists plenty to work on for years to come. The skeleton “differs radically from any other primate, living or fossil, known to science,” Dr. Beard said. “It looks like an odd hybrid, with the feet of a small monkey, the arms, legs and teeth of a very primitive primate and a primitive skull bearing surprisingly small eyes.”

Some of the skeleton’s anatomical characteristics resemble in miniature those of its tarsier lineage. Its head and trunk were less than four inches long, the tail a little more than five inches long. Some characteristics, like the monkeylike heel and ankle, appear to reaffirm the close tarsier connection to anthropoids, which is why the species has been named Archicebus achilles, a reference to the best-known heel bone in Western culture.

The skeleton was found by a farmer a decade ago in a rock near the course of the modern Yangtze River. But it took years of analysis to figure out how to classify the surprising-looking creature. Even today, not all researchers agree. For clues, the researchers looked to the previously oldest primate fossil specimen, a skeleton from Germany that was named in 2009 as Darwinius masillae. But the team concluded that the skeleton from Hubei, which is much smaller, belonged to an entirely separate branch of the primate family tree.

Daniel L. Gebo, a member of the team from Northern Illinois University, said the foot bones “made us stop and think, what is this thing?” Finally, the researchers decided the weight of evidence favored inclusion of the skeleton on the tarsier family tree.

They also agreed that findings strongly supported Asia, not Africa, as the most likely continent of primate origins. No known primate fossils of such antiquity have been found in Africa, which the anthropoids eventually did colonize, evolving into Homo sapiens only about 200,000 years ago.

Dr. Beard imagined that the creature, being so small and metabolically active, must have spent its days (it was not nocturnal, as its tarsier descendants are) as “a kind of frenetic animal, anxious and agile, climbing and leaping around” in the humid tropical world it inhabited. “Amazing,” he said, pausing. “A planet of the apes before there were any apes.

The world inhabited by Archicebus achilles, 55 million years ago, was very different from ours. Non-avian dinosaurs had died out, and mammals were coming into their own. The entire planet was a natural greenhouse, with life-changing tropical or subtropical rain forests growing everywhere and palm trees as far north as Alaska. Philip D. Gingerich, a paleontologist at the University of Michigan, Ann Arbor, who is an authority on primate evolution but was not involved in this research, said in an e-mail that it was “less clear to me that the Archicebus specimen” belonged in the tarsier lineage. Dr. Gingerich agreed, however, that the new skeleton was close to the divergence between the anthropoid and tarsier branches of primates. He said he expected further research to “almost certainly yield additional important information about the skull and dentition, the forelimbs and the hind limbs.

Another member of the group analyzing the new skeleton, John J. Flynn, curator of fossil mammals at the American Museum of Natural History in New York, said the research took 10 years to complete because “we applied rigorous testing to all our ideas and hypotheses — convincing all our own collaborators first.

Some may want to take a shot at us,” Dr. Flynn said, but the analysis involved “intensive international cooperation behind the scenes at several museums.”

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