Fossil of Unusual Human Ancestor Discovered

By GAUTAM NAIK

Researchers have identified a nearly complete skeleton of an unusual, tree-dwelling ancestor of humans that lived 55 million years ago, a discovery that illuminates one of the earliest stages of our evolution.
Unearthed in China, the mouse-sized creature has a mix of anatomical features that make it quite unlike any other primate—living or extinct—known to science. The discovery was published Wednesday in the journal Nature.

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Chinese Academy of Sciences

An artist's depiction of Archicebus achilles.

By about 55 million years ago, the earliest primate ancestors of humans had split into two branches. One branch gave rise to lemurs, lorises and bush babies. The other led to tarsiers, whose enormous-eyed, tree-dwelling descendants still live in Southeast Asia. The second branch also gave rise to anthropoids, including monkeys, apes and humans.
The animal described in the study is a primitive tarsier relative. It is of interest to scientists because it lived at a crucial period, fairly soon after the two branches split.
The discovery gives new insight into very early ancestors of humans: How big they were, what they looked like, what their anatomy was like, what they ate and how they moved.
For example, many scientists believed that the specialized anatomy of a monkey's foot developed much later. But some of that anatomy can be seen in the unearthed skeleton—far, far earlier in evolution.
"It has a monkey-like foot but a body more or less like a tarsier's," said Daniel Gebo, co-author of the paper and anthropologist at Northern Illinois University. "It's the first time we've seen this combination."
The finding also is striking because of the completeness of the skeleton. Until now, scientists only had seen fragmentary remains of these ancient primates, such as teeth, jaws or skulls.
"It's rare because most of the important parts of this skeleton are there," said Jerry Hooker, a paleontologist at London's Natural History Museum, who wasn't involved in the study. "It's only lacking hands and wrists, which is a pity."
In 2003, a farmer in China's Hubei province presented the rock-encased fossil to Xijun Ni, a paleontologist at the Chinese Academy of Sciences. Although other local paleontologists had previously seen the fossil, they failed to recognize its significance.
"They thought it was a rodent or frog," said Dr. Ni, who specializes in mammals. "I realized it was a primate—I was astonished."
The study took a long time to complete partly because scientists initially struggled to get useful images of the fossil. The specimen eventually was sent to the European Synchrotron Radiation Facility in Grenoble, France, where powerful X-rays scanned the specimen and revealed a detailed picture of the animal's skeleton.
Computers then added in the missing parts of the fossil, providing a high-resolution, three-dimensional reconstruction of the creature.
The ancient primate lived in an extremely warm period of earth's history. At that time, known as the early Eocene, there were no ice caps and much of the planet was a rain forest.
Researchers who studied the skeleton say the animal weighed less than an ounce and was small enough to fit in the palm of a human hand. Thanks to slender limbs and a long tail, it apparently was an accomplished leaper.
It had front-facing eyes, a feature found in primates that is important for stereo eyesight, noted Dr. Ni. It likely was active in daytime—more like monkeys than tarsiers—and probably ate insects. (Smaller animals need a lot of energy and hence require high-calorie diets.)
"It shows that the beginning of anthropoid evolution started in a very different way from what we see in living anthropoids" including humans, said Dr. Gebo. Our ancient ancestors' "diets and locomotive patterns were different."
The animal's foot anatomy is peculiar. Like other primitive, arboreal primates, it had grasping big toes, long toes and nailed digits. But it also had the heel bones and long metatarsals that are more typical of monkeys.
"The creature is really bizarre," said Dr. Ni. "Everything is mixed together."
Dr. Ni and his colleagues have named the fossil Archicebus achilles. Archicebus roughly translates to "the first long-tailed monkey." The species name, achilles, alludes to the mythological Greek warrior who inspired the name given to the Achilles tendon.

Write to Gautam Naik at gautam.naik@wsj.com