South African fossils: Between ape and human

Two fossil skeletons of early humans appear to mark a halfway stage between primitive "ape-men" and our direct ancestors.

A year of detailed study has revealed that the skeletons are a hodgepodge of anatomical features: some bones look almost human while others are chimpanzee-like.

The two fossils, an adult female and a juvenile male, were discovered in the Malapa cave system near Johannesburg, South Africa, in 2008.

Both about 1.2 metres tall, they are unusually complete and well-preserved and date from 1,977,000 years ago.

Excavated by Lee Berger of the University of the Witwatersrand in Johannesburg, and colleagues, they were given the name Australopithecus sediba.

Australopithecines were early hominids that lived between 4 and 2 million years ago: the best-known fossil example is a 3.2-million-year-old Australopithecus afarensis skeleton found in Ethiopia and nicknamed Lucy. Unlike chimpanzees and other apes, they walked on two legs, but their brains were still small. Not until Homo erectus evolved, around 1.8 million years ago, did larger brains appear.

Together with a large team of researchers, Berger has spent the last year intensively studying the two A. sediba skeletons. He says they are unusually advanced for an Australopithecus, and may show how the australopiths evolved into humans.
Brainy

One area of particular interest is the brain size. So far, the male skull has been excavated. It's also been named: South African schoolchildren chose the name Karabo, which means "answer" in the Setswana and Sotho languages of southern Africa.

Kristian Carlson of the University of the Witwatersrand and colleagues used synchrotron scans to build a detailed 3D image of the inside of the skull, allowing them to calculate the shape of Karabo's brain.

This was small even for an australopith, with a volume of just 420 cubic centimetres. A. afarensis, by contrast, averaged 459 cc, despite being an earlier species. That suggests there was no overall increase in brain size over the course of australopith evolution.

But Carlson says A. sediba's brain had been subtly reorganised. The orbitofrontal region, which sits just behind the eyes, is a different shape to those of other australopiths and apes, and may have been rewired into a more human-like design.

Carlson draws particular attention to an area called the inferior frontal gyrus, which has bulged out in A. sediba. In modern humans this area is important for language processing, hinting that A. sediba had advanced communication skills.

Carlson remains cautious, however, pointing out that spoken language relies on adaptations like vocal cords. Although neck bones were part of the find, they have not yet been studied in detail, so we don't know if A. sediba was built for speech.

We need to treat the brain studies with great caution, says Robert Barton of Durham University in the UK. "Interpretation of surface features of brains is fraught at the best of times," he says, "but on individual specimens that are merely impressions of the original brain and millions of years old to boot?"