Our closest non-human ancestor lived in South Africa. That's the conclusion of a battery of studies carried out on two strange skeletons discovered near Johannesburg in 2008. They represent a likely stepping stone between the ape-like australopiths and the first members of our own genus.
The origins of the species remain surprisingly obscure. But the two skeletons, unusually, suggest our ancestor may have climbed back up into the trees ? after having evolved to walk on the ground.
It was around 2 million years ago that the first humans evolved from smaller-brained australopiths. The precise species that gave rise to humanity has never been found, but it seems that Australopithecus sediba, discovered in 2008
, was very closely related to it.
A new set of studies, published in Science, confirm that A. sediba carried a bizarre mosaic of ancient australopith and modern Homo features ? making it possible that the fossils are an "ideal ancestral form of the genus Homo", says Peter Schmid at the University of Zurich, Switzerland.
His study of A. sediba's ribcage, along with others of its spine, lower body and teeth, reveal A. sediba to be the most human-like of all australopithecines.
Human-like waist
One piece of evidence is that A. sediba had the same number of vertebrae as Homo species, unlike other species of Australopithecus. And A. sediba's lowermost ribs sweep inwards, narrowing the base of the ribcage into something approaching a human-like waist. Other australopiths are thought to have lacked a waist, says Schmid. A. sediba's waist would have positioned its abdominal muscles to make walking more efficient.
A. sediba's teeth are also remarkably human-like. Joel Irish at Liverpool John Moores University in the UK and Darryl de Ruiter at Texas A&M University in College Station found its canines to be small, like those of modern humans ? and unlike the large and more prominent canines of other australopiths.
On the other hand, the top of A. sediba's ribcage was distinctly unlike the human ribcage. Rather than being barrel-shaped, it tapered towards the shoulders, as in some large apes. This gave A. sediba ape-like shrugged shoulders that would have prevented it from freely swinging its arms when walking or running.
The tooth studies yielded perhaps the most surprising result of the new studies. The heartland of australopiths is in East Africa, where the most famous of them all, Lucy (A. afarensis), emerged 3.9 million years ago. A host of tooth features now suggest A. sediba did not evolve from Lucy. Instead, it is most closely related to A. africanus, which also lived in South Africa, around the same time as A. sediba did.
Ghost lineage
That suggests the roots of both lineages of australopiths ? from East and South Africa ? are even older. "It appears that there may be a 'ghost lineage' of unrecognised hominins that goes back deeper in time than afarensis," says Lee Berger at the University of Witwatersrand in Johannesburg, South Africa, who discovered A. sediba.
Perhaps australopiths found it relatively easy to move between East and South Africa until around 4 million years ago. Changes in the environment at that time may have made it difficult to move between the two, and then the South African australopiths evolved separately.
This isolation may have led to one last unusual feature of A. sediba: its arms and legs show it was far more comfortable swinging in the trees than most australopiths.
Jeremy DeSilva at Boston University in Massachusetts and his colleagues analysed A. sediba's lower skeleton, and found it walked like no other australopith.
"From a distance I'm not sure one would notice differences between sediba and human walking," says DeSilva. "But if you could get closer you might notice something. It wouldn't heel-strike like a modern human does. It would take short, quick steps. And there would also be a tremendous amount of rotation and inward twisting of the foot, knee and hip."
Back to the trees?
The skeletal features ? like a flexible midfoot ? that made for slightly awkward walking would have been useful for moving around in the trees.
That fits quite well with what we know of early members of the Homo genus, says Steven Churchill at Duke University in Durham, North Carolina. Some Homo habilis specimens appear to be more arboreal than Lucy's species, he says.
But this does create a mystery. If australopiths in general spent less time in the trees than their ancestors, why was A. sediba, the most human-like of all australopiths, so well adapted to tree living? "This is the question we are struggling with right now," says DeSilva.
"Is sediba evidence for a return to life in the trees? Or is this body plan evidence for a deeper [tree-dwelling] lineage in South Africa? No matter where these fossils end up on our family tree, they illustrate the wonderful complexity of human evolution."
Journal references: Science. Schmid study, DOI: 10.1126/science.1234598; Irish study, DOI: 10.1126/science.1233062; de Ruiter study, DOI: 10.1126/science.1232997; DeSilva study, DOI: 10.1126/science.1232999
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