This is a 3-D computer graphic image of the skeleton of Archicebus achilles, above a background photo of the fossil, from X-ray computed tomography data. Image: ESRF/P. Tafforeau

A tiny skeleton dating back 55 million years is the oldest primate fossil ever found, shedding new light on human evolution, researchers say.

Found in sedimentary rock from ancient lake bed in Hubei Province, central China, the fossil belongs to a previously unknown genus and species, dubbed Archicebus achilles by the international team who identified it.

The bones are around 7 million years earlier than any previously found primate remains, and could help to further understanding of a pivotal moment in primate and human evolution, when our family tree split into two branches with modern monkeys, apes, and humans (collectively called ‘anthropoids’) on one, and tarsiers (small, nocturnal tree-dwelling primates) on the other.

This graphic by Mark Klingler shows how Archicebus fits into the evolutionary family tree, just before it branched into anthropoids and tarsiers. Credit: M Klingler/Carnegie Museum of Natural History

Archicebus marks the first time that we have a reasonably complete picture of a primate close to the divergence between tarsiers and anthropoids,’ said project leader Dr Xijun Ni of the Institute of Vertebrate Palaeontology and Palaeoanthropology, at the Chinese Academy of Sciences in Beijing. ‘It represents a big step forward in our efforts to chart the course of the earliest phases of primate and human evolution.’

According to findings recently published in NatureArchicebus would have been the size of a modern human’s palm and  weighed around 30g (1oz), making it slightly smaller than any primate living today.

An artist’s impression of how Archicebus may have looked. Image: CAS/Xijun Ni.

‘Archicebus differs radically from any other primate, living or fossil, known to science,’ said Dr Christopher Beard at the Carnegie Museum of National History in Pittsburgh. ‘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. It will force us to rewrite how the anthropoid lineage evolved.’