A fossilised skeleton of a tiny creature with a long tail, sharp teeth and monkey-like feet has turned out to be the oldest-known primate - the group that includes gorillas, chimps and humans.
Scientists discovered the fossil in the solidified sediment of an ancient lake bed in China and have dated it to about 55 million years ago, about 10 million years after the demise of the dinosaurs and seven million years before the date of the previous oldest-known primate.
The researchers said it is one of the most exciting fossil discoveries in recent years because the animal - called Archicebus achilles - is the closest-known relative of the common ancestor to the entire "anthropoid" lineage encompassing monkeys, apes and man.
Archicebus grew no bigger than a modern-day pygmy shrew. Scientists believe it was active during the day.
"Archicebus differs radically from any other primate, living or fossil, known to science," said Dr Christopher Beard of the Carnegie Museum of Natural History in Pittsburgh, co-author of the scientific description, published in the journal Nature.
"It looks like a 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. It will force us to rewrite how the anthropoid lineage evolved," Beard said.