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See Inside Scientific American Volume 308, Issue 6

All in the Family

New evidence shows that a controversial fossil may be the oldest human ancestor
fossil, skull, tomai



RICHARD T. NOWITZ Science Source

A seven-million-year-old skull found in the Djurab Desert in Chad may indeed represent the earliest known member of the human family. Researchers unveiled the specimen back in 2002, assigned it to a new species, Sahelanthropus tchadensis (nickname: Toumaï), and said it was very close to the point at which the human lineage diverged from that of our closest living relative, the chimpanzee. Critics, however, countered that the skull was probably an ape's instead of that of a hominin (a creature on the line leading to us), given its primitive features. Now a new analysis of the skull supports the discoverers' claim that Toumaï is a hominin.

Thibaut Bienvenu of the Collège de France in Paris and his colleagues digitally reconstructed Toumaï's endocast—a cast of the interior of the braincase, which reveals the shape of the brain. They found that Toumaï had a cranial capacity of 378 cubic centimeters—consistent with earlier estimates and within the range of chimp cranial capacity. In comparison, modern humans have brains around three times larger than that. But although Toumaï's brain was apelike in size, it was apparently homininlike in other ways. In a presentation given on April 2 at the annual meeting of the Paleoanthropology Society, Bienvenu reported that the endocast shows strongly posteriorly projecting occipital lobes, a tilted brain stem and a laterally expanded prefrontal cortex, among other hominin brain characteristics.

Previously, Michel Brunet of the Collège de France, whose team discovered Toumaï, and his colleagues argued that Toumaï was a hominin on the basis of traits that included its relatively small canine teeth, which are associated with reduced aggression, and the forward position of its foramen magnum (the spinal cord opening in the base of the skull), which is associated with upright walking. Both these characteristics are considered hallmarks of humanity. The endocast traits bolster the original interpretation.

Bienvenu said Toumaï's endocast offers “a unique window on the first stage of human brain evolution” and shows evidence of brain reorganization toward the human condition well before brain size had begun to expand. He added that this early brain reorganization might be a consequence of the shift to upright walking.

Adapted from Observations at blogs.ScientificAmerican.com/observations

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