Sahelanthropus tchadensis essay


February 12, 2010

Sahelanthropus tchadensis

Sahelanthropus tchadensis
was described in 2002 by French paleontologist Michel Brunet and his team.  It was discovered in Chad from deposits that have been dated by biostratigraphy to between 6 and 7 million years in age.  Central Africa is an unusual place to find hominid fossils, and the conditions under which these paleontological teams work are arduous.  Their efforts are invaluable for documenting the geographic spread of hominids across the African continent.  Many fossils of other animals were recovered at the same site as Sahelanthropus, suggesting that the habitat, a dry desert today, was then a lush lakeshore with extensive forests around it.

The fossil specimen that was found by Brunet’s team was a badly crushed and distorted cranium.  This piece when reconstructed was nicknamed Toumai, which means ‘hope of life’ in the local Goran language.     The cranium housed a small brain, estimated to be around 360 cc in volume.  This is approximately the same size as a contemporary African ape.  It is not surprising that a creature that lived so close to the divergence time of the human and chimpanzee lineages (according to molecular data) should show primitive characteristics.  It is currently placed tentatively on the hominin lineage because of its relatively small canine tooth, which is worn down at the tip.  In contrast, ape canines are large, projecting, and remain pointed throughout life.

Bipedality (walking on two legs) is one of the most diagnostic characteristics of humans and their ancestors.  How can we tell if extinct hominids were bipedal?  The position of the foramen magnum (Latin for ‘great hole’) right under the brain is a trait that indicates walking on two legs.  The spinal cord exits the skull through this large opening, and so the position of the foramen magnum far forward on the base of the skull demonstrates the upright posture seen in all hominids.  In the Toumai skull, the foramen magnum seems to be positioned fairly far forward on the base of the skull.  Brunet has interpreted this as indicating bipedal locomotion in Sahelanthropus, though other paleoanthropologists have questioned this conclusion.  Unfortunately, without any fossil bones from the postcranial skeleton , its locomotion cannot be unequivocally determined.

The very large brow ridges of Toumai are unexpected.  This feature does not appear in human ancestors until Homo erectus, some 5 to 6 million years later.  The relative flatness of the facial features is also unexpected. Both apes and later hominins such as australopithecines are characterized by projecting faces.  With its mixture of primitive, unusual, and advanced traits, Toumai gives us a glimpse into the early history of the human lineage.  However, whether it belongs on our branch or that of the apes cannot be definitively established until more fossils are found.