Researchers have identified nearly 500 tiny finger and toe bones belonging to primates - some half the size of a mouse - that lived about 45 million years ago in what is now China, offering evidence that early human ancestors came from Asia.
Representing nine different taxonomic families of primates and as many as 25 species, the specimens include numerous fossils attributed to Eosimias, the very first anthropoid known to date, and three fossils attributed to a new and much more advanced anthropoid.
The anthropoid lineage would later include monkeys, apes and humans.
"The fossils are extraordinarily small, but in terms of quantity this is the largest single assemblage of fossil primate finger and toe specimens ever recorded," said Dan Gebo, professor at Northern Illinois University in the US.
All of the finger and toe fossils imply tree-dwelling primates with grasping digits in both hands and feet.
Many of the smaller fossils are between 1 and 2 millimetres in length, and the animals would have ranged in full body size from 10 to 1,000 grammes.
"The new study provides further evidence that early anthropoids were minuscule creatures, the size of a mouse or smaller," Gebo said.
"It also adds to the evidence pointing towards Asia as the initial continent for primate evolution. While apes and fossil humans do come from Africa, their ancestors came from Asia," he said.
The newly described fossils were originally recovered from a commercial quarry near the village of Shanghuang in the southern Jiangsu Province of China, about 100 miles west of Shanghai.
Christopher Beard, a paleontologist at the University of Kansas in the US, said the limestone in the quarry is of Triassic age - from the very beginning of the Age of Dinosaurs some 220 million years ago.
Owing to a subsequent phase of erosion, the limestone developed large fissures containing fossil-rich sediments dating to the middle Eocene, after dinosaurs went extinct.
In the early 1990s, more than 10 tonnes of fossil-bearing matrix were collected from the fissures and shipped to the Institute of Vertebrate Paleontology and Paleoanthropology in China and the Carnegie Museum of Natural History in the US.
There, the matrix was washed and screened, yielding fossil bones and teeth from ancient mammals, many of which remain to be identified.
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