A fresh analysis of nearly 2 million years old fossils of Australopithecus sediba has added to debates about the species' place in the human ancestry.
Paleontologists have described the skull, pelvis, hands and feet of the species, discovered three years ago in South Africa. They published five papers in Science on Thursday, saying that Australopithecus sediba may be a leading candidate for an ancient human ancestor.
It was Lee Berger of the University of Witwatersrand in Johannesburg, South Africa, who discovered the fossils. According to him, the species is the most plausible ancestor of archaic and modern humans.
The papers disclosed a curious set of attributes. While some of them were found in Apes and earlier Australopithecus fossils, others were thought to be belonged to Homo erectus and its descendants, including modern humans. Homo erectus was the tall, thin-boned hominin that emerged around 2 million years ago in eastern Africa and colonized Europe and Asia.
Palaeoanthropologists were left unsure by this mix of features about how Australopithecus sediba relates to other ancient human relatives. Berger and his team proposed that the species may have evolved into Homo erectus. However, many other researchers are doubtful of that claim.
Fossils of Australopithecus sediba, about 4 feet tall, were discovered in 2008. It was first reported last year from the Malapa site in South Africa, a series of exposed ancient caves.
The brains were less than one-third the size of a modern human's, but features preserved on their inner skull, the team suggests, resemble precursors to specialized brain areas found in modern humans.
They also had hands capable of using tools, but still strong enough to grasp tree branches. They had a more curved pelvis that would have theoretically accommodated a bigger brain on its way through the birth canal, even though they still had small heads.
The mosaic of features indicates that Australopithecus sediba was at an evolutionary transition point between the tree-climbing australopithecines and upright-walking, often tool-using members of the genus Homo, which includes Homo sapiens.
The fossil remains were discovered in 2008 by Berger, whose then-9-year-old son had accompanied him to a dig site in South Africa. Berger's son had been chasing their dog, Tau, when he stumbled across a rock and immediately saw that it was a fossil.
He took it back to his father, who recognized a collarbone in the block of stone.
The mix of primitive and modern features led Berger and colleagues to suggest that A. sediba could be an ancestor to the genus Homo.
At the same time, the age of the fossils presents a problem. The researchers' isotopic and magnetic dating showed the fossils were 1.977 million years old, about 300,000 years younger than a Homo habilis fossil that should have been their junior.
They're obviously fabulous fossils and I'm cautiously receptive to their big picture overview of it as a predecessor of early Homo, says Dean Falk, a neuroanatomist at Florida State University in Tallahassee and the School for Advanced Research in Santa Fe, New Mexico.