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The surprising link between brain and pelvis

Updated Friday 19th December 2008

What do our ancestors tell us about the relationship between pelvis size and brain development?

One of our ancestors, Homo erectus, appeared in Africa about 1.9 million years ago. Generally fossil specimens of extinct species of humans are skulls and teeth. Other skeletal remains are very rare. In 1984 an almost complete skeleton of Homo erectus was found which, when reconstructed, showed a young male who was perhaps around 6ft tall and slim. He is known as the Nariokotome boy. A spine in a shop window Creative commons image Icon bixentro under CC-BY licence under Creative-Commons license A spine on display in a shop window

Reasoning from the shape of his pelvis suggested that the female would have had a small birth canal which would have limited the brain size of an infant. The brain size - and hence head size - of modern humans is only just narrower than the birth canal, so it is a reasonable assumption that pelvis size and brain development are linked: one can't increase without the other.

What is so interesting about this new find is that on the basis of the cup into which the head of the femur fits, this female was short but the birth canal is large and the whole pelvis is slightly wider than that of a modern female, so she was also broad-hipped. In fact, she could have given birth to at least some modern human babies.

This picture of a short, broad individual isn't the one we would expect from the appearance of the Nariokotome boy and reminds us that variation between individuals occurs in fossil species just as it does in humans. We know that the brain increased in size as Homo erectus evolved. This new find shows us that the pelvis changed too.

Find out more this fossil find in episode 9 of Breaking Science


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