Finds claimed to illuminate evolutionary roots of Homo genus
Nearly 2 million years ago, an adult and a child walking through the South African landscape somehow fell through openings in a partly eroded, underground cave and died. Today, that fatal plunge has led to their identification as representatives of a new hominid species — and a contentious debate among paleoanthropologists over the pair’s evolutionary relationship to modern humans.
In the April 9 Science, anthropologist Lee Berger of the University of the Witwatersrand in Johannesburg and his colleagues assign newly discovered fossils from these ancient individuals to the species Australopithecus sediba. They propose that the species served as an evolutionary bridge from apelike members of Australopithecus to the Homo genus, which includes living people. In a local African tongue, sediba means fountain or wellspring, a reference to this species as a candidate ancestor of the Homo line.
“Australopithecus sediba could be a Rosetta Stone for anatomically defining the Homo genus,” Berger says.
Despite the importance of finding hominid fossils from the poorly understood period between 2 million and 1.7 million years ago, paleoanthropologists familiar with the finds doubt that they will illuminate Homo origins.
“There’s no compelling evidence that this newly proposed species was ancestral to Homo,” remarks Bernard Wood of George Washington University in Washington, D.C.
In an analysis led by geologist Paul Dirks of James Cook University in Townsville, Australia, the investigators place their finds at between 1.95 and 1.78 million years old. That estimate rests on direct dating of sediment just below the fossils, bones from animals such as saber-toothed cats found with the hominids, and measurements of a known reversal of Earth’s magnetic field in fossil-bearing soil.
Homo fossils from East Africa date to 2.3 million years ago, well before A. sediba’s time. Even so, Berger hypothesizes that the new species originated long enough before the time the two newly described individuals fell to their deaths to have sparked Homo evolution. He suspects that A. sediba diverged from an already known South African hominid, Australopithecus africanus, which lived from roughly 3 million to 2.4 million years ago.
Consistent with that scenario, the A. sediba fossils display a general Australopithecus body plan combined with certain features shared by early Homo species such as Homo habilis, Berger says. The two partial skeletons include bones from head to toe, although each is far from complete.
His team identifies the fossil child’s remains as those of a 12-year-old boy and the adult’s bones as those of an approximately 30-year-old female — at least if they grew at a rate comparable to people.
As in other Australopithecus species, A. sediba individuals possessed brains slightly larger than those of chimpanzees and small bodies, as well as long arms suited to tree climbing. Berger estimates that the hominid child would have grown to an adult height of about 4 feet, 7 inches. The ancient female stood nearly that tall, he adds, suggesting that A. sediba males were generally not much larger than females.
He also cites a number of Homo-like traits on the new finds, including small front teeth, distinctively shaped brain cases, narrow faces with signs of chin development and hips designed for a long, smooth stride.
“I don’t see anything here that tells us much about human evolution,” comments Owen Lovejoy of Kent State University in Ohio. Most of the Homo-like traits cited by Berger appear on the child’s skeleton and would have likely changed as that individual got older, Lovejoy says.
Selected Homo-like pelvic measurements, such as those documented for A. sediba, can vary greatly across individuals and don’t necessarily have evolutionary implications, Lovejoy adds. He argues that Berger’s finds most likely represent a late-surviving form of A. africanus.
Without more fossil material, it’s hard to know whether Berger’s finds represent a new species or A. africanus, notes Harvard University’s G. Philip Rightmire. “These fossils won’t clear up the question of where Homo came from,” Rightmire says (SN: 3/1/03, p. 131).
Berger’s discoveries could just as likely denote an early form of Homo that died out without influencing the evolution of other Homo species, says Susan Antón of New York University.
Berger rejects those criticisms. The A. sediba boy stood “on the cusp of adulthood,” he contends, and thus provides an accurate reflection of his species’ mature anatomy. Also, he adds, A. sediba displays a Homo-like skull shape that distinguishes it from A. africanus and remarkably long arms and a small brain case that fall outside the realm of Homo — putting it in an Australopithecus species of its own.
Berger’s team is now excavating at least two more A. sediba skeletons at the South African cave.
Berger, L.R. et al. 2010. Australopithecus sediba: A new species of Homo-like Australopith from South Africa. Science, 328(April 9): 195-204.