Tiny bones tell evolution story

Ankle bones: Small and perfectly formed

By BBC News Online science editor Dr David Whitehouse

Tiny ankle bones, about 42 million years old and the size of a grain of rice, are yielding important clues about the evolution of ourselves and our close relatives, the apes and monkeys.

The bones, which were dug up in China, are claimed to come from a creature that sits at the bottom of our particular branch of the evolutionary tree.

Called Eosimias, it fits in somewhere between prosimians, such as lemurs and tarsiers which leap and cling to trees, and anthropoids, such as monkeys, apes and humans which walk on four or two limbs.

Professor Daniel Gebo of Northern Illinois University and colleagues describe the fossil bones in the journal Nature. They recovered the fossils from a limestone quarry 160 km (100 miles) west of Shanghai and along the Yellow River, about 560 km (350 miles) southeast of Beijing.

50-50 creature

"They are half prosimians and half anthropoids. They really do make that connection. Much of the debate in the field has been to figure out which of those early prosimian fossil primates gave rise to anthropoids."

"We needed something that is 50-50 and that's what we think Eosimias is."

Fossils from this creature are rare.

We know very little about it. All scientists have had to go on are a few teeth, jawbones, one piece of a skull and guesswork.

The discovery of other parts of the skeleton - especially parts of the limbs - will help scientists understand what the creature looked like as well as make deductions about its lifestyle.

Some doubt

The bones suggest that the creature, like anthropoids in general, tended to use horizontal foot postures, and walk on horizontal surfaces, more than would be typical of prosimians.

"The most interesting aspect of these new foot bones is that they represent a mosaic," Professor Gebo said. "They possess primitive lower-primate features as well as several advanced or higher-primate characteristics."

But there are doubts. Some scientists question whether the bones come from Eosimias at all.

Professor Gebo and colleagues feel confident that they do.

They say the bones come from the same deposits that have yielded only two other creatures, and that the teeth and jaws of only one of these - Eosimias itself - would have been sufficiently small to match the tiny ankle fossils.