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Brain Shape Confirms Controversial Fossil as Oldest Human Ancestor

The views expressed are those of the author and are not necessarily those of Scientific American.

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Sahelanthropus tchadensis, also known as Toumaï, had a tiny brain, but one that had nonetheless undergone some reorganization toward the human condition. Image: Didier Descouens, via Wikimedia Commons

HONOLULU–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 and made quite a splash with their claim that the ancient fossil was our ancestor. They 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. But a new analysis of the skull—specifically, its braincase—supports the discoverers’ claim that Toumaï is a hominin.

Thibaut Bienvenu of the Collège de France and his colleagues reconstructed Toumaï’s endocast—a cast of the interior of the braincase, which reveals the shape of the brain. Because the fossil skull is distorted and filled with a highly mineralized matrix, they had to do their reconstruction virtually, which meant imaging it with 3D X-ray synchrotron microtomography and then feeding that data into a program that allowed them to remove the matrix and correct the distortion on screen.

The resulting virtual reconstruction of the endocast reveals that Toumaï had a cranial capacity of 378 cubic centimeters—consistent with earlier estimates. This puts it within the range of chimp cranial capacity. In comparison, modern humans have brains around three times larger than that. But though Toumaï’s brain was apelike in its small 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 brainstem, 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 including his relatively small canine teeth, which are associated with reduced aggression, and the forward position of his foramen magnum (the spinal cord opening in the base of the skull), which is associated with upright walking. Both of these characteristics are considered hallmarks of humanity. But skeptics argued that other features, such as the hulking brow ridge and aspects of the rear and base of the skull, signaled that the fossil represents an ape. The endocast traits bolster the original interpretation.

Bienvenu said that 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 was probably a consequence of the shift to upright walking.

04/10/13 Posted updated to identify Brunet.

Kate Wong About the Author: Kate Wong is an editor and writer at Scientific American covering paleontology, archaeology and life sciences. Follow on Twitter @katewong.

The views expressed are those of the author and are not necessarily those of Scientific American.

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  1. 1. Sauce23 10:51 am 04/4/2013

    Kate, Have you seen the fossil and data or is your report based upon the paper presented? Was there any discussion about how one gets from Toumai to Ardi?

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  2. 2. OdinsAcolyte 12:34 pm 04/4/2013

    It was a hominid ape. Our great-grandpa. Ancestor of

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  3. 3. nicholasjh1 1:22 pm 04/4/2013

    It makes sense that the reorganization would take place first, then as conditions were set up the brain slowly got bigger as that became advantageous. Without the conditions of the brain being different then a chimp brain there wasn’t much reason that it would simple get bigger. Smarter first then bigger makes sense to me.

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  4. 4. kwong 2:03 pm 04/4/2013

    Sauce23: I saw the original fossil years ago, although that doesn’t really matter because I’m not a paleoanthropologist. The new findings described in this story came from a presentation given at the Paleoanthropology Society meeting a couple days ago.

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  5. 5. greenhome123 2:38 pm 04/4/2013

    How can this fossil be 7 million years old when the earth is only 6,000 years old :-) But seriously – it is a shame that there is such a big percentage of the US population who doesn’t believe in evolution or that the earth is older than 6,000 years. Viva La Evolucion

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  6. 6. jtdwyer 3:33 am 04/5/2013

    Kate Wong,
    So this report has not been subjected to peer review for journal publication? Just curious.

    I’m not a paleoanthropologist, either, but I suspect higher intelligence is determined more by brain structure than size. Otherwise pygmy mammoths for example, whose brain case is a small fraction of other mammoths, would have been so dumb they couldn’t have fended for themselves…

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  7. 7. Torbjörn Larsson, OM 5:59 pm 04/6/2013

    Interesting, reorganization with more upright walking.

    @OdinsAcolyte: The same science that tells us it was hominid tells us there never were any single breeder pair on the human lineage. The smallest population was ~ 4 000 breeder pairs, and no less than 1 200.

    @jtdwyer: It is a continuous analysis, but sensor and motor neurons correlate with size (so dwarf species aren’t necessarily “dumb”) while reorganization has happened a lot (so, say, corvids are smart) and especially in hominins.

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  8. 8. RoedyGr 4:23 am 04/7/2013

    How many genetic mutations separate this creature from us? In that list must be the secret to our superior intelligence. It can’t very well be all that complicated. It is probably something as simple as expanding existing areas of the brain.

    This suggests figuring out how a mouse brain works will get us 99% of the way through to the theory needed to surpass our own intelligence.

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  9. 9. N a g n o s t i c 2:20 pm 04/10/2013

    Amazing how much can be gleaned from a squished up fossil. It gives me hope that information can be retrieved from black holes.

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  10. 10. BobWalton 7:14 am 04/12/2013

    Basically, you’re saying that the serendipitous relocation of the foramen magnum associated with an upright posture made room for ‘reorganization’ of the brain. I’m slightly uncomfortable with the connotations of the term ‘reorganization’, as it implies some sort of master plan. Perhaps we need to expand on the definition, as we could be giving creationism a toe-hold here. By the way, I’m not a creationist, more of an agnostic evolutionist.

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  11. 11. NucPal 12:56 pm 04/12/2013

    I’m also surprised that such an important finding is not published in a refereed journal! Do you know if there are other remains related to Tumai?

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  12. 12. Vivi Freitas 10:57 pm 01/1/2014

    It’s an amazing discovery, but I couldn’t find any other sources about this information. It’s a pity, because I would like to read more about the features of this skull.

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