Light Years

Ancient fossils question human family tree

At a family reunion of the direct evolutionary predecessors of our species, there would be a lot of arguing over whether Australopithecus sediba gets in the door.

Australopithecus sediba is the name of an ancient species discovered in South Africa in 2008. Researchers now have substantial evidence, published in this week's edition of the journal Science, that Australopithecus sediba could be a direct ancestor of the Homo genus, of which humans are a part (we are Homo sapiens). If that's true, it means our family tree may have to be redrawn, with Australopithecus sediba at the stem of the Homo line.

But that's just one possibility, and a controversial one at that.

Researchers studied two partial skeletons, a young male named Karabo and an adult female who has not yet been named, which were found in the remains of a collapsed cave. "Australopithecus" means "southern ape," and "sediba" is "natural spring" or "fountain" in the Sotho language. The team announced the discovery of the previously unknown species in 2010.

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Scientists have several theories about what these skeletons might mean for human evolution.

The earliest undisputed Homo genus member is Homo erectus, which researchers estimate to be about 200,000 years younger than Australopithecus sediba, so Homo erectus could theoretically be the direct evolutionary descendant. Alternatively, Australopithecus sediba could be the direct ancestor of Homo habilis, considered to be a toolmaker because its hand bones were found next to stone tools, or of Homo rudolfensis, a contemporary of Homo habilis of disputed evolutionary origin. Australopithecus sediba could be related to both of them, and perhaps their current labels are inaccurate. Or, of course, it could be a dead end, although researchers say the skeletons' human-like features suggest otherwise.

Species as experiments in evolution

It makes sense that there seem to have been many variations in anatomical form evolving around 2 million years ago, said Lee Berger, paleoanthropologist at the University of the Witwatersrand in Johannesburg, South Africa, who led the discovery of the fossils.

"As you’re beginning to have the stresses and stressors of environmental change and the things going on in Africa around 2 million years (ago), you would think that many experiments would emerge, Sediba just being one of those," he said.

There can be only one species that gave rise to Homo erectus, which is our direct ancestor, however. To demonstrate stronger evidence, Berger said, archaeologists would have to find fossils that come before and after Australopithecus sediba in the evolutionary lineage.

Based on the variety of Australopithecus forms that have been found, Ian Tattersall, paleoanthropologist and curator at the American Museum of Natural History in New York, who wasn't involved in the study, said he believes Australopithecus sediba probably did not give rise to Homo. Instead, it represents the context in which our true ancestor, in whatever form it was, did arise: during a time when there were many different forms of upright creatures. About 2.6 million years ago there was a huge change in the African fauna, with more grasslands arising; these kinds of environmental factors probably shaped the evolution of different species.

"In some population, some genetic novelty became established which basically set the patent for the genus Homo in a short period of time," Tattersall said. "What we’re not going to see, I think, is the gradual modification over millions of years of an Australopith into Homo."

Exploring the skeletons

Based on the two skeletons studied so far, Australopithecus sediba represents a curious, unique combination of human-like and primitive features.

For instance, consider the brain: Australopithecus sediba's skull shows a cranial capacity of 420 cubic centimeters, whereas a chimpanzee's is about 380 cubic centimeters. Homo erectus is about 200,000 years younger than Australopithecus sediba, and its cranial capacity would be a whopping 900 cubic centimeters. If Australopithecus sediba is the direct evolutionary ancestor of Homo erectus, it suggests a more rapid expansion of brain size over the course of evolutionary history than previously believed. Also, the overall shape of the brain appears to resemble a human's more than a chimp's.

Then there's the matter of feet. Australopithecus sediba has a mostly human-like ankle joint, but the heel bone is mostly ape-like. That's surprising because the species of Lucy, the famous 3-million-year-old skeleton classified as Australopithecus afarensis, has a more advanced heel than Australopithecus sediba. If Berger's skeletons descend evolutionarily from Lucy's species, that would mean that heel anatomy would have evolved from advanced to primitive to advanced again - which is unlikely. Alternatively, Lucy's species may be more of a cousin to Australopithecus sediba, and to our genus, on the evolutionary tree, rather than an immediate relative.

"If that's the case, then there may very well be a ghost lineage," Berger said. In other words, there are probably more fossils out there to explain where these species came from.

At the same time, Australopithecus sediba's pelvis is the most human-like of any found in the Australopithecus genus, Berger said. While females of Lucy's species have wide, stable platforms with a birth outlet, the human pelvis is more bowl-like and curves around the body, and Australopithecus sediba's pelvis is closer to that.

Researchers have good evidence from the hands and feet that Australopithecus sediba was spending a decent amount of time climbing in trees. And the hands, which have grasping capabilities, are more advanced than those of Homo habilis, suggesting it, too, was an early tool-user.

"Sediba and the other early bipedal apes were creatures of relatively small stature that retained a lot of climbing features, particularly in their upper body skeleton, so they spent a lot of time in the trees even though, when they came to the ground, they walked on two legs," Tattersall said.

What led to the deaths of these possible proto-humans that Berger's team studied? They appear to have fallen, perhaps while looking for water, Berger said. But further investigation will reveal more precise details.

From the other evidence that hasn't yet been unearthed or examined, Berger promised his team will also likely discover the dietary habits of Australopithecus sediba and whether they were hairy. Researchers may already have found evidence of soft tissue. And they've got more skeletons to explore from the same area.

"What makes this really exciting is that this is opening this whole question of where the genus Homo came from to re-examination. What they have is a wonderful sample of individuals, of a kind that we don’t really expect to find in the human fossil record. Just one complete skeleton is rare, let alone a whole bunch of individuals," Tattersall said.