Australopithecus sediba may be an ancestor of modern humans

Researchers say two skeletons found in a cave in South Africa may belong to a species that was the direct ancestor of Homo erectus, and hence modern humans.

It was a traumatic and lingering death. The adult female and young male probably fell through a fissure in a cave roof and remained alive for days or weeks with little or no food before finally meeting their end. The pair – possibly a mother and her son – were then washed by a rainstorm into an underground pool where they gradually solidified into rock.

Their unusual demise nearly 2m years ago, and the preservation of most of their fossilised skeletons, has given scientists a unique glimpse of what kind of creature they were. The researchers who have studied them in detail believe they may be direct ancestors of modern humans.

The ancient bones were recovered from sediments in a subterranean cave at Malapa, South Africa, 25 miles (40km) from Johannesburg.

The discovery of the partial skeletons was made public last year, but in a series of papers published in the US journal Science on Thursday, researchers report the first comprehensive analysis of the individuals' anatomy.

Through a combination of high resolution scans and precision measurements of the skull, pelvis, hand and foot, the authors argue that Australopithecus sediba, or the "southern ape", was an immediate ancestor of Homo erectus, the ancient form from which modern humans arose.

Lee Berger, a palaeoanthropologist at the University of Witwatersrand in Johannesburg who led the team, said the skeletons possessed an extraordinary mix of primitive, ape-like features alongside traits that define modern humans today.

"What is remarkable about Australopithecus sediba is that, as a field, it is a discovery we never thought would be made: a bona fide transitional species," Berger told the Guardian.

"It is a humbling experience. These are skeletons that you realise are going to be studied by humans for as long as humans study themselves. And that gives you some pause," he added.

At least 25 other animals died alongside them, including sabre-toothed cats, hyenas, woodland antelope and at least one primitive form of zebra. Around the cave was a sub-tropical alpine forest, with mixed woodlands and forests, Berger said.

A. sediba walked upright and stood around 1.3m tall. It had a chimp-sized body, long arms similar to those of orang-utans, and was adept at climbing.

But other features appear distinctly human, Berger said. "The pelvis is shaped like a human pelvis, but longer, almost like a Neanderthal's. The hand is incredibly human-like, with short fingers and a long thumb. And then there is the brain," he added.

Researchers used a powerful x-ray scanner at the European Synchrotron Facility in Grenoble, France, to create exquisitely detailed maps of the interior of the skull of one of the individuals. The bumps and other contours revealed the imprint of a small brain, only 420 cubic centimetres in volume, but one that was apparently reorganising from a primitive structure into a more modern form.

Kristian Carlson, a colleague of Berger's who worked on the brain scans, said some areas of the organ appeared more developed than expected.

"There are areas above and behind the eyes that are expanded and they are responsible for multitasking, reasoning and long-term planning. These are changes that mirror the differences that humans exhibit from chimpanzees," Carlson said. The discovery challenges the previously held theory that our ancient ancestors grew large brains before they reorganised to resemble the modern human brain.

Further measurements of the brain, skull and hand suggest that the creature may have been intelligent enough to wield tools and even communicate non-verbally, Berger said. "They could probably smile, and that is something unique to humans that chimps cannot do, they grimace. Australopithecus sediba has the beginning of our face," he said.

Other palaeontologists have yet to be convinced that the creature was an immediate ancestor of H. erectus - and hence our own species H. sapiens. But if Berger is correct, the fossils fill a gap between Lucy, the 3.2m-year-old hominin unearthed in Ethiopia, and H. erectus, which lived from 1.8m to 1.3m years ago and likely gave rise to modern humans in Africa.

"No matter where this species is eventually put in the family tree, whether you agree with our idea that it's the best candidate ancestor of Homo erectus, or whether it is a species that mimicked the developmental processes that led to our genus, or whether it turned out to be an evolutionary dead end, these are some of the finest transitional fossils that have ever been discovered for any mammal species, and I don't say that lightly," Berger told the Guardian.

