The skull fragments were discovered amongst the scorching sands of the Djurab Desert in Chad, Africa, in 2002. At that time, the anthropologists who discovered ‘Toumai’ (as the fossil hominin was dubbed) were already claiming it as a human ancestor — possibly the oldest ever found at roughly 7 million years old. This date is close to the latest estimated date that our ancient ancestors diverged from our nearest primate relative, the chimpanzees.
But critics noted that the claimed anthropoid creature (officially named Sahelanthropus tchadensis), despite its small canine teeth, had many primitive features that were distinctly ape-like, such as a thick brow ridge and certain other features at the rear and base of the skull. The skull also had a cranial capacity of about 380 cubic centimeters, which is similar to that of chimpanzees (but which would not be inconsistent with an early hominin recently diverged from its primate cousin).
And so, the fossilized skull became an object of intense study with supporters on both sides of the primate debate.
But now a new digital analysis of the brain case lends credible support to the claim of “oldest human ancestor ever found.”
Thibaut Bienvenu and colleagues at the Collège de France sought to reconstruct Toumaï’s endocast (a cast of the interior of the braincase) so as to reveal the shape of its brain. However, the shape of the real skull had become distorted over time and the interior filled with a highly dense matrix of minerals.
So, in order to make the endocast, the scientists resorted to a technology called 3D X-ray synchrotron microtomography. This microtomography data was then inputed into a program that allowed the scientists to distinguish actual skull bone from the mineral matrix. At that point, they could then digitally remove the matrix, leaving only the “true” brain case behind. Any distortion in the case could be corrected by the computer program.
The virtual reconstruction of Toumai’s braincase revealed a cranium capacity of just 378 cubic centimeters (which is consistent with earlier estimates), putting it in line with a chimp’s cranial capacity. However, the researchers noted that the endocast was hominin-like in several other ways: it had strong, posteriorly projecting occipital lobes (at the back of the brain), a tilted brainstem, and a laterally (sideways) expanded prefrontal cortex, among other hominin brain characteristics.
These newly observed features help buttress other evidence noted by Toumai’s original discoverers (Michel Brunet et al, also of the College de France), such as the relatively small canine teeth (considered a sign of decreased aggression), and a more forward positioning of the foramen magnum (the opening at the base of the skull that accommodates the spinal cord). This latter feature is significant because it is a strong sign of up-right, bipedal locomotion. Amongst primates, these two features are considered uniquely human.
Still, until this recent reconstruction, enough contraposing evidence existed for critics to claim that Toumai was an ape. Now, little doubt remains. The digital endocast of Toumai’s skull offers “a unique window on the first stage of human brain evolution”, says Bienvenu.
Most tellingly, according to the anthropologists, it shows an evolutionary push towards more human physiological traits well before anthropoid brains began to expand, probably driven by our slow shift to bipedalism (upright walking). This find helps resolve another old debate: whether our brains grew first (enabling upright posture and other homo traits), or, whether we first stood upright, and then grew bigger brains. The latter scenario seems to be the case; the proof is in the braincase.
The evidence supporting the claim of ancient human ancestry was presented on April 2 at the annual meeting of the Paleoanthropology Society.
Some source material (and quote) for this post came from the Sci Am article: Brain Shape Confirms Controversial Fossil as Oldest Human Ancestor by Kate Wong.