Science: Are we really what we ate?

An American scientist has challenged the view that Australopithecines owed their heavy facial features to the chewiness of their food. By Steve Connor

HE WAS not a pretty sight. Blessed with a thick-set jaw, jutting eyebrows, eyes a touch too close together and a pin-sized brain, he was not the sort of handsome conversationalist you would like to have round for dinner. Yet Australopithecines were our ancestral bridge with the great apes, and what they looked like was very much dictated by what they ate - or was it?

The conventional view of the thick jawbones and general facial thuggishness of the Australopithecine hominid was that it was a supreme adaptation to some pretty rough chewing habits. Tough nuts, raw vegetables and other chewy fruits of a primitive feeding habit meant that Australopithecines had to have big teeth, thick jaws and strong facial muscles. A new interpretation published in the journal Science suggests, however, that the thick jawbone may have been an unavoidable byproduct of something that happened during the beast's embryonic development.

Australopithecines are the earliest known hominids in the evolutionary line from ape to humans. They are believed to have lived from about five million years ago but, although they walked upright and were clearly distinct from their ape ancestors, they did not share enough human traits to be classed as members of the Homo genus. They did not, for instance, use primitive flaked tools and it is most unlikely that they had either language or fire.

There are several types of Australopithecus, but they can be broadly divided into two groups. One was thick-set and "robust", the other had a lighter body and was more "gracile" in appearance - it is from this group that the human line evolved.

The most famous specimen of Australopithecus is "Lucy", an almost complete skeleton of a female, discovered in the 1970s by archaeologists excavating near Laetoli in Tanzania. Lucy lived about 3.6 million years ago and owes her nickname to the Beatles' song "Lucy in the Sky with Diamonds", which happened to be playing at the time she was discovered. Her formal name is A afarensis.

Several other species of Australopithecus have also been discovered, including A africanus - a slender, gracile type which is likely to have been one of our direct ancestors - and the thick-set A robustus. The conventional view is that all these species share a common ancestor, but Melanie McCollum, an physical anthropologist from Case Western Reserve University in Cleveland, Ohio, challenges this theory with a study of the facial characteristics of the robust Australopithecines.

She notes that the robusts have a very thick palate - the bone at the roof of the mouth - which was thought to be an adaptation to some heavy- duty plant chewing. However, McCollum argues, a thick palate might also have resulted as a byproduct of how the face formed during embryonic development.

McCollum says in her paper in Science that the 20 or so traits which are commonly thought to place the three species of robust Australopithecines into the same related group may actually be the incidental byproducts of just two facial features - a tall mandible bone and a forward- growing nasal bone called the vomer. In short, McCollum believes that the common characteristics were are believed to have linked all the robust Australopithecines may not in fact derive from a common ancestor - they are, in effect, less related than we once thought.

If McCollum's interpretation is right, it seems that the thick-set jaw and general neo-brutal looks of at least one of our distant ancestral cousins may be more to do with incidental side-effects of embryo development than a product of evolutionary adaptation to eating chewy food.

It just so happened that the same traits made it easy to eat what was available at the time.