Coloured scans of the Dikika skull

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Australopithecus afarensis: Human ancestors had slow-growing brains just like us

Australopithecus afarensis was made famous by a skeleton known as Lucy, found 1974 in Ethiopia. Lucy proved that her species - one of our early human relatives - walked on two legs.

Ancient fossils are revealing even more about this species' evolution.

A new study, led by paleoanthropologists Philipp Gunz and Simon Neubauer from the Max Planck Institute for Evolutionary Anthropology in Germany, reveals that Lucy's species Australopithecus afarensis had an ape-like brain.

But surprisingly, it still took a prolonged, more human-like amount of time to grow, providing the crucial basis for longer childhood learning.

Prof Fred Spoor, Calleva Research Leader in Human Evolutions at the Museum and a senior author on the study says, 'Our new results show that the size of the brain is not necessarily linked with how long it grew after birth.'

A cast of Lucy, the partial skeleton of an Australopithecus afarensis female found at Hadar, in the Afar region of Ethiopia. The fossil is slightly less than 3.18 million years old.

 

A. afarensis lived in East Africa more than three million years ago. Understanding these early humans is important because it helps us understand how we - Homo sapiens - evolved as well. Lucy's fellow hominins are likely to be our ancestors.

Prof Zeresenay Alemseged from the University of Chicago was also an author on the study. He says, 'Lucy and her kind provide important evidence about early hominin behaviour. They walked upright, had brains that were around 20% larger than those of chimpanzees, and may have used sharp stone tools.'

Inside the mind of the Dikika child

To study the shape and growth of the Australopithecus afarensis brain the researchers CT scanned the fossil skull of the Dikika child, a 3.32 million-year-old skeleton of a young female found in 2002 in the Dikika region of Ethiopia by Dr Alemseged.

They used synchrotron microtomography at the European Synchrotron Radiation Facility (ESRF) in France, which allowed them to take a closer look at the skull than previous studies.

In addition, seven other fossil skulls from the nearby Hadar site in Ethiopia were investigated.

With the help of technology and several years of painstaking fossil reconstruction, the brain size and shape were worked out from the imprints left on the inside of the fossil skulls. Moreover, by counting microscopic growth lines inside the teeth the researchers could obtain a precise age at death of the Dikika child.

Coloured scans comparing the Dikika skull to an ape's

Brains do not fossilize, but as the brain grows, the tissues surrounding its outer layer leave an imprint in the bony braincase. The Dikika child’s endocranial imprint reveals an ape-like brain organization, and no features derived towards humans. License: CC BY-NC-ND 4.0; Image by Philipp Gunz.

 

These data shed new light on two questions that have been controversial. The first is whether there is evidence for human-like brain evolution in A. afarensis, and the second is whether the pattern of brain growth in A. afarensis more like that of chimpanzees or that of humans.

Australopithecus afarensis had an ape-like brain

Contrary to previous claims, the brain imprints of A. afarensis were found to be ape-like, lacking key features that uniquely evolved in humans. Australopithecus was thought by some to show a modern human-like arrangement of brain structures linked with more complex behaviours like tool manufacture and vocal communication. However, the new evidence for the Dikika child and another fossil unambiguously shows that the arrangement, marked by a particular brain groove, is the same as in apes.

Humans are unique in that they grow their large brain over a much longer period of childhood than apes. It would therefore be expected that A. afarensis, with a brain size about 20% larger than chimpanzees, would only have a slightly prolonged pattern of brain growth.

The Dikika child offered a unique opportunity to test this assumption because synchrotron computed tomographic scans made it possible to determine how old it was when it died, and how large its brain was at this age. Counting the tiny growth lines inside the teeth, the team's dental experts could calculate an age at death of 2.4 years, younger than the 3 years estimated previously. Combined with its small brain size, this finding surprisingly indicates that the brain growth of A. afarensis was as slow as in modern humans.

It shows that A. afarensis had a more ape-like brain, that nevertheless developed over a longer period, similar to modern humans. This extended period of brain growth may have resulted in a long dependence on caregivers. As such it provided a basis for the subsequent evolution of the brain and social behaviour in later human ancestors, and was likely critical for the evolution of a long period of childhood learning.