Big debate grows from tiny discovery

- The discovery of incredibly tiny monkeylike primates, some the size of a human thumb, is cranking up the monumental debate over how monkeys, humans and great apes evolved.

Paleontologists working in China have been recovering bones of three tiny primate species that lived 42 million years ago, two of them the smallest primates known to science, weighing one-third and one-half ounce, respectively. The discovery may represent a critical link between higher- and lower-order primates. And it also could cast doubt on the long-held theory that such species arose in Africa.

The ankle bones of the pint-size primates are about as big as grains of rice. They are key evidence of the evolution of anthropoids (monkeys, apes and humans), according to Daniel Gebo, the Northern Illinois University anthropologist who heads the research team recovering the bones in China.

His team is working in limestone quarries that 42 million years ago lay under a steaming rain forest that was home to a kingdom of Lilliputian animals, including a rhinoceros which--if alive today--might make a dandy house pet. The primate bones are what is left of the animals after being eaten by owls.

"Few would have predicted such a diminutive monkeylike creature at such a key branch of evolution," Gebo said of the discovery being chronicled Thursday in the British journal Nature.

"These are the smallest primates ever discovered, alive or extinct," Gebo said. "Some of these fossils are one-third the size of the living mouse lemur from Madagascar, which at one ounce is the smallest known primate."

All three of the primates lived in the upper canopies of the primeval rain forest trees. The largest of the three, Eosimias, a creature weighing about 5 ounces, is particularly fascinating to scientists, as it is the oldest anthropoid fossil ever found.

In the Nature article, Gebo's team lays out evidence that Eosimias fills a previous gap in the fossil record, linking lower (prosimian) and higher (anthropoid) primates. The team describes the two smaller primate species in an article to be published in April in the Journal of Human Evolution.

"Nobody thought a primate could get so small, so this is a totally novel and surprising discovery. We have broken the sound barrier of how small anthropoids could get," said L. Christopher Beard, a paleontologist at Pittsburgh's Carnegie Museum, one of Gebo's team members and a co-author of the articles.

Gebo's team has spent years combing through piles of nearly microscopic bones and bone fragments found in mud and debris that filled fissures that had opened in the limestone bed under the rain forest 42 million years ago. The bones were a part of that debris, now being recovered by Chinese laborers in the limestone quarry.

Using a process similar to panning for gold, Gebo and his team sifted through the debris to recover the fossilized bones of the small creatures that lived in the forest back then, a place similar to the Amazon or Borneo, much hotter than today's China. In addition to finding the primate bones, they also discovered remains of a dozen other pint-size animals, but none of any large animals.

"It was an interesting place," said Beard. "There were little mouselike rodents, small bats, tiny rabbits and even an extinct rhino that was about the size of a dog."

Researchers have not found complete skeletons of Eosimias or the smaller, unnamed primates, but the diminutive stature of the animals tells researchers a lot about how the creatures lived and died.

"These were such amazingly small creatures," Gebo said, "that their rate of metabolism would have had to have been tremendously high, like hummingbirds. To compensate for loss of body heat, they'd have to be moving about constantly and feeding constantly on high-calorie diets, probably lots of insects and sugary tree saps."

They probably were solitary, nocturnal animals that bounded about on upper branches of the forest, he said, the perfect prey for ancient owls.

The bones found are broken in the same way owls break bones of their prey before swallowing them, Gebo said. Markings on the bones bear the kind of marks left by owl digestive acids.

"Owls digest the flesh of their prey, but their stomachs form pellets out of the fur and bones of their food, and they regurgitate the pellets like hairballs," Gebo said. "We think our fossil bones were in those owl pellets that somehow got washed into the fissures of the limestone."

Gebo's team includes his wife, Marian Dagasto, a molecular researcher at the Northwestern University Medical School; Chinese paleontologist Qi Tao; and Beard.

Beard, in fact, found teeth and jaw bones of Eosimias in the same Chinese quarry sites several years ago. Many fellow scientists rejected Beard's theory that Eosimias was a primate, and because he had found only teeth and jaw bones, he had little else to underscore his theory.

Gebo and Dagasto are experts in fossil primates--Gebo specializing in their foot structures--so Beard subsequently asked Gebo to head a team to look for more anatomical evidence of the early, miniature primates. The rigors of examining thousands of fossil bones under microscopes paid off with the identification of dozens of ankle bones no bigger than a grain of rice, including those that confirm Eosimias was a primate.

"These are very interesting findings," said Duke University paleontologist Richard Kay. "It is an animal that lived in China 45 million years ago, and the oldest identified monkeys and apes in Africa are 37 million years old.

"There has been a big debate about where and from what continent the monkeys and great apes originated. The traditional view has been Africa, but there always has been doubt about that. This does not prove they originated in Asia, but it raises the level of discussion."

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