Fossil Trove Sheds Light On a Stage of Evolution
Scientists in South Africa announced a hoard of fossil finds Thursday documenting a puzzling forerunner to modern mankind that lived nearly two million years ago, with human-like hands and ape-like feet.
Unearthed near Johannesburg, the extensive collection of fossils—including the most complete early specimen of a hand known—highlights a sparsely documented era of evolution when four or more ape-like hominid species roamed Africa, each one a natural experiment in anatomy and dawning intelligence. The new finds all belong to a prehuman species of that time calledAustralopithecus sediba, discovered in 2008.
From head to toe, the bones reveal an unexpected patchwork of primitive and advanced traits, the researchers reported in the journal Science. The tiny skulls, long arms and diminutive bodies were all chimp-like; yet the hands, ankles and pelvis were surprisingly modern.
A model of the creature's grapefruit-size brain, based on a three-dimensional X-ray scan of its fossilized cranium, hinted that its structure had been growing more advanced, heralding perhaps the first glimmer of sophisticated mental abilities.
"It's as if evolution is caught in one vital moment, a stop-action snapshot of evolution in action," said paleoanthropologist Richard Potts, director of the Smithsonian Institution's Human Origins Program, who wasn't involved in the discoveries.
Based on its analysis, the international research team of 80 scientists and technicians, led by Lee Berger of the University of the Witwatersrand in Johannesburg, said the species was the most probable ancestor of the family to which all modern humans belong, the genus Homo.
"They do represent a model that could lead to the genus Homo," Dr. Berger said.
Prof. Lee Berger with the Australopithecus sediba cranium.
Four independent experts in the field of human origins, however, strongly discounted that claim. The species, which stood about four feet tall, was more likely an evolutionary dead end, they said.
"Just because it shares a bit of anatomical morphology with Homo does not mean it is Homo or ancestral to Homo," said anthropologist Bernard Wood at George Washington University. "It looks increasingly that these bits of morphology are appearing more than once, independently, in the tree of life."
Even so, the bones are invaluable. "They're stunning," Dr. Wood acknowledged. The fossils challenge some assumptions about human origins and are certain to prompt years of scholarly debate. "This is an incredible trove for anything that early in time," said anthropologist Ian Tattersal, at the American Museum of Natural History, in New York.
So far, Dr. Berger and his colleagues have discovered 220 bones from skeletons of five individuals, including infant, juvenile and adult remains representing both sexes, in the Malapa Cave at the United Nations Cradle of Humankind World Heritage Site, about 28 miles from Johannesburg.
They dated the finds at 1.977 million years old, based on a laboratory estimate of the rate of decay of uranium traces in the cave sediments.
The cranium of the juvenile skeleton of Australopithecus sediba.
The creatures may have belonged to a family group that died by falling into the deep cave. Their remains came to light when erosion exposed the ancient sediments that had buried them. Some of the bones were found still connected to each other, as they would have been in life.
The creature's right hand featured a long thumb and short fingers able to grip tools, yet the curvature of the hand and the gangling arms appear more suitable for swinging on branches. There were no stone tools in evidence.
In the same way, the arch of its foot and its leg bones were well suited for human-like upright walking—with a slightly knock-kneed gait—while its heel and shin bone were more like those of a chimpanzee, better configured for climbing trees.
To the researchers' surprise, the primitive creature's pelvis was "the most human-like" ever discovered among a prehuman species, Dr. Berger said.
Australopithecus sediba skull reconstruction.
Most scientists have longed believed that the pelvis evolved to accommodate the birth of big-brained infants. But this early species had very small brains, leaving researchers wondering whether the need to walk more efficiently, rather than the demands of childbirth, shaped the biomechanics of the pelvis and hip joints.
"This hand is wonderful. The foot is fine. And the pelvis is spectacular," said anthropologist Philip Rightmire at Harvard University's Peabody Museum. "Evolution is more convoluted than we thought."
Write to Robert Lee Hotz at sciencejournal@wsj.com
Decade of Discoveries
2001: Three previously unknown prehuman species arev discovered in Africa, dating back almost six million years.
2003: The oldest known fossils of modern humans, Homo sapiens, are found in Africa, dating back 160,000 years.
2006: The remains of a 4.2-million-year-old prehuman species, Australopithecus anamensis, are found in an area of Ethiopia, where seven other human-like species also once lived.
2008: The first fossil of Australopithecus sediba, dating back almost two million years, is found by a 9-year-old boy in South Africa.
2009: A 4.4-million-year-old skeleton of an early human ancestor called Ardipithecus ramidus is unveiled after a decade of study.
2010: The earliest fossil evidence of modern humans in eastern Asia is found, dating back at least 100,000 years.
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