Chris Stringer, head of human origins at the Natural History Museum in London, said: "For the last 30 years, attention has focused on East Africa as the place where the first humans evolved, with a possible transition from Australopithecus to Homo erectus, via the intermediate species Homo habilis occurring there about 2 million years ago.

"In that view, the South African australopithecines were side-branches in human evolution, leading only to extinction. These new and detailed descriptions of the skeletons of two individuals from the Malapa site return the spotlight to South Africa as the possible location for the postulated transition from Australopithecus to Homo.

"Australopithecus sediba resembles its presumed local ancestor, Australopithecus africanus, in its ape-sized brain, ape-like body shape, and the form of the shoulders and arms. Yet despite the fact that the hands had a powerful grip, they show more human proportions, suggesting greater dexterity. And the shape of the front of the brain cavity, the face, teeth, pelvis and legs also show more human
characteristics, confirming that sediba is the most human-like australopithecine yet discovered, providing valuable clues to the evolutionary changes that led to the genus Homo.

"However, it is possible that australopithecines in different parts of Africa were taking up tool-making, meat-eating and travelling longer distances overground, which could have driven the parallel evolution of human-like features," he said.

"Whatever you call these things, there seem to be a number of different species running around at the same time – a number of experiments in being hominin," Carol Ward, a palaeoanthroplogist at the University of Missouri, Columbia, told the journal Science.

Ian Sample, science correspondent   
Guardian
http://www.guardian.co.uk/science/2011/sep/08/australopithecus-sediba-ancestor-modern-humans

All Non-Africans Part Neanderthal, Genetics Confirm

If your heritage is non-African, you are part Neanderthal, according to a new study in the July issue of Molecular Biology and Evolution. Discovery News has been reporting on human/Neanderthal interbreeding for some time now, so this latest research confirms earlier findings.

Damian Labuda of the University of Montreal's Department of Pediatrics and the CHU Sainte-Justine Research Center conducted the study with his colleagues. They determined some of the human X chromosome originates from Neanderthals, but only in people of non-African heritage.

"This confirms recent findings suggesting that the two populations interbred," Labuda was quoted as saying in a press release. His team believes most, if not all, of the interbreeding took place in the Middle East, while modern humans were migrating out of Africa and spreading to other regions.

The ancestors of Neanderthals left Africa about 400,000 to 800,000 years ago. They evolved over the millennia mostly in what are now France, Spain, Germany and Russia. They went extinct, or were simply absorbed into the modern human population, about 30,000 years ago.

Neanderthals possessed the gene for language and had sophisticated music, art and tool craftsmanship skills, so they must have not been all that unattractive to modern humans at the time.

"In addition, because our methods were totally independent of Neanderthal material, we can also conclude that previous results were not influenced by contaminating artifacts," Labuda said.
This work goes back to nearly a decade ago, when Labuda and his colleagues identified a piece of DNA, called a haplotype, in the human X chromosome that seemed different. They questioned its origins.

Fast forward to 2010, when the Neanderthal genome was sequenced. The researchers could then compare the haplotype to the Neanderthal genome as well as to the DNA of existing humans. The scientists found that the sequence was present in people across all continents, except for sub-Saharan Africa, and including Australia.

"There is little doubt that this haplotype is present because of mating with our ancestors and Neanderthals," said Nick Patterson of the Broad Institute of MIT and Harvard University. Patterson did not participate in the latest research. He added, "This is a very nice result, and further analysis may help determine more details."

David Reich, a Harvard Medical School geneticist, added, "Dr. Labuda and his colleagues were the first to identify a genetic variation in non-Africans that was likely to have come from an archaic population. This was done entirely without the Neanderthal genome sequence, but in light of the Neanderthal sequence, it is now clear that they were absolutely right!"

The modern human/Neanderthal combo likely benefitted our species, enabling it to survive in harsh, cold regions that Neanderthals previously had adapted to.

"Variability is very important for long-term survival of a species," Labuda concluded. "Every addition to the genome can be enriching."

http://news.discovery.com/human/genetics-neanderthal-110718.html