An ancient relative of humans discovered in a cave in Africa is a scandidate for the immediate ancestor to the human lineage, an international team of scientists said Thursday, April 8, 2010. The skeletons - a adolescent male and an adult female that lived nearly 2 million years ago - are well-preserved and were found near the surface in the remains of a deeply eroded limestone cave system.
It's possible they died after falling into the cave.
The hominids had longer arms than we do, and smaller brains. Their faces were human-like, and scientists say the discovery represents an important look into our pre-human past. Researchers stopped short of calling the new species, dubbed Australopithecus sediba, a missing link.
Australopithecus means 'southern ape.' Sediba means "natural spring, fountain or wellspring in Sotho, one of the 11 official languages of South Africa," said researcher Lee Berger, a paleoanthropologist at the University of Witwatersrand in South Africa. This was "deemed an appropriate name for a species that might be the point from which the genus Homo arises," Berger said.
The incomplete skeletons were found near Johannesburg at a site called Malapa, which means "homestead" in Sotho, in an area named the Cradle of Humankind.
"This is one of the richest fossil sites in Africa," said researcher Daniel Farber, an earth scientist at the University of California at Santa Cruz. Nearly a third of the entire evidence for human origins in Africa come from just a few sites in this region.
The sex of the fossils was determined from the shape of the jaws and hips, while analysis of the teeth suggest the juvenile male was about 12 years old and the adult female in her late 20s or early 30s. Since these specimens died at or about the same time as each other - from hours to weeks apart - the researchers suggest they would almost certainly have known each other in life and may very well have been related.
Both stood upright a little more than 4 feet high (1.2 meters). "The female probably weighed about 33 kilograms (72 lbs.) and the child about 27 kilograms (59 lbs.) at the time of his death," Berger said. The male was "right on the cusp of adulthood."
In many ways, the skeleton appears to be a mix of features, but also resembling members of the human family tree and others more like those of earlier ape-like hominids. (A hominid includes humans, chimpanzees, gorillas and their extinct ancestors, while hominins include those species after the human lineage split from that of chimpanzees.)
"The brain size of the juvenile was between 420 and 450 cubic centimeters, which is small when compared to the human brain of about 1,200 to 1,600 cubic centimeters," Berger said. "It would look almost like a pinhead."
But "the shape of the brain seems to be more advanced than that of australopithecines," Berger noted. Indeed, a number of skull features, such as certain wide, broad lines in the bone, "are ones you tend to attribute to early members of genus Homo," Berger said.
A number of facial and dental features resemble those of early human species, such as small teeth and a projecting nose. At the same time, "it had very long forearms - in fact, as long as an orangutans," Berger said, similar to other members of the genus Australopithecus. Its fingers were curved, ideal for climbing trees, yet relatively short, like in humans.
Its legs were relatively long and the ankles seem to be intermediate between modetn humans and earlier hominids. Still, its pelvis and hip were more advanced than other australopithecines, approaching the hip structure of the extinct human species Homo erectus. This indicates that Australopithecus sediba was able to walk upright in a striding manner.
Despite the differences in sex, the male and female skeletons physically resembled each other, something they seem to have had in common with the human family tree but not with more distant relatives, such as chimpanzees. This could mean that Australopithecus sediba leaned toward social behavior "where you don't necessarily have a dominant alpha male and you are lowering violence between males who are probably working more cooperatively in a group," Berger claimed.
A combination of dating techniques determined the rocks encasing the fossils are 1.95 million to 1.78 million years old.
"This fits in a critical moment in time," Berger explained. The human lineage is thought to have originated between 1.8 million to 2 million years ago, but the hominid fossils unearthed so far from that period have proven remarkably poor, giving scientists a great deal of room for speculation as to how the humanity tree evolved.
The researchers believe it is a convincing candidate for the immediate ancestor to the genus Homo due to Australopithecus sediba's age and physical traits. Based on its physique, they suggest its appearance signified the dawn of more energy-efficient forms of walking and running.
Many scientists believe the human genus Homo evolved from Australopithecus a little more than 2 million years ago, but that possibility has been widely debated, with other experts proposing an evolution from the genus Kenyanthropus. This new species might help clear up that controversy.
"These fossils give us an extraordinarily detailed look into a new chapter of human evolution, and provide a window into a critical period when hominids made the committed change from dependency on life in the trees to life on the ground," Berger said. "Australopithecus sediba appears to present a mosaic of features demonstrating an animal comfortable in both worlds."
