When our ancient human ancestors leapt out the trees to begin exploring the vast plains of the African savanna on two feet, what exactly were they thinking?
Anthropologists have been debating the origins of human intelligence and evolution of the brain for a long time – and now there’s a new theory in town.
A new study suggests that blood flow to the brain, rather than just the size of the brain, may have driven the evolution of human intelligence.
A University of Adelaide-led team of researchers analysed 35 skulls or skull casts from 12 ancient human species, and determined that blood flow to the brain increased dramatically over evolutionary time. The study was published today in Royal Society Open Science.
“What our work shows is that, aside from an increase in size over the three million years, there has been a disproportionate increase in rate of blood flow to the brain,” lead researcher Professor Emeritus Roger Seymour tells SBS Science.
“The assumption up until now has been that the development of cognitive ability, or thinking ability, is related directly to the size of the brain.”
Feeding the brain
One of the main theories on the evolution of human intelligence states that the size of the brain proportionate to body size is a key indicator of cognitive abilities – as hominin brain size increased, so too did intelligence.
The earliest human ancestor Homo habilis evolved around 2.8 million years ago and was the first hominin to make use of stone tools.
Our next closest ancestor, Homo erectus, popped up in the fossil record about 1.9 million years ago and is thought to be the first to have used fire and complex tools before the modern human, Homo sapiens, emerged around 200,000 years ago.
Using measurements of holes in the base of the skull that feed two main arteries, known as the internal carotid arteries, to the brain, the researchers calculated that the rate of blood flow increased about 600 per cent across human evolution, whereas brain size increased around 350 per cent.
Increased blood flow indicates that the brain’s oxygen and nutrient requirements also increased, which infers that the connections between neurons were becoming more complex and metabolically costly.
Professor Maciej Henneberg, an anthropologist also from the University of Adelaide, says he is impressed by the new study.
“It’s a good study,” says Henneberg. “They are supporting what I was saying since 2009 [..] that the human mind evolved not as a result of increase in brain size but changes in the way neurotransmitters operate inside the brain.”
Size is not everything
According to Henneberg, Seymour and others, brain size is an inaccurate and incomplete way of measuring human intelligence – some have argued that increased brain complexity and organisation drove this progression, whereas other studies have suggested that the demands of raising human offspring, which are born far more immature than offspring of other species, helped drive human intelligence.
“We can’t judge intelligence of modern humans or past humans based on just the size of their brains, which is very well supported by the fact that for at least 1.5 million years, we humans produced complex tools, we used fire and we managed our environment in increasingly efficient ways,” says Henneberg.
However, Associate Professor Darren Curnoe, director of the Palaeontology, Geobiology and Earth Archives Research Centre at UNSW, is more cautious and says the study is “too speculative”.
“I don’t think it’s a groundbreaking piece of science or a game-changer in our understanding of human brain evolution,” he told SBS Science in an email.
He says one of the key limitations of the study is that it makes a number of assumptions about how the human brain evolved - such as evolving in a linear fashion with changes preceding one another when this may not have been the case, as well as about the increasing complexity and organisation of the brain.
“My main concern is that we know so little about variation in brain organisation [..] among the different human species that it is unreasonable to assume that major differences actually existed,” he says.