(CNN) -- Researchers using a new method to evaluate the cognitive abilities of species stumbled across a surprising finding: Our evolutionary ancestor, Australopithecus, may have been less intelligent than modern-day great apes.
It has long been theorized that a bigger brain means greater cognitive ability, which is generally defined as an individual's overall mental capability and how they reason, solve problems, plan, think in the abstract, and learn from life experiences.
If this were the case, humans would have superior cognitive power, followed in the distance by Australopithecines (the pre-human group that the famously fossilized Lucy belongs to) and gorillas, with chimpanzees and orangutans bringing up the rear of the great ape species.
This may align with what we observe to be true about our own cognitive abilities, but when it comes to those of the other species, things are far less certain.
Scientist Roger Seymour, who led the study at the University of Adelaide's School of Biological Sciences in Australia, hypothesized that a different set of physiological characteristics may, in fact, provide clues to the cognitive capabilities of species, both past and present.
It's not just about size. It's about energy.
Seymour's method, which was outlined in the journal Proceedings of the Royal Society B on Wednesday, relies on the assumption that cognitive ability can be assessed, not simply by brain size, but by the energy that is required for the brain to perform its cognitive functions -- or the brain's metabolic rate.
This is because much of the brain's cognitive power can be found at the synapses between neurons, where the cells interact with one another. "The synapses are like little computers in the brain," he says. And they require 70% of the brain's energy.
Assessing the metabolic rate starts with a measurement of the holes in the skull called the carotid foramina, through which arteries snugly pass to carry blood to the brain. The size of these holes indicates the size of the arteries -- and therefore the rate of blood flow that was once supplied to the living, computing brain.
Why is the rate of blood flow important? "Every organ in the body demands oxygen and it requires blood to supply that oxygen," said Seymour. The more energy the brain spends on cognition, the more oxygen it requires.
It's like when an athlete grows big muscles, Seymour continued. "As the athlete trains and grows bigger, stronger muscles, those muscles demand more oxygen and the artery supplying the muscle will increase in size."
So, foramina size tells us about the blood flow rate, which tells us about the metabolic rate -- which, if Seymour's hypothesis holds true, tells us about cognitive ability.
After comparing the foramina sizes of Australopithecines, great apes and humans, Seymour found that Australopithecines had the lowest blood flow rate through the carotid foramina. This is despite having a brain volume roughly on par with gorillas, and larger than the other great ape species.
Tying it all together
According to these findings, our ancient relative, Lucy, may have had less cognitive ability than today's great apes.
So what does this all mean? Is brain size out and blood flow rate in when it comes to measuring cognitive ability?
Not necessarily. There is no way to definitively test which characteristic, if either, underlies cognitive ability. But Seymour has found reason to doubt that brain size alone is the determinant.
"Some very small animals appear to be extremely intelligent but don't have large brains," he says.
That means the opposite is true as well: some of the largest brain-size-to-body-size ratios can be found in fish.
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