Palaeontologists on Wednesday said they had found the fossilised remains of a tiny tree-dwelling creature that lived around 55 million years ago, making it the oldest primate ever found.
The discovery will help chart the evolution of primates, a family that includes humans, and should strengthen a once-contested theory that primates originated in Asia, they said.
"This skeleton will tell us a lot of the story about the origins of primates and about our remote ancestors," Xijun Ni of the Chinese Academy of Sciences in Beijing, who led the study, said in a teleconference.
The tale of the find dates back 10 years, when Ni was doing fieldwork in Hubei province and was shown a fossil that had been discovered by a farmer at a quarry near Jingzhou, not far from the Yangtze River.
The fossil had been encased within a rock which had been split down the middle, yielding the skeleton and impressions of it on both sides.
But it took years of patient work, using 3D scanning technology, to extract a complete, detailed view of the specimen.
What has emerged is a strange primate that was just few centimetres tall and barely more than 30 grammes (one ounce) in weight, making it even tinier than the pygmy lemur, the Madagascar-dwelling primate that is the smallest primate around today.
Judging from its skeleton, it was clearly adapted to living in trees. With slender limbs, a long tail, and skinny fingers, "it would have been an excellent arboreal leaper, active during the daytime, and mainly fed on insects," said Ni.
It has been called Archicebus achilles, a compound name that means "first long-tailed monkey." The "achilles" is derived from the mythical Greek warrior to draw attention to its unusual ankle anatomy.
"Archicebus differs radically from any other primate, living or fossil, known to science," said co-researcher Chris Beard of the Carnegie Museum of Natural History in Pittsburgh, Pennsylvania.
"It looks like an odd hybrid, with the feet of a small monkey, the arms, legs and teeth of a very primitive primate, and a primitive skull bearing surprisingly small eyes."
The nearly-complete skeleton is the oldest fossil primate ever found, beating the previous record-holders -- species called Darwinius and Notharctus, respectively found in Messel, Germany and in Bridger Basin, Wyoming -- by seven million years.
Archicebus is important because it lived close to the time when primates started to diverge.
One branch led to today's tarsiers, a group of small nocturnal tree-dwellers, and the other led to anthropoids, a group that includes apes, monkeys and humans.
The quarry was once a lake, and has become a treasure trove of ancient bird and fish fossils from the Eocene epoch, when Earth was literally a hothouse -- a planet where there was no ice at the poles and palm trees grew in Alaska.
Tropical and sub-tropical forests flourished over much of the planet. "It was literally a great time to be a primate," said Beard.
He added that it was "not a coincidence" that the fossil had been found in China.
Asia was a place of riotous biodiversity in the Eocene and very likely produced the first primates, he said.
"Recent palaeontological advances have really indicated that the first and most pivotal steps in primate evolution, including the beginnings of anthropoid evolution, almost certainly took place in Asia, rather than Africa, which is the received wisdom that we all thought roughly two decades ago," said Beard.
Early anthropoids migrated to Africa during the Eocene, reaching it around 38 million years ago, eventually providing the source material for hominids, he theorised.
The big moment for us was when apes and humans in Africa diverged into separate lineages between five and 10 million years ago.
The study appears in the British journal Nature.