Humans specialise in being smart. It wasn’t our sharp teeth that allowed us to rise to dominance chain but our brains. It provides us with the intelligence needed to build a metaphorical ladder to the top of the food chain. Yet what is it about our brains that makes us so smart? Is there something unique to the lump of organic matter in my head that distinguishes it from a chimp’s lump of organic matter or are they just variants of the same basic brain?
The most obvious difference between the two is brain size. Despite the fact that both Homo sapiens and Pan troglodytes have a very similar body size our brain is nearly thrice the size of a chimp brain. In other words our brain is bigger than would be expected given how big our body is. This is known as encephalisation and humans are indeed highly encephalised with our brains being over 7 times bigger than would be expected.
This increase in size is best explained by the social brain hypothesis, which postulates that it evolved larger to facilitate larger groups. This allowed them to better defend against predators, forage the landscape and so forth. Eventually the brain was sufficiently big that it could also be used for other things, like making fire, and these acted as additional benefits compounding the evolution of bigger brains.
So our brain is big, but is that that the key difference? If I were to magically make a chimp’s brain three times bigger (and also use my new found magic powers to make their skull appropriately bigger) could they develop the cognitive abilities of a human? Many have suggested that this would not be the case and our brain has many other unique components. According to these individuals a human brain is not simply a giant chimp brain.
Suggested differences between human brains and chimp brains have included everything from relative numbers of neurons (with humans, of course, having more) to the number of non-neuron cells(with humans, of course, having more of those too). And lets not forget the shape, with humans apparently having bits of their brain relatively bigger than would be expected if you just scaled up a monkey brain.
However, new research purports to disprove many of these differences between human and primate brains. Suzana Herculano-Houzel used a technique she created back in 2005 called the “isotropic fractionator.” Whilst I’m not familiar with the specifics it purports to offer a reliable way of counting the number of cells in a brain. And counting brain cells is what she’s been doing, documenting the number of cells in over 20 species.
Her results revealed that there is a relationship between the number of neurons and brain size. Which is kind of obvious. What’s less obvious (and thus more interesting) is that different groups of animals have differing relationships between the two. A rodent brain, for example, has less neurons than a similarly sized primate brain.
This relationship can be expanded and you can use it to predict the number of neurons in a hypothetically sized brain (or vice versa) of a particular type of animal. Returning to the rodent/primate example, if you increase the number of neurons in a rodent brain 10-fold you wind up with a brain that’s 35 times larger! Conversely increasing the number of neurons by the same amount would only get you a 10 times larger primate brain.
The data provided even more interesting information, hinted at in the above image: different parts of the brain had different rules governing the relationship between brain size and number of neurons. Of particular interest is the cerebral cortex since it is often associated with intelligence (and especially large in humans).
However, in primates the relationship between the number of neurons in the cerebral cortex and the size of that cortex were very similar to the relationship present in the rest of the brain. Increasing the number of neurons in the cerebral cortex 10 times increases the size of the cerebral cortex ~10 times. Compare that to a rodent where increasing the number of neurons in the cerebral cortex 10-fold increases its size 50-fold! Further, there was also a link between the size and the cerebral cortex and the size of the cerebellum.
Whilst neuron numbers varied across different groups of animals and in different areas of the brain, non-neuron numbers had a relatively stable relationship with brain size. Increase the brain size by x and the number of non-neuron cells also increase by x in a nice, linear fashion. This result is relevant given that, although they don’t get as much press as neurons, non-neuron cells also appear to be involved in intelligence.
All of this suggests two key things. Firstly that primates have evolved a much more efficient neuron set-up than other animals, being able to pack many more neurons into a much smaller area. This explains why a non-primate with a brain the same size as a primate is often not as intelligent. Secondly, non-neuron cells are much more restricted in how they can evolve. The lack of variation in their relationship with brain size indicates that any such variation is detrimental and so being organised how they currently are is either necessary for the brain to work or optimal.
As well as these two general points about the brain the researcher was also able to make more specific statements about the primate brain. They then looked humans to see if our brain also followed the same rules. In other words, if you would get a human brain by simply scaling up a primate brain. She found:
- Primates have ~1:1 ratio between neurons and non-neuron cells. Humans also have this, with 85 billion neurons and 86 billion non-neurons in our brain
- The size of the primate cerebellum increases linearly with number of neurons in it (i.e. increase the size by 10-fold and you increase the number of neurons in it 10-fold). Humans also follow this trend
- The size of the primate cerebral cortex increases linearly with number of neurons in it. Humans also follow this trend.
- The size of the primate cerebral cortex is related to the size of their cerebellum. The same is true of humans.
So essentially humans have giant chimp brains. It turns we aren’t as unique as many have suggested we are. However, if that’s got you feeling down, remember that we still have a large brain. Combined with our high density of neurons this means that we probably have the most neurons of any species!
That said, I have two major qualms with the study. The first is that I couldn’t find a description of the “isotropic fractionator” either in this paper or her citation for it. As such I don’t know how reliable it is. The second is the small sample size, with only 7 non-human primate species being examined and only 1-3 members of each species being studied.
However, the “isotropic fractionator” (which is sounding more like an evil geniuses super-weapon the more I say it) has been employed by other scientists so there are clearly a number of individuals who have deemed it to be reliable. And at the rate Suzana Herculano-Houzel is measuring brains the small sample size will soon be rectified but until then the conclusion should be taken with a grain of salt.
A bigger concern I have (and did not list earlier, what a twist!) is that other research is revealing other changes to the human brain that would not be picked up by just counting cells. SRGAP2 mutations, for example, seem to have changed the very cells themselves! As such there may well be “hidden” differences between humans and primates.
So where does that leave us? Well this study shows that many of the alleged differences between human and chimp brains are non-existent. Relatively speaking we have the same number of neurons, non-neurons and all the various bits are the same size. However, this doesn’t show that the human brain is essentially a giant chimp brain but it does make us a lot less special. As a result its evolution isn’t necessarily that special either.
|Suzana Herculano-Houzel, Christine E. Collins, Peiyan Wong, Jon H. Kaas (2007). Cellular scaling rules for primate brains Proceedings of the National Academy of Sciences, 104 (9), 3562-3567 DOI: 10.1073/pnas.0611396104|
|Suzana Herculano-Houzel (2012). The remarkable, yet not extraordinary, human brain as a scaled-up primate brain and its associated cost Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.1201895109|
I didn’t post last week because I was on holiday. I would apologise for this except I’m not really sorry as I had a very nice time.