We humans may like to think that we have grown ever more intelligent as our brains have grown larger over the past several million years, but that is not necessarily the case, research revealed last week.
Neurobiologist William Calvin told the conference of the American Association for the Advancement of Science in Washington that the expansion of the human brain did not automatically equate to a rise in human intelligence.
Professor Calvin pointed out that our ancestors went through two periods of more than a million years when their brains were getting bigger but their tool-making techniques were not improving and they plodded on with what had been done before.
He added that Homo sapiens was walking around Africa 165,000 years ago with a brain the size ours is today. But we spent most of the next 100,000 years doing "more of the same", with the same level of problem-solving as Neanderthals.
Professor Calvin, an affiliate professor of psychiatry and behavioural sciences at the University of Washington, said: "Everyone assumes that with brains, bigger is smarter is better. People tend to think this is an automatic process. But evolution doesn't work as gradually as Darwin originally thought."
He said that for three quarters of his time on earth, Homo sapiens showed no signs of thinking and problem-solving in the way that we do today.
Professor Calvin's work is now focusing on what really drove brain size to increase. Possible reasons include the development of language, the emergence of concepts of social organisation and the development of more accurate throwing ability.
Professor Calvin explained that two-year-old children today provide a good model of what the language skills of our Homo sapiens ancestors might have been like.
"This is a proto language but with no long sentences or complex thoughts. To string thoughts together, you need a structural ability, which may have required some reorganisation (of the brain)," he said.
He added that some brain development may have been necessary to deal with things such as sharing: "We have a need to keep track of who owes what to whom in order to avoid being tricked by the cheaters."