Tuesday, April 12, 2016

Brain size: African elephants and us

Why are humans so smart? Is it the sheer number of neurons we have, or the global architecture of those neurons? The brain of the African elephant is three times as heavy as ours, but what's the neuron count? Suzana Herculano-Houzel in Nautilus:
Lo and behold, the African elephant brain had more neurons than the human brain. And not just a few more: a full three times the number of neurons, 257 billion to our 86 billion neurons. But—and this was a huge, immense “but”—a whopping 98 percent of those neurons were located in the cerebellum, at the back of the brain. In every other mammal we had examined so far, the cerebellum concentrated most of the brain neurons, but never much more than 80 percent of them. The exceptional distribution of neurons within the elephant brain left a relatively meager 5.6 billion neurons in the whole cerebral cortex itself. Despite the size of the African elephant cerebral cortex, the 5.6 billion neurons in it paled in comparison to the average 16 billion neurons concentrated in the much smaller human cerebral cortex.

So here was our answer. No, the human brain does not have more neurons than the much larger elephant brain—but the human cerebral cortex has nearly three times as many neurons as the over twice as large cerebral cortex of the elephant. Unless we were ready to concede that the elephant, with three times more neurons in its cerebellum (and, therefore, in its brain), must be more cognitively capable than we humans, we could rule out the hypothesis that total number of neurons in the cerebellum was in any way limiting or sufficient to determine the cognitive capabilities of a brain.

Only the cerebral cortex remained, then. Nature had done the experiment that we needed, dissociating numbers of neurons in the cerebral cortex from the number of neurons in the cerebellum. The superior cognitive capabilities of the human brain over the elephant brain can simply—and only—be attributed to the remarkably large number of neurons in its cerebral cortex.
And coking has something to do with it:
As it turns out, there is a simple explanation for how the human brain, and it alone, can be at the same time similar to others in its evolutionary constraints, and yet so different to the point of endowing us with the ability to ponder our own material and metaphysical origins. First, we are primates, and this bestows upon humans the advantage of a large number of neurons packed into a small cerebral cortex. And second, thanks to a technological innovation introduced by our ancestors, we escaped the energetic constraint that limits all other animals to the smaller number of cortical neurons that can be afforded by a raw diet in the wild.

So what do we have that no other animal has? A remarkable number of neurons in the cerebral cortex, the largest around, attainable by no other species, I say. And what do we do that absolutely no other animal does, and which I believe allowed us to amass that remarkable number of neurons in the first place? We cook our food. The rest—all the technological innovations made possible by that outstanding number of neurons in our cerebral cortex, and the ensuing cultural transmission of those innovations that has kept the spiral that turns capacities into abilities moving upward—is history.
H/t 3QD.

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