martes, 22 de octubre de 2013

One happy family: Dmanisi cave controversy

Over the years, the site of Dmanisi in Georgia, has yielded an impressive sample of hominid cranial and postcranial remains, documenting the presence of Homo outside Africa around 1.8 million years ago. In 2005, the find of a complete cranium – Dmanisi D4500 – brought the sample size to five, providing direct evidence for wide morphological variation within and among early Homo paleodemes.
A spectacular skull
The find in 2005 included a mandible and was the most complete skull from this period. The delicate parts of the face were still intact – which was rare in other finds – making it “the world’s first completely preserved adult hominid skull” of such antiquity.
Combined with skulls found earlier at Dmanisi, it appears to show that these ancient individuals looked very different from each other but were still members of one species. This hypothesis has implications for the perplexing patchwork of Homo fossils found in Africa and is forcing a huge paradigm shift in the field of paleoanthropology. It now seems that species diversity two million years ago was much smaller than presumed thus far.
Single contemporaneous group
A new study used 3D computer imaging to examine the differences in “shape” among the five ancient Dmanisi skulls and found it no more pronounced than observed between five living humans or five chimpanzees.
The latest fossil is the only intact skull ever found of a human ancestor that lived in the early Pleistocene, when our predecessors first walked out of Africa. The skull adds to a haul of bones recovered from Dmanisi that belong to five individuals, most likely an elderly male, two other adult males, a young female and a juvenile of unknown sex.
The site was a busy watering hole that human ancestors shared with giant now extinct cheetahs, sabre-toothed cats and other beasts. The remains of the individuals were found in collapsed dens where carnivores had apparently dragged the carcasses to eat. They are thought to have died within a few hundred years of one another.
“Nobody has ever seen such a well-preserved skull from this period,” said Christoph Zollikofer, a professor at Zurich University’s Anthropological Institute, who worked on the remains. “This is the first complete skull of an adult early Homo. They simply did not exist before,” he said. Homo is the genus of great apes that emerged around 2.4m years ago and includes modern humans.
Odd dimensions
The remains at Dmanisi are thought to be early forms of Homo erectus, the first of our relatives to have body proportions like a modern human. The Dmanisi fossils show that Homo erectus migrated as far as Asia soon after arising in Africa.
The latest skull discovered in Dmanisi was the largest of the haul and had a long face with big, chunky teeth. But at just under 550 cubic centimetres, it also had the smallest braincase of all the individuals found at the site.
The odd dimensions of the fossil prompted the team to look at normal skull variation, both in modern humans and chimps, to see how they compared. They found that while the Dmanisi skulls looked different to one another, the variations were no greater than those seen among modern people and among chimps.
The scientists went on to compare the Dmanisi remains with those of supposedly different species of human ancestor that lived in Africa at the time. They concluded that the variation among them was no greater than that seen at Dmanisi. Rather than being separate species, the human ancestors found in Africa from the same period may simply be normal variants of H erectus.
“Everything that lived at the time of the Dmanisi was probably just Homo erectus,” said Prof Zollikofer. “We are not saying that palaeoanthropologists did things wrong in Africa, but they didn’t have the reference we have. Part of the community will like it, but for another part it will be shocking news.”

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