The human brain is a ‘scaled up primate brain’


Both of these headlines appeared in yesterday’s news:

Obama proposes brain mapping project

“As humans we can identify galaxies light years away, we can study particles smaller than the atom, but we still haven’t unlocked the mystery of the 3lb of matter that sits between our ears,” he said.

One in four Americans think Obama may be the antichrist, survey says

The study revealed that 13% of respondents thought Obama was “the antichrist”, while another 13% were “not sure” – and so were at least appeared to be open to the possibility that he might be. Some 73% of people were able to say outright that they did not think Obama was “the antichrist”.

I find the juxtaposition interesting.

Perhaps ‘we still haven’t unlocked the mystery of the 3 lb of matter that sits between our ears’ because the key to this ‘mystery’ does not lie within the brain but in its relationship to the rest of the body and to the social relations within which that body lives.

Speaking as a primate, let me tell you that humans consider their brains to be remarkable because they consider themselves to be the pinnacle of mammalian development and can only think that this must be due to their ‘remarkable’ brains. There is no truth in either assumption.

What follows is taken from the abstract and conclusion of just one of the many papers which question the celebrity status of the human brain:

“Neuroscientists have become used to a number of “facts” about the human brain: It has 100 billion neurons and 10- to 50-fold more glial cells; it is the largest-than-expected for its body among primates and mammals in general, and therefore the most cognitively able; it consumes an outstanding 20% of the total body energy budget despite representing only 2% of body mass because of an increased metabolic need of its neurons; and it is endowed with an overdeveloped cerebral cortex, the largest compared with brain size.

These facts led to the widespread notion that the human brain is literally extraordinary: an outlier among mammalian brains, defying evolutionary rules that apply to other species, with a uniqueness seemingly necessary to justify the superior cognitive abilities of humans over mammals with even larger brains.

These facts, with deep implications for neurophysiology and evolutionary biology, are not grounded on solid evidence or sound assumptions, however.

Our recent development of a method that allows rapid and reliable quantification of the numbers of cells that compose the whole brain has provided a means to verify these facts. Here, I review this recent evidence and argue that, with 86 billion neurons and just as many nonneuronal cells, the human brain is a scaled-up primate brain in its cellular composition and metabolic cost, with a relatively enlarged cerebral cortex that does not have a relatively larger number of brain neurons yet is remarkable in its cognitive abilities and metabolism simply because of its extremely large number of neurons.

Despite our ongoing efforts to understand biology under the light of evolution, we have often resorted to considering the human brain as an outlier to justify our cognitive abilities, as if evolution applied to all species except humans. Remarkably, all the characteristics that appeared to single out the human brain as extraordinary, a point off the curve, can now, in retrospect, be understood as stemming from comparisons against body size with the underlying assumptions that all brains are uniformly scaled-up or scaled-down versions of each other and that brain size (and, hence, number of neurons) is tightly coupled to body size. Our recently acquired quantitative data on the cellular composition of the human brain and its comparison to other brains, both primate and nonprimate, strongly indicate that we need to rethink the place that the human brain holds in nature and evolution, and to rewrite some basic concepts that are taught in textbooks. The human brain has just the number of neurons and nonneuronal cells that would be expected for a primate brain of its size, with the same distribution of neurons between its cerebral cortex and cerebellum as in other species, despite the relative enlargement of the former; it costs as much energy as would be expected from its number of neurons; and it may have been a change from a raw diet to a cooked diet that afforded us its remarkable number of neurons, possibly responsible for its remarkable cognitive abilities.

Suzana Herculano-Houzel. ‘The remarkable, yet not extraordinary, human brain as a scaled-up primate brain and its associated cost.’ Proceedings of the National Academy of Sciences of the United States of America. April 2012.

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