Nature skull cast of 'Toumai' - oldest human ancestor

Published on April 14th, 2013 | by Michael Ricciardi

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Confirmed: 7 Million Year Old Skull Belonged To Oldest Human Ancestor

skull cast of 'Toumai' - oldest human ancestor

Cast of the Sahelanthropus tchadensis holotype cranium TM 266-01-060-1, dubbed Toumaï, in facio-lateral view.
Image credit: Didier Descouens

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.

Six views of Toumai's skull

Six views of Toumai’s skull (image credit: Didier Descouens); Specimens from Laboratoire d’Anthropologie Moléculaire et Imagerie de Synthèse de Toulouse; (order: top row to bottom row / left to right: A : frontal view B : posterior view C : right lateral view D : left lateral view E : superior view F : inferior view (Size 182.5x105x97 mm)

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.




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About the Author

Michael Ricciardi is a well-published writer of science/nature/technology articles as well as essays, poetry and short fiction. Michael has interviewed dozen of scientists from many scientific fields, including Brain Greene, Paul Steinhardt, Arthur Shapiro, and Nobel Laureate Ilya Progogine (deceased). Michael was trained as a naturalist and taught ecology and natural science on Cape Cod, Mass. from 1986-1991. His first arts grant was for production of the environmental (video) documentary 'The Jones River - A Natural History', 1987-88 (Kingston, Mass.). Michael is an award winning, internationally screened video artist. Two of his more recent short videos; 'A Time of Water Bountiful' and 'My Name is HAM' (an "imagined memoir" about the first chimp in space), and several other short videos, can be viewed on his website (http://www.chaosmosis.net). He is also the author of the (Kindle) ebook: Artful Survival ~ Creative Options for Chaotic Times



  • Miles Gearhound Jamell

    This is an extinct ape and is not related to human ancestry. Scientists can only hypothesize based on analysis of ape and human skulls, which are very different. There is no such thing as a transitional fossil linking ape to man. Fossils of austrolopethicus, which is an extinct ape, do not speak. Saying that a fossil of an extinct ape has similarities to ancient man therefore it’s a transitional fossil proving that humans were once ape and slowly evolved, mutated and transitioned to human is not a scientific fact. Evolution is a theory that is not an absolute fact. Scientists have no way of knowing with 100% certainty that we were once ape. Just like archaeopteryx and Tiktaalik, which were incorrectly called transitional fossils, scientists always jump the gun and incorrectly call certain fossils transitional when they are not. Archaeopteryx and Tiktaalik are an extinct species of bird and fish respectively.

  • Raimo Kangasniemi

    The “critics” were the somewhat unhinged finders of the Orrorin tugenensis hominid who wanted to sideline all other early hominids to promote their own find while trying to hide Orrorin fossils from other scientists.

    Orrorin is a very interesting hominid, but instead of representing a new “true” line leading to humans, it’s almost certainly descended from Sahelanthropus (or a very closely related species) and ancestral to Ardipithecus kadabba(5.6-5.2 million years ago). Ardipithecus genus(5.6-3.4 million years ago) was very likely ancestral to Australopithecines(4.2-1.2 million years ago), which gave birth to our own genus Homo 2.3 million years ago.

    • http://www.chaosmosis.net Michael Ricciardi

      RK:

      Thanks for your comment and the anthropo-chronology; with so many early hominin (or proto-hominin) discoveries, it can be difficult to visualize a clear succession (I almost said ‘progression’) of our genus down through the ages.

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