New Ancestor May Be Ape-Human Link

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Australopithecus sediba walked on two legs and had hips similar to ours, but still retained more primitive features, such as longer arms and a smaller brain.
Brett Eloff courtesy of Lee Berger and the University of the Witwatersrand

THE GIST:

- A new member of the human family tree may be a transitional species between ape-like men and our direct human ancestors.

- Australopithecus sediba could have given rise to either Homo erectus or Homo habilis.

- Climate change in South Africa may have helped to drive our ancestors out of the trees and onto land.

A new member of the human family tree recently found in South Africa is a good candidate for being the transitional species between ape-like beings and our direct human ancestors, according to a paper in the latest issue of Science.

The 1.95- to 1.78-million-year-old hominid, named Australopithecus sediba, walked on two legs and had hips similar to ours, but still retained a few more primitive features, such as longer arms and a smaller brain. This human relative may have given rise to Homo erectus, "Upright Man," or to Homo habilis aka "Handy Man," who was the earliest known human.

"Further studies will tell, but A. sediba should act as a Rosetta Stone to unlocking just what makes up the genus Homo," project leader Lee Berger told Discovery News.

"Being that we have very complete specimens of different sexes, derived from a single moment in time, we have an opportunity to explore questions that relate both to ancestral forms, as well as to potentially descendant species," added Berger, a senior research officer and director of the School of Geosciences at the University of Witwatersrand.

He and his team made the discovery after investigating caves at several locations in South Africa. The nine-year-old son of one of Berger's post-doctoral students, Job Kibii, found the partial skeleton of a 60-pound, nine- to 13-year-old male hominid at what is known as the Malapa site.

Further exploration led to the discovery of the remains for an adult female in her late twenties or early thirties weighing around 73 pounds. Both she and the excavated young male were identified as being members of the new Australopithecus species, with sediba meaning "wellspring."

Since the fossils for at least 25 animals, including saber-toothed cats, a wildcat, a brown hyena, a wild dog, antelopes, and a horse, were also found in the cave, the researchers believe the deep site acted like a "death trap" for individuals seeking water. The now fossilized woman and boy appear to have "taken a significant fall" before plunging, or being later washed by rainwater, around 164 feet. Their bodies were then buried by a roof cave-in.

Analysis of their bones shows they had longer, more orangutan-like arms, short powerful hands, a hip bone similar to that of modern humans, and long legs capable of striding, and possibly even running, as people do today. They also had human-like faces, noses and back teeth. Their brains, however, were probably about one-third the size of a Homo sapiens' brain. 

The new hominid species reveals that the evolutionary transition from our small-bodied, tree-dwelling ancestors to larger-bodied, full-striding walkers happened in gradual steps.

Climate Change a Key?

A climatic event in South Africa around 2 million years ago -- possibly a period of cooling -- may have helped to fuel at least part of the change, driving this new Australopithecus member to depend more on being able to walk distances as it sought out food, water and shelter.

"However, it does appear that sediba kept a 'safety chute' in the form of long arms and keeping the ability to climb, but in a different way to the (other) australopithecines," Berger said.

Chris Stringer, research leader in human origins at the Natural History Museum in London, told Discovery News, "The two partial skeletons from Malapa in South Africa are extremely important additions to the fossil record of human evolution." Stringer believes this is "the most human-like australopithecene yet discovered."

Francis Thackeray, director of the University of Witwatersrand's Institute for Human Evolution, said the new species "confirms the view that there is no clear boundary between Australopithecus and Homo."

"The time has come to reassess the variability in all hominin fossils from Africa, without necessarily 'pigeon-holing' fossils into one or other species," Thackeray told Discovery News.

In the not-too-distant future, we may also learn much more about how our earliest ancestors lived. Berger said that because the Australopithecus sediba fossils are so well preserved, "it won't be very long before we will tell you a great deal about their lifestyle and diet."

He added, "Watch this space!"

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