Ancient fossil, Archicebus achilles, key link in chain leading to apes, man
Mouse-sized creature that roamed what’s now China offers clues to evolutionary step
A 55-million-year-old fossil of a mouse-sized primate with a human-like face has been identified as a crucial evolutionary link in the chain that led to apes and man.
Just 10 centimetres long, with a 12-centimetre tail, Archicebus achilles probably thrived for millions of years during a warm period of earth's history, feasting on insects and leaping around in canopies of trees that surrounded a tropical lake in what is now China, according to a report published online on Wednesday by the journal Nature.
"It was probably kind of a frenetic animal," study leader Chris Beard, a palaeontologist at the Carnegie Museum of Natural History in Pittsburgh, US, said. "You could even think anxious - an animal that moves around a lot, very active, searching for its next meal, very agile in the trees, climbing and leaping around in the canopy."
The remarkably complete fossil of Archicebus - derived from Latin and Greek for "ancient monkey" - helps make the case that primates first arose in Asia, soon after the extinction of the dinosaurs about 66 million years ago, even though the lineage leading to man later flourished and diversified in Africa.
The fossil is about eight million years older than any other extensive primate skeleton, bringing scientists closer to pinpointing a pivotal event in primate and human evolution: the divergence between the lineage leading to anthropoids, which include modern monkeys, apes and humans, and the one leading to tarsiers.
"If we go along a tree to the point where all the primates began to evolve, they all point to Asia," said palaeontologist Ni Xijun of the Chinese Academy of Sciences, who led the fieldwork in a fossil-rich area of Hubei province better known for its fish fossils. Ni said the creature had "a rounded face and brain case, very short snout, and front-facing eyes unlike with other small creatures".
Richard Kay, a Duke University anthropologist who has studied early primate evolution, said A. achilles lies tantalisingly close to the first primate of the evolutionary tree. "This specimen is not the earliest primate but very close to it, perhaps within a million years or so," said Kay. "This animal is the perfect model for what I think the basal primate was like."
A distinctive heel bone is a characteristic A. achilles shares with the branch of primates leading to apes, monkeys and people, inspiring the "achilles" moniker. Its teeth are also similar to those of ancient marmosets, according to the Nature report.
The analysis of the fossil took 10 years, said palaeontologist Paul Tafforeau, who scanned the fossil at a research facility in Grenoble, France. "When you see this new fossil, I would say you have no room for doubt" that it is an extremely early primate, Tafforeau said. "The discussion can only go on details. There should not be a big controversy."
Beard said the ancient animal was "very close to the divergence" between small tree-dwelling primates, such as tarsiers, and higher primates, such as monkeys, apes and humans.
A. achilles probably leant a bit towards the tarsier lineage that remained isolated to Asia, and probably lived alongside our ancestral anthropoids, Beard said.
How those ancient anthropoids made it to Africa - then a solitary continent drifting towards western Asia - and developed into apes and humans is uncertain. Palaeontologists suggest they swam via island chains that were eventually overrun by Africa as it docked with Asia in the present-day Middle East.
Additional reporting by the New York Times