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New York. An ape-like creature with human features, whose fossil bones were discovered recently in a South African cave, is being greeted by paleoanthropologists as a likely watershed in the understanding of human evolution.

The discoverer of the fossils, Lee Berger of the University of Witwatersrand in Johannesburg, says the new species, known as Australopithecus sediba, is the most plausible known ancestor of archaic and modern humans. Several other paleoanthropologists, while disagreeing with that interpretation, say the fossils are of great importance anyway, because they elucidate the mix-and-match process by which human evolution was shaped.

Berger’s claim, if accepted, would radically redraw the present version of the human family tree, placing the new fossils in the center. The new species, in his view, should dislodge Homo habilis, the famous tool-making fossil found by Louis and Mary Leakey, as the most likely bridge between the australopithecenes and the human lineage. Australopithecenes were ape-like creatures that walked upright, like people, but had still not forsaken the trees.

Berger and his colleagues present this claim in five articles in the current edition of Science. As is common in paleoanthropology, the discoverer of a new fossil is seeking to place it as close as possible to the direct line of human descent, while others are resisting that interpretation.

In this particular case, there are many uncertainties regarding the fossil record from that time, including when the human lineage first emerged and how Homo habilis fits in the picture.

The principal significance of the new fossils is not that Australopithecus sediba is necessarily the direct ancestor of the human genus, other scientists said, but rather that the fossils emphasize the richness of evolutionary experimentation within the australopithecine group.

“This is really exciting new material,” said Ian Tattersall, a paleoanthropologist at the American Museum of Natural History in New York. “I think it holds the possibility of flinging wide open the question of what Homo is.”

Besides two skulls reported last year, researchers led by Berger have since retrieved an almost complete right hand, a foot and a pelvis. The bones are especially well preserved because their owners apparently fell into a deep cave and a few weeks later were swept into a sediment that quickly fossilized their bones. The rocks above the cave have gradually eroded away, bringing the fossils to the surface, where one was found by Berger’s 9-year-old son, in 2008, while chasing his dog.

That fall into the cave happened 1.977 million years ago, according to dating based on the rate of decay of uranium in the rock layer that holds the fossils.

In the articles in Science, Berger’s team describes novel combinations of ape-like and humanlike features in the hand, foot and pelvis of the new species. The hand, for instance, is ape-like because it has long, strong fingers suitable for climbing trees, yet is also humanlike in having a long thumb that in combination with the fingers could have held tools in a precision grip. A cast of the inside of the skull shows an ape-like brain, but one that had taken the first step toward being reorganized on human lines.

This mixture of ape-like and human-like features suggests that the new species was transitional between the australopithecines and humans, the researchers said. Given its age, Australopithecus sediba is old enough to be the ancestor of Homo erectus, the first species that researchers agree belonged to the human ancestry and which existed 1.9 million years ago.

But the fossils are significant even if sediba is not a direct human ancestor. They are evidence that a ferment of evolutionary experimentation was going on at the time, out of which the human lineage emerged. “If you take sediba as a metaphor for evolutionary change, it is a whole lot more powerful than the claim for direct ancestry,” Tattersall said.

A similar view was taken by Bernard Wood, a paleoanthropologist at George Washington University. “I think these are some of the most interesting papers that have been published in recent years,” Wood said.

Both Wood and Tattersall see Berger’s discovery as pointing to the great variety of australopithecine apes, from which it will be very difficult to select the particular species that gave rise to humans. Tattersall believes the leap to humans may have been brought about suddenly, perhaps by a few critical genetic changes, which is why the transition is so hard to trace in the fossil record.

Tattersall said that Berger had been “incredibly forthcoming” in giving other researchers access to his fossils instead of hoarding them for his private use, as other paleoanthropologists have been known to do.
 
The New York Times