Friday, October 17, 2008

Big Giant Heads

She studied him for a long moment. He was twice her size, with an enormous head. Yet his ears were small and his nose was stubby, like an afterthought. "Yes, they were more our size. Their heads were human and –"
"Ur-human," the man corrected absently, as though he was distracted.
"Oh, I am sorry, We term your kind Ur-human, since you are the earliest form available."
Her mouth whitened. "And what do you call yourselves?"
"Ah, humans," he said uncomfortably.
~ Beyond the Fall of Night, Arthur C. Clarke and Gregory Benford.
Before transhumanism became all the fashion, science fictional depictions of far future often gave our human descendants fantastic mental powers along with giant brains. But there is a serious problem with that idea: human brain size at birth is limited by the size of the opening in the pelvis, and those far future women never seem to have extra-wide hips to go along with their giant heads. A solution, of course, is assume that human birth will ultimately require medical technology. And it appears possible that even technology we use today might allow humans to start heading down that evolutionary path.

At The Panda's Thumb PZ Myers looks at the suggestion that the wide availability of relatively safe Cesarean sections in developed countries allows the evolution of bigger-headed humans.
Babies have very big heads that squeeze with only great difficulty through a relatively narrow pelvis, so the relationship in size between head diameter and the diameter of the pelvic opening has been a limitation on human evolution. We know this had to be a factor in our evolution: the average newborn mammal has a cranial capacity that is roughly 50% of the adult size, chimpanzee babies have heads about 40% of the adult size, but human babies have crania that are only 23% of what they will be in adults. While our brains have gotten larger over evolutionary time, they have not gotten proportionally larger in utero, because large-headed babies increase the difficulty of labor and cause increased mortality in childbirth. If childbirth could bypass the pelvic bottleneck, that would allow for fetal heads to grow larger without increasing the risk of killing mother and/or child.
There is some circumstantial evidence that bigger babies are being born in the US, and that the increased size has a genetic component. That's certainly not proof that humans are evolving along those lines, but it does make the idea plausible.

But is bigger necessarily smarter? Given a similar genetic background larger brain size doesn't seem to improve cognitive abilities. It's the connections between neurons that have the greatest effect on brain function. A recent review of human brain evolution described some of the other potential problems of increasing brain size.
For one thing, a big brain is a metabolic drain on our bodies. Indeed, some people argue that, because the brain is one of the most metabolically expensive tissues in our body, our brains could only have expanded in response to an improved diet. Another cost that goes along with a big brain is the need to reorganise its wiring. “As brain size increases, several problems are created”, explains systems neurobiologist Jon Kaas (Vanderbilt University, Nashville, Tennessee, United States). “The most serious is the increased time it takes to get information from one place to another.” One solution is to make the axons of the neurons bigger but this increases brain size again and the problem escalates. Another solution is to do things locally: only connect those parts of the brain that have to be connected, and avoid the need for communication between hemispheres by making different sides of the brain do different things. A big brain can also be made more efficient by organising it into more subdivisions, “rather like splitting a company into departments”, says Kaas. Overall, he concludes, because a bigger brain per se would not work, brain reorganisation and size increase probably occurred in parallel during human brain evolution. The end result is that the human brain is not just a scaled-up version of a mammal brain or even of an ape brain.
So our brains may get bigger, but they won't necessarily get better. And while both brain size and intelligence are both influenced by genetics, we haven't yet found any genes that affect both characteristics. For example, two genes that affect brain size, ASPM and Microcephalin, appear to have been evolving adaptively in modern human populations, there is no consistent correlation between the different genetic variants and IQ. But there is a correlation between specific variants of those genes and populations that use tonal languages, so perhaps their effect on brain function is more subtle than development of intelligence.

So will humans evolve Talosian-like giant brains? There is no way for us to know. What we can say is that elimination of the requirement for that babies' heads pass through the pelvis makes that development more plausible. And even if humanity does become big-headed, we may not end up any smarter - and almost certainly not psychic.

There is more discussion at Pharyngula.

Related Articles:
Bradbury J. "Molecular Insights into Human Brain Evolution" PLoS Biol 3(3):350 (2005)

Mekel-Bobrov N. et al. "The ongoing adaptive evolution of ASPM and Microcephalin is not explained by increased intelligence." Hum. Mol. Genet. 16(6): 600-608 (2007)

Schoenemann PT et al. "Brain size does not predict general cognitive ability within families." PNAS 97(9):4932-4937 (2000)


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