The human brain is spectacular. It's absolutely massive. But beyond that size, is there anything special about brain evolution in our family? A growing body of evidence suggests that its development wasn't that special. Relatively speaking, we've got no extra neurons, no extra white or matter, and no extra big bits. But might this focus on numbers and size hide some extra special brain evolution? This is something Derek asked me:
What about the idea that we have more synapses per neuron than our ape relatives? Might this make a significant difference to our brain function even if we don't have a greater density of neurons per-se? You wrote a bit about this in 2012: http://www.evoanth.net/2012/05/17/mutation-brain-evolve/
As he suggests, there is some evidence that the human brain isn't as mundane as you might think. Unfortunately, I had to point out this was something difficult to study:
There's some potential there, but it's difficult to study over the course of human evolution given that neurons don't fossilise.
Without fossil evidence, it's difficult to tie in these changes with our evolution. It's all well and good pointing out we've got special neurons, but for all we know there's no correlation with our family's rising intelligence.
Fortunately, some researchers seem to have heard my complaint. Shortly after I wrote an article about how the evolution of our brain isn't that special, new research revealed that it is. And these researchers managed to get some evidence of this from the fossil record!
And whilst I have all these researchers' attention, might I also ask for them to investigate belly button fluff. There was some interesting early work on the subject, but there's been no follow-up.
Bloodthirsty brain
Our brain is a substantial organ. Neurons and the other cells in our noggin are amongst some of the hungriest in the body. In fact, roughly 1/4 of your daily recommended calories go straight to your head. For context, that's an entire big mac (but not the fries. Those go straight to your hips).
All of these bananas, oxygen, and other stuff your brain needs is transported into the brain through the blood. So as you might expect, our brain receives more blood than a species which needs fewer big macs to fuel their noggin. This is where things start to get interesting. Blood doesn't typically fossilise, unfortunately for John Hammond and his mosquitoes. However, the route it takes into the brain does.
One of the routes our blood takes into the brain is through the internal carotid artery. That's a fancy medical term you may or may not know (or care about). The key point is that it passes into our cranium through a hole in the base of the skull. This is called the carotid foramen (which is latin for "carotid hole". Suddenly it doesn't seem like such a fancy medical term).
Crucially, this is a hole in the skull itself. And so whenever that part of the skull will fossilise, we gain information about the size of the hole. By extension, palaeoanthropologists can infer the size of the internal carotid artery and how much blood the brain is getting. As you might imagine, it did increase over the course of human brain evolution.
Unique brain evolution
All of those holes seem interesting. After all, they've even been circled (and with a size bar) like a proper scientific diagram. But what does it all mean?
Obviously, these holes get bigger over time. The oldest species depicted ( Australopithecus afarensis on the left) has the tiniest carotid foramen. If human brain evolution hasn't been that unique, except for its size changes, then the changes in the foramen should be consistent with those size changes. It's a well-established fact that bigger brains need more blood, to the point it's almost a perfect correlation. A brain double the size needs 1.9x as much blood.
Over the course of human brain evolution, our noggin has grown 3.5 times as large as it started. And . . . drumroll please . . . the relative amount of blood flow indicated by these holes has increased by 6 times. Our brain is getting much more blood than it should need, given its size. Clearly, there has been some unique changes to it beyond simply being ruddy large.
And that's not all this tells us. Since we're dealing with fossils we can track this change over time.
As the graph shows, things remain relatively consistent for a good chunk of human brain evolution. The brain continued to draw the amount of blood expected for its size until ~2 million years ago. This is despite the fact that size increased over that time period. Then, our genus emerged and (after a bit) began developing larger and larger brains. Blood flow began to increase more than expected, even given these changes in brain size.
What made the brain so special?
All of this raises the question: what was that change? Why did our brain suddenly start needing all that extra blood?
The researchers who made this discovery have some ideas. They point to the fact that neurons interacting is what takes up half of the energy our brain needs. Could a dramatic increase in that interaction be responsible? By coincidence, that was what Derek suggested, back at the start of this. Or maybe it isn't a coincidence and scientists are copying me.
Derek pointed to some key genetic mutations which happened over the course of human brain evolution. These were duplications and modifications of an existing gene ( SRGAP2) at various points in our family history. The second duplication, producing SRGAP2C has risen to fixation in modern humans and seems to play a big role in neuronal development. Specifically, it leads to more growth of dendrites; which are the parts of neurons that interact.
Genetic estimates suggest SRGAP2C mutated around 2.4 million years ago, which puts it tantalisingly close to when the brain got bloodthirsty.
Of course, that close relationship doesn't mean the gene and the blood demand really are linked. More work is needed. It also doesn't mean these are the only explanations in play. The researchers studying the brain flow also posited some other factors; which aren't necessarily mutually exclusive with the role of SRGAP2C. Other possibilities include the way our brain has become right-left specialised.
As an interesting aside, this extra energy consumption likely means the brain would take longer to grow. It would take more time to become big and powerful enough to demand that extra blood. Humans have a long maturation, but Neanderthals have a slightly faster one. Curiously, their brain also seems to have used less blood. Could this hint at some cognitive differences between our two species?
Conclusion
Over the course of human brain evolution our noggin has become increasingly bloodthirsty. Literally. It needs more blood than would be expected given our brain's size. This would indicate that there have been some unique developments, beyond its large size. Curiously, this stands in contrast with many other measurements concerning our brain. Several aspects of our thinking organ have only changed in a way consistent with our increase in brain size.
Of course, there are still many other ways our brain may have developed in a unique way. Neurones likely communicate more than expected, the specialisation of our hemispheres might have something to do with it. And I'm sure many people will think of all sorts of other speculation.
References
Dennis MY, Nuttle X, Sudmant PH, Antonacci F, Graves TA, Nefedov M, Rosenfeld JA, Sajjadian S, Malig M, Kotkiewicz H, Curry CJ, Shafer S, Shaffer LG, de Jong PJ, Wilson RK, & Eichler EE (2012). Evolution of Human-Specific Neural SRGAP2 Genes by Incomplete Segmental Duplication. Cell, 149(4), 912-22
Dennis, M.Y. and Eichler, E.E., 2016. Human adaptation and evolution by segmental duplication. Current opinion in genetics & development, 41, pp.44-52.
Seymour, R.S., Bosiocic, V. and Snelling, E.P., 2016. Fossil skulls reveal that blood flow rate to the brain increased faster than brain volume during human evolution. Royal Society Open Science, 3(8), p.160305.
