On July 19th of 2001, fossil hunters in Djurab Desert in Chad, Africa (more than 1,500 miles from more familiar fossil deposits) discovered a cranium section, two lower jaw fragments and three teeth that could very well be the most important discovery in human evolution thus far. The pieces themselves were found by Ahounta Djimdoumalbaye, a life-sciences student and member of a thirty-eight member team of researchers. The team was led by Michel Brunet and Patrick Vignaud, from the University of Poitiers in France, and Mackaye Hassane Taisso, from the University of N'Djamena. The discovery came after the team had spent eight years working in areas that had been long-dismissed by other researchers.

Cranial Capacity: approximately 350 cc
Habitat: Forest

The fossils found clock in at around six or seven million years old. Lack of volcanic ash in the area prevents scientists from using normal, and more conclusive dating techniques, but the estimated age was hypothesized based on comparisons of other, more recognizable fossils from the site with similar fossils found in East Africa. If the dating is correct, then tchadensis (nicknamed Toumai or 'hope of life' in Goran) is by far the oldest pre-human fossil ever found, predating Orrorin tugenensis by almost a million years. It is also three million years older than the next-oldest hominid skull ever found.

The skull and fragments found (representing five individuals) present an interesting mix of primitive and modern traits. The braincase is ape-like, with a cranial capacity similar to that of modern chimpanzees, but the facial structure, brow ridges and teeth are closer to later hominids (for example the small canines). Scientist have estimated the tchadensis’s size at slightly larger than modern chimps. Because no post-cranial remains have been found, it is difficult to ascertain whether or not the creature was bipedal. However, the position of the foramen magnum does suggest an upright walker but not necessarily a habitually bipedal animal.

There are two reasons that this find is so important. Firstly, the location. Tchadensis was discovered in an area thought to be unimportant in the scheme of human evolution. Bipedalism has been widely held to have developed on the savannah as an adaptation for seeing long distances over high grass. Because of this, it was assumed that evolution of pre-humans was based solely in east and south Africa where those habitats existed. With tchadensis, a possible bipedal creature, evolving in a lush, forested habitat*, science needs to formulate other theories on the development of bipedalism, as well as recognize that pre-human evolution may have been a pan-African occurrence. This means more diversity and transitional species in our family tree.

Secondly, the age of Sahelanthropus tchadensis, in conjunction with its distinguishing features, indicate that the path of human evolution and the split from chimpanzees may not have been as linear as was first suspected. The discovery of a hominid older than Orrorin tugenensis pushes back the dates originally estimated for the divergence of chimps and pre-humans (which had originally been placed as early as five million years ago). Tchadensis may be a very close ancestor of the creature from which pre-humans and chimpanzees both stemmed.

But the real puzzle of tchadensis as it relates to human evolution is its mix of traits. Toumai is seven million years old with a facial structure closer to Homo habilis (around 2 mya) than to Australopithecus afarensis (around 3.2 mya), which had chimp-like facial features. Based on previous hominid discoveries, it would have been assumed that any fossil dated at before Lucy’s time would have had similar features. If tchadensis is in fact a direct ancestor of afarensis, then two evolutionary reversals must have occurred (extremely unlikely, according to many scientists).

Another two possibilities exist. One, that Sahelanthropus tchadensis may have been the precursor to an entirely different species, which then gave rise to the Homo line, making Australopithecines a dead-end line. Or two, that a less linear and more untidy model of the evolutionary progression might be explored in which there may exist a diverse range of creatures with chimp-like, human and unknown traits.

Sources:
http://www.nytimes.com/2002/07/11/international/africa/11FOSS.html
http://newsday.com/news/nationworld/wire/sns-ap-ancient-skull0710jul10.story
http://newsday.com/news/health/ny-hskin112780653jul11.story


* In typical evolutionary models of the development of bipedal locomotion, pre-humans that migrated to grasslands, or were forced there by declining forested areas, developed into bipedal Australopithecines, while those that remained in the forests evolved into today’s apes.

Note: More information will be added as it becomes available.

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