Tiniest Primate Discovered
Fossil Find Stirs Debate on Asia's Place in Theory of Human Origins
By Guy Gugliotta
They lived 42 million years ago in China, and they are the smallest primates ever found. The tiny, now extinct creatures were about the size of a human thumb. The smallest weighed about one-third of an ounce and the largest weighed about one-half ounce. The mouse lemurs of Madagascar, the smallest primates living today, weigh about one ounce.
Northern Illinois University paleontologist Daniel Gebo announced the discovery of the two tiny primates yesterday, along with the identification of the first-ever heel and ankle bones from a previously discovered small early primate that dates back to the same period known as Eosimias.
The Eosimias find offers further evidence that that creature, which weighed no more than about 4 ounces, is the oldest known anthropoid, the more advanced primate branch that includes monkeys, apes and humans.
The new finds add to a growing body of evidence of a rich evolutionary history of primates in Asia, raising questions about the long-held view that human origins can most probably be traced to Africa. The oldest remains of human ancestors ever found are 2 million to 4 million years old, and were found in East Africa.
The primate finds may also help forge linkages in the evolutionary chain between early primates, which made their appearance about 66 million years ago, and anthropoids, which evolved 40 million to 50 million years ago.
Remains of all three animals were found in a limestone quarry at Shanghuang, about 100 miles west of Shanghai. Additional Eosimias remains were found in the central Chinese province of Shanxi. Gebo and his colleagues reported on their research this week in the Journal of Human Evolution and in today's issue of the journal Nature.
Gebo said the tiniest primate is probably an advanced "prosimian," or lower primate, while the second primate and Eosimias are anthropoids with prosimian characteristics. In this light, all three animals could help bridge the transitional gap between the two great primate branches. Neither of the new primates has yet been named.
The smallest primate is "about the size of a tiny mouse," Gebo said. It is somewhat like today's prosimians--the lemur, loris or bush baby, he added, but is perhaps most akin to the modern tarsier, a rat-like tree-dweller thought to occupy the intermediate stage between lower primates and anthropoids.
Still, he said, the connection between the new primate and the tarsier "is not very strong." The smallest animal was identified from a single heel bone examined under a microscope by Northwestern University's Marian Dagosto, Gebo's co-researcher and wife.
"The other big point [is] the implications of small body size," Gebo continued. "Your metabolic rate is going crazy. You can't eat grass. You have to eat insects, fruit and nectar, because you're burning calories so fast."
This meant that the newly discovered creatures "didn't have a lot of time to be social," in contrast to most primates, Gebo said. They had to have been loners and night hunters, able to leap from tree to tree, and vulnerable to nocturnal predators. "In Madagascar," he added, "mouse lemurs are eaten like popcorn."
Gebo said the primate fossils are much younger geologically than the soft Shanghuang limestone where they were found. The stone is threaded with fissures and small caverns carved by millions of years of rainwater. Mud containing the bones of tiny animals washed into the interstices and solidified.
Gebo noted that there were no whole skeletons, suggesting that the animals had been killed and eaten by owls, who regurgitated feathers and bones in "owl pellets," carried away by the rain.
Most of the bones, found in 1996, are broken, and had to be tediously examined and sorted under a microscope, Gebo said. Both Gebo and Dagosto are foot bone specialists, and in addition to primates, they found remains of rodents, lizards, birds, frogs, rabbits and other small creatures.
In the four years since the original Shanghuang discovery, researchers have catalogued "perhaps 10 percent" of the findings, stored in cigar boxes at Beijing's Institute of Paleontology and Paleoanthropology, Gebo said.
One puzzle that confounds the researchers is the site's almost total lack of large animal bones. Forty-two million years ago the quarry was a tropical forest, Gebo said, a habitat which ordinarily cannot support many species of small animals.
"But we think we have maybe 12 to 16 species [of primates] in all," Gebo said. "It's unheard of to find a lot of animals of the same size competing against each other. You go to any other forest, in the Amazon, in Indonesia, in Wyoming, you won't find them. It's quite odd."
Examination of the Eosimias bones supported the theory that these small creatures were primitive anthropoids with prosimian characteristics, possibly a "transitional" species between the primate family's two great branches, Gebo said.
While prosimians "leap and cling" to trees, anthropoids travel horizontally on the tops of branches using all four limbs, he said. The bones examined by the team confirmed this "quadripedalism," although they also showed residual prosimian characteristics.
While the researchers did not find any skulls or teeth to match the foot bones, their analysis showed that the foot size was consistent with already discovered Eosimias jaws and teeth.
Gebo acknowledged, however, that current research into ancient primates has become quite controversial because it calls into question the theory that humans originated in Africa. Until Eosimias, the oldest known anthropoids were about 37 million years old, and came from a famous excavation at Fayum, in Egypt. The smallest primates recovered there weighed a bit less than a pound.
"And now these fossils are coming out of Asia," Gebo said. "The continent is weird, their size is weird and their morphology is really weird. If Eosimias is at the base of the pyramid, then without Eosimias, you don't have humans."
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