The evolution of the human brain may have occurred far more gradually than previously believed.
A new study on nearly 100 fossils from several different human species has found that brain size tripled over the course of the last three million years, in a process that was likely slow and consistent, as opposed to a series of ‘step-like increases.’
Experts say the shift was driven by increasing complexity, as humans developed culture, language, and the ability to make tools.
According to the researchers, the process can be likened to a rugby team that trains hard.
The average weight goes up as heavier players are recruited – or lighter players are cut.
‘So it is with brains – we found existing species evolved larger brains, larger-brained species appeared and smaller-brained ones went extinct,’ said Dr Andrew Du, of Chicago University.
His team looked at 94 fossils from 13 different human species dating back to when Australopithecus afarensis – whose most famous member ‘Lucy’ was dug up in Ethiopia – roamed the Earth.
Modern human brains are three times larger than those of chimpanzees – our closest living relative.
But how that dramatic enlargement came about has puzzled evolutionary experts for decades.
Dr Du said: ‘When we looked at nearly a hundred human fossils that preserve some evidence of brain size, we found that average brain size increased gradually and consistently over the past three million years.’
The study published in Proceedings of the Royal Society B showed this was driven by evolution within populations, introduction of larger-brained species and extinction of smaller-brained ones.
First author Dr Du said: ‘Brain size is one of the most obvious traits that makes us human.
‘It’s related to cultural complexity, language, tool making and all these other things that make us unique.
‘The earliest hominins had brain sizes like chimpanzees – and they have increased dramatically since then. So it’s important to understand how we got here.’
Senior author professor of human origins Bernard Wood at George Washington University said it was similar to choosing to walk up a ramp rather than take the steps.
He said: ‘Think about the entrance to a building. You can reach the front door by walking up a ramp, or you can take the steps.
‘The conventional wisdom was that our large brains had evolved because of a series of step-like increases each one making our ancestors smarter.
‘Not surprisingly the reality is more complex – with no clear link between brain size and behaviour.’
Dr Du and colleagues compared data on the skull volumes of specimens – beginning with Australopithecus from 3.2 million years ago to pre-modern species.
These included Homo erectus from 500,000 years ago when brain size began to overlap with that of modern-day humans.
Descending from a common ancestor the researchers saw the average brain size increased gradually over three million years.
Dr Du compared it to how a football coach might build a squad of bigger, strong players.
One way would be to make all the players hit the weight room to bulk up. But the coach could also recruit new, larger players and cut the smallest ones.
He said: ‘That’s exactly what we see going on in brain size – the dominant process is like the players hitting the gym.
‘They’re evolving larger brains within a population.
‘But we also see speciation events adding larger-brained daughter species, or recruiting bigger players, and we see extinction, or cutting the smallest players too.’
