Man’s ancestors who lived more than three million years ago had an ape-like brain structure but human-like brain growth, according to new research.
The findings are based on an analysis of eight fossil skulls belonging to Australopithecus afarensis – the species to which the famous early human ancestor “Lucy” belongs.
Researchers consider this “mosaic” of ape and human features in A.afarensis to be “a hallmark of evolution”.
Philipp Gunz, a biological anthropologist at the Max Planck Institute for Evolutionary Anthropology in Germany and lead author on the study, said: “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.”
An international team of scientists, which included researchers from the the Natural History Museum in London and University College London, used state-of-the-art scanning technology to examine the ancient fossil skulls of A.afarensis collected from the archaeological sites Dikika and Hadar in Ethiopia.
While brains do not fossilise, they leave imprints on the inside of the skull, which can reveal information about the structure and development of the organ.
Analysis of these brain imprints revealed key differences in the structural organisation of human and A.afarensis brains.
For example, the team found the placement of the lunate sulcus – a fissure that separates the anterior and posterior parts of the brain – closer to the front of the brain in A.afarensis, like chimpanzees.
In humans, this fissure is pushed further down in the brain.
The researchers calculated the endocranial volume, or brain mass, of an A.afarensis infant and found evidence of a prolonged period of brain development compared with chimpanzees.
They believe the findings show that brain growth in A.afarensis was protracted, suggesting their children, like those of modern humans, had a long dependence on caregivers.
Fred Spoor, a scientist at the Natural History Museum and a co-author on the study, said: “Our new results now show what their brain looked like, and how it grew after birth.”
A.afarensis inhabited East Africa more than three million years ago, and is widely accepted to be ancestral to all later hominins, including humans.
The 3.2 million-year-old ape Lucy was the first A.afarensis skeleton ever found and is considered to be the world’s most famous early human ancestor.
Zeresenay Alemseged, a paleoanthropologist from the University of Chicago in the US and senior author on the study, said: “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, may have used sharp stone tools.”
He added: “Our new results show how their brains developed, and how they were organised.”
– The findings are published in the journal Science Advances.