First humans 'small brained'

Larger brain size was probably not the only driving force behind the exodus of early humans from Africa.

A third skull found at the camp of some of the first humans to leave the continent is much smaller than the others.

It would have housed a brain less than half the size of that of modern humans.

Many scientists believe that early humans arose in Africa, then went on to conquer the world.

How such primitive humans were able to adapt to the new environments they encountered on their travels is a central question of the Out-of-Africa hypothesis.

One theory is that they had larger brains and were more intelligent. Fossils found at an archaeological site in Dmanisi, Georgia, have until now supported this idea.

Unique site

Three skulls some 1.75 million years old have been uncovered there - the biggest collection of well-preserved early human fossils known anywhere in the world.

The first two skulls have room for relatively large brains - 800 cubic centimetres, or about three-quarters that of modern humans.

But the brain of the third find is only about 600 cubic centimetres, and came from a more petite individual, with a short nose, thin brow and huge teeth.

Professor David Lordkipanidze of the Georgian Academy of Sciences in Tbilisi is one of the scientists who discovered the latest fossil.

He says the three probably belong to the same species, Homo erectus, thought to be the first to leave Africa.

Like modern humans, there may be natural variation in shapes and sizes of these primitive humans, he says.

It is also possible that the smaller, more lightly built individual is female.

First footsteps

The small brain size of the new skull "suggests that enlargement of the brain was not the only reason to leave Africa," says Professor Lordkipanidze.

"My feeling is there should be a combination of reasons, not just one reason, that forced people out of Africa."

The new fossil shows how primitive early humans were in their small brain size and great physical variation, says Professor Chris Stringer, head of human origins at London's Natural History Museum.

Commenting on the find, revealed in the journal Science, he told BBC News Online: "It is still unclear whether this indicates that more than one species of primitive human is represented at Dmanisi."

Early humans
Homo habilis: The first species in the genus Homo, dating back 1.9 million years, and found only in Africa
Homo erectus: The first species in the genus Homo to leave Africa

The new skull showed clear resemblances to the smaller-sized group of fossils attributed to Homo habilis from East Africa, he added.

This might well provide a clue to the ancestry of the Dmanisi humans, said Professor Stringer.

"Their small brain sizes and primitive technology suggest that the first exodus of humans from Africa around two million years ago did not necessarily require special new adaptations or evolutionary changes but may have related more to extensions of African habitats into western Asia," he said.

"Perhaps these early humans initially underwent range expansions in African-like environments that were rather familiar to them, rather than making pioneering moves into the unknown."