The human family split from the other apes about 7 million years ago. At that time our brain was about the size of the chimp. Since then, our thinking organ has undergone a lot of evolution, growing 4 times the size. In the process, it gained many amazing abilities. So surely the human brain must have also developed some unique features. How else could it give us language, technology, advanced social skills, and much more?
Maybe just by being super big? A growing body of evidence indicates that increased size – called “encephalisation” – is all that separates us from the chimps. Many of the unique changes beyond just “being big” are being disproven. Even those which are being vindicated are having their significance undermined.
Prefrontal cortex & primates
The prefrontal cortex is kind of a big deal. It’s been linked to our personality, planning capabilities, and decision-making processes. In humans, these characteristics are important and highly developed. As such, it looked like the prefrontal cortex must have undergone some unique evolution. If you enlarged a chimp brain to our size, it should surely differ from the human brain.
One key difference seems to be the relative size of our prefrontal cortex. Previous research has indicated that part of the brain has relatively more neurons than it should. However, this idea seems destined for the dustbin. A new study has examined our entire cerebral cortex in great detail. It reveals that the prefrontal cortex has no extra neurons after all.
This study dissected a human brain, along with brains from 8 primate species.Samples were taken from the front to the back of the brain. These sections were then fed through isotropic fractionation. This is a method that essentially makes the brain sample consistent. Thus, you only need to examine the number of neurons in a small bit and extrapolate the results. The alternative is counting all the neurons in the prefrontal cortex. We have 1.3 billion, making isotopic fractionation a far more appealing option.
And, as I just hinted, this study revealed that the prefrontal cortex has no extra neurons after all.
The boring human brain
All of this doesn’t mean our brains aren’t unique. The prefrontal cortex has 1.3 billion neurons. Meanwhile, Macaques only have 137 million. The absolute number of neurons in the human brain is incredible. It’s just that if another primate somehow got a brain our size, they’d have just as many too.
And it’s not just our prefrontal cortex that seems boring. Previous work (by some of the same people who just poo-pooed our prefrontal cortex) show that other parts of our brain follow the same pattern. Generally speaking, our brain has as many neurons as would be expected for a primate with a giant brain. In relative terms, we just aren’t that special.
That’s not to say there’s nothing special about our brains. The shape of certain regions can be unusual, with some bits larger than others. Although this doesn’t impact the relative number of neurons in those bits, so the significance of these changes is debatable. The overall shape of the human brain is unusual as well. It’s typically described as “globular”. Which means what it sounds like it means. Our brain is rounder than our relatives’.
This globular brain is often used to help define modern humans. Yet the significance of this might be overstated as well. A recent analysis of how skull shape influences brain shape revealed some interesting results. Namely, that this globularisation might simply be a side effect of our small faces. Trying to combine a large primate brain with such a setup naturally produces such a weirdly shaped brain.
Yet another bit of uniqueness that can be ticked off.
The human brain is unique: it’s absolutely massive. As a result, we have 85 billion neurons. This seems to be more than any other known species. Granted, we might not have relatively more neurons than other primates. However, when our absolute number is just that high who cares?
The human brain is just a giant primate brain. Yet such a “minor” change has produced something spectacular.
The human brain is linked to everything that makes us unique. So was its evolution also special? New research reveals it wasn’t that unusual
Barton, R.A. and Venditti, C., 2013. Human frontal lobes are not relatively large. Proceedings of the National Academy of Sciences, 110(22), pp.9001-9006.
Gabi, M., Neves, K., Masseron, C., Ribeiro, P.F., Ventura-Antunes, L., Torres, L., Mota, B., Kaas, J.H. and Herculano-Houzel, S., 2016. No relative expansion of the number of prefrontal neurons in primate and human evolution. Proceedings of the National Academy of Sciences, p.201610178.
Herculano-Houzel, S. and Lent, R., 2005. Isotropic fractionator: a simple, rapid method for the quantification of total cell and neuron numbers in the brain. The Journal of neuroscience, 25(10), pp.2518-2521.
Schoenemann, P.T., Sheehan, M.J. and Glotzer, L.D., 2005. Prefrontal white matter volume is disproportionately larger in humans than in other primates. Nature neuroscience, 8(2), pp.242-252.
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