A tiny fossil is the oldest known primate fossil, researchers announced. The mouse-sized fossil found in China helps explain what happened during the earliest days of evolution.

The fossil is a whopping 55 million years old, making it the oldest primate fossil ever found. The scientists have named it the Archicebus, which means, roughly, "ancient monkey".

The fossil has important applications to evolution. It sits at the base of two branches of the evolutionary tree - it's related to both.

The monkey is the ancestor of taristers, a kind of small animal that lives in trees. They're only found in Southeast Asia now. But because it's so old, it's also related to arithopods, the primates that include monkeys, apes, and humans.

"We are all very curious about the ancestors of primates, including those of human beings," said Dr. Xijun Ni from the Chinese Academy of Sciences, Beijing, China.

The archicebus lived on a warm "forest planet", scientists say. It ate insects and seems to have been a daytime creature, rather than nocturnal, although it has large eye sockets.

"At this time, 55 million years ago, the Earth was a jungle planet," Nature editor Henry Gee told BBC News. "The whole Earth was covered with tropical jungle - full of trees for little scampery things to climb up and down on. It was an ideal time for primates to be evolving."

The skeleton was almost complete, and the finding is significant. Most of the archicebus' key bones remain well preserved in the slate slab, including excellent imprints of its feet and rear limbs. This is particularly important because the foot emphasized the gap between the two lineages.

"The heel, and the foot in general, was one of the most shocking parts of the anatomy of this fossil when we first saw it; because, frankly, the foot of this fossil primate looks like a small monkey, specifically like a marmoset." Dr Chris Beard from the Carnegie Museum of Natural History in Pittsburgh told press.

"What is means is that the common ancestor of tarsiers and anthropoids had some features that looked more like anthropoids than tarsiers. And I guess we shouldn't be so surprised by this."

Scientists found the specimen in China ten years ago, but have waited deliberately to assess its importance. They used x-rays and microscopes to image the fossil.