Fossil Skeletons May Be Human Ancestor

2 Million Years Ago

Two Million Years Ago

A new-found ancient relative of humanity discovered in a cave in Africa is a strong candidate for the immediate ancestor to the human lineage, an international team of scientists said today.

The remarkably well-preserved skeletons — a juvenile male and an adult female that lived nearly 2 million years ago — were found near the surface in the remains of a deeply eroded limestone cave system.

Scientists don't know how they died, but it's possible they fell into the cave.

The hominids had longer arms than we do, and smaller brains. But 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 paleo-anthropologist 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 partial skeletons were found near Johannesburg at a site called Malapa, which means "homestead" in Sotho, in an area named the Cradle of Humankind and "one of the richest fossil sites in Africa"  according to 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 young male was about 12 years old and the adult female in her late 20s or early 30s. Since these specimens apparently died at or about the same time as each other — anytime 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 noted. The male was "right on the cusp of adulthood."

In many ways, the skeleton appears to be a mishmash of features, with some 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.)

For instance, "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."

Still, "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 told LiveScience — that is to say, our lineage.

Human-like faces

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 modern humans and earlier hominids. Critically, its pelvis and hip were more advanced than other australopithecines, approaching the hip structure of the extinct human species Homo erectus.

This indicates that A. 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 A. 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 suggested.

Time machine

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 our family tree evolved.

Due to A. sediba's age and physical traits, the researchers believe it is a convincing candidate for the immediate ancestor to the genus Homo. 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."

Not exactly The Missing Link

Based on its age and overall details of its body, researchers suggested A. sediba descended from Australopithecus africanus, which lived between 2 million and 3 million years ago and seemed to have eaten mostly soft foods like fleshy fruits, young leaves and perhaps some meat. This new species appears more similar to humans than do Australopithecus afarensis, most famed for Lucy, or Australopithecus garhi, which was discovered in 1996.

"We are perhaps at the beginning of a more coherent view of the diversity of the earliest South African hominids," said paleo-anthropologist Ian Tattersall at the American Museum of Natural History in New York, who did not take part in this research. These specimens provide "a better position to perceive the larger evolutionary patterns among hominids in a critical part of the timeframe."

As intriguing as the new fossil is, "it's not everything the rumor mill said it was going to be," said paleoanthropologist John Hawks at the University of Wisconsin at Madison. "It's not a missing link."

One of the biggest mysteries in human evolution is when the human genus Homo arose.

"What sets us apart most from the australopithecines is the size of our brain," Hawks said. With this new fossil, "while it has a somewhat Homo-like face, it doesn't have a Homo-like brain — it's smaller than the average for the earlier [Australopithecus] africanus."

LiveScience Article