Average brain size of humans increased gradually and consistently over the past three million years, growing to over three times larger than our closest living relatives, chimpanzees and bonobos.
The study, published in the journal Proceedings of the Royal Society B, shows that the trend was caused primarily by evolution of larger brains within populations of individual species, but the introduction of new, larger-brained species and extinction of smaller-brained ones also played a part.
"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," said Andrew Du, a postdoctoral scholar at the University of Chicago in the US.
"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," said Du.
"Think about the entrance to a building. You can reach the front door by walking up a ramp, or you can take the steps," said Bernard Wood, from George Washington University (GW) in the US.
"The conventional wisdom was that our large brains had evolved because of a series of step-like increases each one making our ancestors smarter," said Wood.
"Not surprisingly the reality is more complex, with no clear link between brain size and behaviour," he said.
Researchers compared published research data on the skull volumes of 94 fossil specimens from 13 different species, beginning with the earliest unambiguous human ancestors, Australopithecus, from 3.2 million years ago to pre-modern species, including Homo erectus, from 500,000 years ago when brain size began to overlap with that of modern-day humans.
They saw that when the species were counted at the clade level, or groups descending from a common ancestor, the average brain size increased gradually over three million years.
Looking more closely, the increase was driven by three different factors, primarily evolution of larger brain sizes within individual species populations, but also by the addition of new, larger-brained species and extinction of smaller-brained ones.
The team also found that the rate of brain size evolution within hominin lineages was much slower than how it operates today, although why this discrepancy exists is still an open question.