Lucy had an ape-like brain, but prolonged brain growth like humans

Prof Fred Spoor, Calleva Research Leader in Human Evolutions at the Natural History 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.”

The species Australopithecus afarensis inhabited East Africa more than three million years ago, and occupies a key position in the hominin family tree, as it is potentially ancestral to the human lineage. “Lucy and her kind provide important evidence about early hominin behaviour. They walked upright, had brains that were around 20 per cent larger than those of chimpanzees, and may have used sharp stone tools,” explains Prof Z. Alemseged from the University of

Chicago, who directs the Dikika field project in Ethiopia where the skeleton of an Australopithecus child was found in the year 2000 and another senior author of the study.

To study the shape and growth of the Australopithecus afarensis brain Spoor and Alemseged took the fossil skull of the Dikika child for CT scanning to Dr Paul Tafforeau of the European Synchrotron Radiation Facility (ESRF) in Grenoble, France. Says Spoor: “when we were given the opportunity to use this ‘Rolls-Royce’ of scanning facilities a completely new universe of details emerged inside this beautiful little child skull”. For comparison seven other fossil skulls from the nearby Hadar site in Ethiopia were investigated as well, using conventional high-resolution CT scanning.

With the help of this state-of the-art technology and several years of painstaking fossil reconstruction the brain size and shape could be studied 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.

These data shed new light on two questions that have been controversial. (1) Is there evidence for human-like brain evolution in Australopithecus afarensis, and (2) was the pattern of brain growth in A. afarensis more similar to that of chimpanzees or that of humans?

Contrary to previous claims, the brain imprints of Australopithecus afarensis were found to be ape-like, lacking key features that uniquely evolved in humans. In particular, Australopithecus was thought by some to show a modern human-like arrangement of brain structures linked with more complex behaviours (e.g., tool manufacture, mentalizing, 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 (the ‘lunate sulcus’), 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 Australopithecus afarensis, with a brain size about 20 per cent larger than chimpanzees, would only have a slightly prolonged period of brain growth.

“The Dikika child offers a unique opportunity to test this assumption”, notes Fred Spoor,  “because synchrotron CT 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, headed by Paul Tafforeau, could calculate an age at death of 2.4 years, substantially younger than the 3.0 years estimated previously. Combined with its small brain size of only 275ml this finding surprisingly indicates that the brain growth of Australopithecus afarensis was slow as in modern humans.

“Our data show that Australopithecus afarensis had a more ape-like brain, that nevertheless developed over a longer period of time, more similar to modern humans” concludes lead author Philipp Gunz. This extended period of brain growth in Australopithecus afarensis may have resulted in a long dependence on caregivers. As such it provided a basis for 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.

The paper is published in Science Advances on 1 April 2020.

Human Evolution Research at the Natural History Museum is funded by the Calleva Foundation.

ENDS

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