March 19, 2004
Primate Frontal Cortex Hyperscales

Across primates the size of the frontal cortex goes up faster than the size of the rest of the brain.

"In primates, having a bigger brain means you have a disproportionately larger frontal cortex," said Eliot Bush, a PhD candidate at Caltech who worked on the study.

Large human total brain size results in much larger frontal cortex size.

PASADENA, Calif.--Everybody from the Tarzan fan to the evolutionary biologist knows that our human brain is more like a chimpanzee's than a dog's. But is our brain also more like a tiny lemur's than a lion's?

In one previously unsuspected way, the answer is yes, according to neuroscientists at the California Institute of Technology. In the current issue of the Proceedings of the National Academy of Sciences (PNAS), graduate student Eliot Bush and his professor, John Allman, report their discovery of a basic difference between the brains of all primates, from lemurs to humans, and all the flesh-eating carnivores, such as lions and tigers and bears.

The difference lies in the way the percentage of frontal cortex mass increases as the species gets larger. The frontal cortex is the portion of brain just behind the forehead that has long been associated with reasoning and other "executive" functions. In carnivores, the frontal cortex becomes proportionately larger as the entire cortex of the individual species increases in size--in other words, a lion that has a cortex twice the size of another carnivore's also has a frontal cortex twice the size.

By contrast, primates like humans and apes tend to have a frontal cortex that gets disproportionately larger as the overall cortex increases in size. This phenomenon is known as "hyperscaling," according to Bush, the lead author of the journal article.

What this says about the human relationship to the tiny lemurs of Madagascar is that the two species likely share a developmental or structural quirk, along with all the other primates, that is absent in all the carnivores, Bush explains. "The fact that humans have a large frontal cortex doesn't necessarily mean that they are special; relatively large frontal lobes have developed independently in aye-ayes among the lemurs and spider monkeys among the New World monkeys."


The hyperscaling mechanism is genetic, and was presumably present when the primates first evolved. "Furthermore, it is probably peculiar to primates," says Allman, who is Hixon Professor of Neurobiology at Caltech.

Given that brain size differs between humans it would be interesting to know whether this same mathematical relationship exists when comparing humans with each other/.

From the abstract of the paper:

Primate frontal cortex hyperscales relative to the rest of neocortex and the rest of the brain. The slope of frontal cortex contrasts on rest of cortex contrasts is 1.18 (95% confidence interval, 1.06-1.30) for primates, which is significantly greater than isometric. It is also significantly greater than the carnivore value of 0.94 (95% confidence interval, 0.82-1.07). This finding supports the idea that there are substantial differences in frontal cortex structure and development between the two groups.

Whatever may have been the cause of this hyperscaling? Is there a reason why the way all primates function causes this to be advantageous?

Share |      Randall Parker, 2004 March 19 02:29 AM  Trends, Human Evolution

Stefan Jones said at March 21, 2004 12:12 AM:

Something about staying in the trees a bit longer than the rest of the mammalian gang?

This may not necessarily be a universally useful change, but one that when neutral doesn't incur any particular cost and is thus not selected against. It's not like the trait automatically selects for a larger brain, which has a metabolic cost . . . just a differently structured brain.

So . . . if we pop this into carnivores, do we get really dangerous, forethoughtful leopards and such?

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