Saturday, 10 September 2011

Australopithecus Sediba May Rewrite Human Evolution

It has long been thought that Australopithecus africanus is the last ancestor to Homo gautengensis, the first ever species of human. The world famous skeleton of Lucy, a young Australopithecine, underlined this connection due to the exceptional completeness of the skeleton from which logical deductions and calculations were made. Yet research into fossils from its evolutionary brother Australopithecus sediba could potentially rewrite human evolution.

The newly discovered fossil hand
of Australopithecus sediba
A set of recent discoveries from Australopithecus sediba's archetypal fossil site, which include the most complete fossil hominid hand ever, have shown that this species had far more advanced characteristics than previously thought. It could be the ancestor to more advanced hominids such as Homo ergaster; but earlier forms such as Homo gautengensis and Homo habilis were not ancestral forms and simply died out,s without leaving behind any descendants within the Homo genus.

Its pelvis, ankles and feet show that it was capable of proper upright strides, possibly even running, a characteristic that defines our genus. Other evidence was discovered in the hands. The extraordinary manipulative skills of the human hand is also a hallmark of our species. The recently discovered fossil hand has a longer thumb than its ancestors which suggests that it evolved to be able to grip tools and other such objects with opposable thumbs. These theories are  backed up by new discoveries of arm and wrist bones.

The 3D computer rendering of an
Australopithecus sediba skull
The final new piece of evidence which underpins the whole theory is a new and incredibly detailed scan of the holotype skull of Australopithecus sediba. X ray analysis of the fossil at the European Synchotron Facility in Grenoble, Switzerland, using a machine that produces radiation powerful enough to penetrate solid rock, has revealed startling new data about the brain of the species. A 3D computer model with 3D pixel detail down to 45 microns, just below the width of a human hair, sheds light on the patchy area of brain evolution from Australopithecine to human.

The brain of Australopithecus sediba overall was closer to that of a human than an ape or ancient hominid. The human brain is around 4 times the size of that of a chimpanzee. While Australopithecus sediba's brain only had a volume 40 cubic centimetres, larger than that of a chimp, it had a surprising mix of characteristics which make it a likely ancestor to later members of the human genus, which would give rise to our species. Scientists believe that its brain evolved as a result of its advanced pelvis and hands.

The skull of Australopithecus sediba is actually closer
to that of the later hominid Homo ergaster in shape, size
and facial sloping
Walking upright requires an extraordinary amount of co-ordination and balance and the amount of information provided by hands with opposable thumbs is phenomenal. Therefore a more advanced brain would have been required. All three of these characteristics are present in later hominids such as Homo ergaster. 'Indeed, one of our major discoveries is that the shape and form of sediba's brain is not consistent with a model of gradual brain enlargement, which has been hypothesised previously for the transition from Australopithecus to Homo,' stated Dr Kristian Carlson from the University of the Witwatersrand, who is the main author of the paper.

The next few years may be very exciting for the fields of palaeoanthropology and human evolution as Australopithecus sediba's place within our family tree is re-examined. Its abnormally advanced cranium, pelvis and hands do indeed provide strong evidence for its place as the ancestral form to our species. More tests will be conducted alongside comparisons with other hominid skulls both early and late in hominid evolution. In just a few years, another piece of the  human evolutionary puzzle may be solved.