An “amazingly complete” 55-million-year-old fossil found in China is the oldest primate skeleton ever discovered, researchers say.

The new species illustrates a critical juncture on the primate family tree: the split that saw monkeys, apes, and humans — a group collectively known as anthropoids — travel down a separate evolutionary path from the lineage that led to modern-day tarsiers, a bug-eyed, tree-dwelling primate that lives in Southeast Asia.

The research team named the new species Archicebus achilles and concluded that it is the earliest known member of the tarsier family — in other words, it is not a direct ancestor of humans.

But Archicebus displays a mishmash of tarsier-like and monkey-like features, meaning it must have existed close to the split between the two groups. The “achilles” in its name is a reference to its heel bone, which is similar to a modern marmoset’s.

Christopher Beard, one of the study’s authors, doesn’t like the term “missing link” because it’s so loaded. But he says this fossil certainly could qualify, in some respects.

“It’s a hybrid or mosaic — it’s a fossil that shows a combination of features that we’ve simply never seen before in any living or fossil primate.”

Beard added that the discovery also sheds light on human evolutionary history, showing that the anthropoid line that eventually produced Homo sapiens is at least 55 million years old. Apes split from humans 5 to 10 million years ago.

“If you just do a little bit of math, what you see is that the larger lineage, the anthropoid lineage, is between five and 10 times as ancient as the human lineage itself,” said Beard, curator of vertebrate paleontology at the Carnegie Museum of Natural History.

“So if you want to think about it in a philosophical sense, we share much more in common with our anthropoid brethren than the differences between us — on a scale of about nine to one.”

The oldest known primate fossil prior to this discovery is approximately 7 million years younger. And those slightly younger specimens belong to the lineage the produced lemurs, a more distant relative of humans.

Archicebus achilles lived during the Eocene, in particular during a “greenhouse” period when temperatures were at a global maximum and the earth was filled with lush tropical forests.

The slender-limbed, long-tailed Archicebus would have only weighed around 30 grams. Among living primates, the only one as small is the pygmy mouse lemur of Madagascar. Archicebus’ trunk and head span less than 10 centimetres, though its tail adds another 13 centimetres to its body length.

Because of its size and metabolic demands, the team concluded that it probably ate insects and spent much of its day leaping energetically from tree to tree in the forest.





The fossil was discovered in an ancient lake bed in Hubei province, in east-central China. The geography of the find adds, the researchers say, to a growing body of evidence that early primate evolution exploded in Asia.

Later, Beard says, primates made their way to Africa, where the human lineage split from the ape family.

“We still don’t know how these Asian anthropoids made it to Africa, but we know it could not have been easy,” says Beard. Africa was still an island continent, so the primates had to navigate open water.

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The Archicebus specimen, besides being beautiful, is “really exciting,” said Jonathan Bloch, associate curator of vertebrate paleontology at the Florida Museum of Natural History. “We’ve had bits and pieces of very closely related primates for a long time — teeth, fragmentary jaws, isolated postcranial bones, parts of the skeleton. But nothing like this.”

Bloch, an expert on early primate evolution who was not involved in the study, added: “These kinds of fossils really break things open and allow us to address all kinds of interesting questions.”

The study was led by Xijun Ni, a scientist at the Chinese Academy of Sciences and research associate at the American Museum of Natural History, and supported by scientists in the U.S. and France. It is published in this week’s issue of the journal Nature.