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Teeny-weeny monkey may be our oldest relative

Thursday, March 16, 2000

By Bob Batz Jr., Post-Gazette Staff Writer

Bonehunters from the Carnegie Museum of Natural History are pointing to a tiny creature's even tinier anklebones to bolster their claim that it's the earliest known ancestor of human beings.


The creature is Eosimias, a mouse-sized monkey that lived 40 million to 45 million years ago in what is now China.

New evidence, described today in the British journal Nature, confirms that Eosimias is a very primitive member of the lineage that includes monkeys, apes and humans, say the paper's authors, who include Carnegie paleontologist K. Christopher Beard.

Their new findings -- specifically, fossil ankle bones, some as small as grains of rice -- suggest that the tiny creatures walked on all fours on horizontal surfaces, such as the tops of branches. That's a trait that links them to monkeys and other anthropoids, or higher primates, says Beard, the museum's associate curator of vertebrate paleontology.

Other aspects of their foot structure link the creatures back to prosimians, or lower primates, such as tarsiers and lemurs, which instead tend to leap and cling to vertical tree trunks.

That's why Beard describes Eosimias as a "missing link," since little previously was known about what happened on the primate family tree between 55 and 35 million years ago. "We're filling in this huge gap in the fossil record," he said.

    Online graphic:

The primate family tree

On the 'Net:

The full text of the Nature paper, as well as the Journal of Human Evolution paper, plus additional materials are available at the following Internet sites:

Carnegie Museum of Natural History

Northern Illinois University

The Nature site

The Journal of Human Evolution site


Beard is one of the scientists who originally discovered and named Eosimias, which means "dawn monkey."

He's been the leader of field expeditions in China that the Pittsburgh museum began in 1983 with its Beijing counterpart, the Institute of Vertebrate Paleontology & Paleoanthropology of the Chinese Academy of Sciences.

Beard and some of his Chinese colleagues found fossilized remains of the previously unknown creature in the summer of 1993 in the southeastern province of Jiangsu, and first described it in an April 1994 article in Nature. Basing their conclusions on part of a lower jaw and three teeth, they posited that the animal -- so small you could hold one in the palm of your hand -- was the precursor of the first monkeys.

They hypothesized that it had a face like a modern monkey but a body more like a lemur, a long-tailed primate now found on Madagascar. So different was it from any living or extinct creature that they gave it a new genus and family name, Eosimias sinensis.

And on the primate family tree, they placed it at the beginning of the limb that runs from the lower primates to the higher primates.

As is often the case with new scientific theories, this one drew disagreement: Some scientists disagreed with where they put Eosimias on the primate family tree, and others didn't think it was a primate at all.

But Beard presented more evidence in 1996. In a paper published in the American journal Science, he, Carnegie Curator of Vertebrate Paleontology Mary Dawson and others described additional fossils, including a nearly complete set of jaws, that they'd found at a different site along the Yellow River in the central Chinese province of Shanxhi.

Younger than the previous specimens by about 5 million years, they appeared to belong to a different species, which Beard named Eosimias centennicus, in honor of the museum's 1995 centennial. Still, everything pointed to Eosimias being our earliest known ancestor, and more scientists started to concur.

With today's paper, Beard and his co-authors hope to convince even more with their new evidence: multiple ankle bones found at both sites in China.

The paper's lead author, Dan Gebo, a professor of anthropology at Northern Illinois University in Dekalb, is an expert at identifying foot bones. As he notes, "The most interesting aspect of these new foot bones is that they represent a mosaic. They possess primitive lower primate features as well as several advanced or higher primate characteristics."

In other words, some microscopic details of the bones show that the creature's feet would be more horizontally oriented than in prosimians, while other more primitive features reflect a vertically oriented foot.

This "weird half-and-half morphology," he adds, is not shared by any other fossil primate from the Eocene age, the post-dinosaur period from about 55 to 34 million years ago when mammals started to come into their own.

Beard's group marks the evolutionary start of higher primates far earlier than other theorists do. "We think the monkey, ape and human lineage is very ancient" -- at least 20 million years older than lemur-like early monkeys that have been found in Egypt.

The search for more Eosimias samples continues in China and in the tons of material that has been shipped from there to the Oakland institution.

"Every day in the basement of the Carnegie Museum we find more fossils of these things," says Beard. Some of this material was analyzed by Gebo and his wife, Marian Dagosto, who works at Northwestern University Medical School. Also co-authoring the paper were Qi Tao and Wang Jingwen of the paleoanthropology institution in Beijing.

The researchers have no complete skeleton. They connect the ankle bones to Eosimias because they say it is one of just two primates found at the Shanxhi site, and the other primate is too big.

As far as the general significance of the tiny creature, Beard says:

"At the most fundamental level, why do people care about human evolution at all? They care because people want to know how, where and when did we evolve, how did people come into existence. It's almost a metaphysical question."

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