Thursday, February 28, 2008

Oldest Hominid Fossil Discovered Placed At 7 Million Years Old

toumai fossilFrench fossil hunters have pinned down the age of Toumai, which they contend is the remains of the earliest human ever found, at between 6.8 and 7.2 million years old.

The fossil was discovered in the Chadian desert in 2001 and an intense debate ensued over whether the nearly complete cranium, pieces of jawbone and teeth belonged to one of our earliest ancestors.

Critics said that Toumai's cranium was too squashed to be that of a hominid -- it did not have the brain capacity that gives humans primacy -- and its small size indicated a creature of no more than 120 centimetres (four feet) in height, about the size of a walking chimp.

In short, they said, Toumai had no right to be baptised with French researcher Michel Brunet's hominid honorific of Sahelanthropus tchadensis -- he was simply a vulgar ape.

Toumai's supporters used 3D computer reconstructions to show that the structure of the cranium had clear differences from those of gorillas and chimps and indicates that Toumai was able to walk upright on two feet, something our primate cousins cannot do with ease.

If Toumai is truly an early human, that means that the evolutionary split between apes and humans occurred far earlier than previously thought.

And pinning down his age is key to redrawing the evolutionary map.

"The radiochronological data concerning Sahelanthropus tchadensis ... is an important cornerstone both for establishing the earliest stages of hominid evolution and for new calibrations of the molecular clock," Brunet wrote in a study which will appear in the March 4 edition of the Proceedings of the National Academy of Sciences.

"Thus, Sahelanthropus tchadensis testifies that the last divergence between chimps and humans is certainly not much more recent than 8 Ma (million years ago.)"

Toumai also probably lived "very close in time to this divergence contrary to the unlikely 'provocative explanation,' which recently suggested a 'possible hybridization in the human-chimp lineage before finally separating less than 6.3 (million years ago)," the authors concluded.

If Toumai -- the name means "hope of life" in the local Goran language -- is accepted as a human, the implications are profound.

The fossil was found some 2,500 kilometers (1,500 miles) west of the Great Rift Valley. If that is still seen as humankind's ancestral home, it implies the early hominids ranged far wider from East Africa, and far earlier, than previously thought.

The discovery also implies hominids evolved quickly from apes after they split from a common primate ancestry.

Hominids are considered the forerunners of anatomically modern humans, who appeared on the scene about 200,000 years ago.

Still unclear, though, is the exact line of genealogy from these small, rather ape-like creatures to the rise of the powerfully-brained Homo sapiens.

Story here.

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