Scientific Controversy: The Earliest Human Ancestor?

For many, the holy grail of paleoanthropology is to find the earliest human ancestor. For the last decade there have been three primary candidates: Ardipithecus kadabba, Orrorin tugensis, and Sahelanthropus tchadensisA. kadabba lived in the Middle Awash Valley of Ethiopia between 5.8 and 5.2 million years ago, O. tugensis was found in the Tugen Hills of central Kenya between 6.2 and 5.8 million years ago, and the oldest, S. tchadensis lived in Chad between 7 and 6 million years ago.

Cases have been made for and against these three species and anthropologists can not agree on our earliest ancestor. Brunet and his team, the fossil’s discoverers, suggested the S. tchadensis (nicknamed Toumaï) lived just after human’s split from chimpanzees, our nearest ancestor. They put forth Toumaï’s relatively small canine teeth and forward foramen magnum as evidence. Critics believed that Toumaï was cranially too primitive and too old, and therefore came before the split. However, more information has come to light. New analysis of the braincase suggests that Toumaï is more similar to us than originally thought.

At the Collège de France, Thibaut Bienvenu and his team have reconstructed the distorted skull and revealed the key to Toumaï’s braincase: the endocast. An endocast is a mold of the inside of the braincase and allows scientists to measure the approximate size and shape of Toumaï’s brain. By virtually reconstructing the endocast, Bienvenu’s team could correct for the distortion and produce an accurate representation of what the brain was actually like. The reconstruction showed that while Toumaï had a relatively small cranial capacity (only 378 cubic centimeters), morphologically it was similar to that of a hominin (human ancestors).

This new information could help to shine light on our earliest human ancestor and help us to piece together our evolutionary history. However, science is rarely straightforward and this is probably not the last that we have heard of this debate.

For more information, check out Kate Wong’s Scientific American blog!

Brain Shape Confirms Controversial Fossil as Oldest Human Ancestor

Smithsonian: Human Evolution

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