Why our brains are big

ResearchBlogging.orgOne of the biggest mysteries regarding human origins is why our brains became so big (or “encephalised” in fancy shmamcy science language). Over the past ~2.5 million years our thinking organ has gradually tripled in size, but what was the advantage such growth conferred that increased our ancestor’s “fitness”?

The most important graph in human history

Did their brains increasing in size allow them to keep track of where fruit can be found and when that fruit is ripe? The extra calories gained from this knowledge might have given an individual with a larger brain an advantage, allowing them to out-compete others.

Perhaps then, recent government campaigns to get people to eat more fruit & veg isn’t an effort to keep the populace healthy but to ensure our continued dominion over the chimps.

Perhaps our ancestors’ bigger brain gave them better tool making abilities. With a larger brain they might be able to imagine an object that does not yet exist and thus create it, rather than merely altering existing forms like other animals.  This would open a whole new world of potential as our predecessors could create something suited to their needs, giving them an advantage. And from there, natural selection would do the rest.

Whilst an iphone might not be especially useful on the Savannah – even though there probably is an antelope hunting app – certain technological advances might be. Sharp stones, eventually leading to spears and arrows, would ensure our eventual conquest of the natural world and they all need to be envisaged in our mind’s eye before they can be made.

In a life or death situation this is this is better than both Windows and Mac.

Or maybe it’s to allows us to maintain a larger group size. With a bigger brain we could keep track of more individuals, enabling our ancestors to live in larger groups since they can remember whose in it.

The advantages of living in a larger group are many. You can maintain a larger territory, have more people to fend of predators and you’ll be the biggest ape gang on the block – able to take what you want when you want. And this is all enabled by the fact you can form and maintain relationships with more of your monkey brethren.

The pinacle of human evolution?

So, which of these is the reason we got big brains? Well, some scientists went and compared the brain size – specifically the size of their neocortex relative to the rest of the brain – of living (or “extant”  in the aforementioned in fancy shmamcy science language) primates with the amount of fruit in their diet, the complexity of their toolkit and the amount of chums they hung out with.

The idea being that if the advantage conferred by a larger brain is the ability to eat more fruit, one would expect to see primates with see a bigger brain eating more fruit – the same for tool complexity, group size etc.

However, they found there was little correlation  between brain size and the amount of fruit in a primate’s diet, suggesting having  a bigger brain doesn’t enable an animal to eat more fruit. This doesn’t seem so surprising when one remembers other primates are already extraordinarily good at recalling the location of resources – even remembering when fruits are ripe. So, being better at fruit hunting doesn’t seem to be the advantage large brains gave our ancestors.

I'm not looking at funny monkey pictures, I'm doing science!

They also found there was no correlation between the complexity of a primate’s toolkit and their brain size. Having a bigger brain doesn’t mean your tools are going to be better. It might sound surprising that better tools aren’t the main advantage of better brains – humanity, after all, considers technology to be one of its strong points – until you take a deeper look at the fossil record.

By the time brain growth really got going, fairly sophisticated technology had already been developed: the first modified stone tools appeared ~2.6 million years ago but encephalisation didn’t start until ~2 million years ago. And then there’s everyone favourite hobbit: Homo flourisiensis. The diminutive cousin of humanity had a brain size similar to a chimp, yet was able to produce stone blades as complex as those being made by their big-brained counterparts during this period.

There was going to be a joke about the power of his stick, but it couldve been taken the wrong way.

They did, however, find a rather strong correlation between brain size and group size. In other words those with a larger brain had a larger group. This correlation is strong enough to suggest that there is a link – that having a bigger brain enables a bigger group. This is known as the “social brain hypothesis” and seems to be reason our brains are the size they are.

This is the graph from the journal. I don't know why there are random pictures of monkeys.

So, 3-2 million years ago our ancestors accrued a mutation (or some other change) which enabled their neocortex to grow a bit larger. This meant that they could remember more relationships, enabling them to have larger groups. These larger groups were advantageous, providing a better defence from predators, allowing them to better compete with other ape populations and to control a larger territory.

These benefits allowed our ancestors to be more reproductively successful, spreading their new genes throughout the population so all future generations had this advantage. The brain increased in size again at some point, it conferred the same advantages and so it was spread throughout the population again. This kept happening, resulting in a gradual growth in brain capacity over time until we wound up with the nogin we have today.

Janson, C.H. & Byrne, R., 2007. What wild primates know about resources: opening up the black box. Animal Cognition, 10(3), pp.357-367.
Robin Dunbar (1998). The social brain hypothesis Evolutionary Anthropology

9 thoughts on “Why our brains are big

  1. My question here is what size do they estimate for human groups when these brains emerged?
    In ‘Race and Human Evolution’, Milford Wolpoff & Rachel Caspari estimate the maximum Neanderthal population at any one time across Europe and the Middle East at 15,000 individuals. That is a very low density suggesting pretty small groups. J. Diamond in ‘Third Chimpanzee’ suggests that pre ‘modern’ humans had far smaller and wider displaced populations than ‘modern’ humans. Modern HG bands range about 20 – 35 adults, about 100 total individuals where chimp troops can number over 200. It looks like the larger groups they reference are far more modern than our brains.

    • Although individual hunter-gatherer bands are quite small, they make up a larger whole. The entire community could consist of several hundred individuals (possibly up to 10,000) although they would only aggregate once a year or so during particularly bountiful times (although they may meet up with other bands in smaller numbers more regularly). If we use the modern lifestyle as a model of the distant past then it would appear that large populations are a sufficiently old phenomenon (although nobody has calculated quite how large they were back then).

      Indeed, having to remember large groups even when you only see them once a year would require additional cognitive power and may be another factor which caused our brain to evolve.

      • I really don’t see the pieces fitting for this theory.
        I don’t think we can project the ‘modern’ potlatch or annual reunions into pre-modern behavior. Further I think they represent festivals of convenience and do not represent a competitive advantage (or have evolutionary ‘pressure’). It is my understanding that these get suspended if game is short or there is strife among the clans. The very times a competitive advantage is most useful, these meetings are not held. Their existence is also fairly random but, I think, concentrated in areas of low population stress. If it represented an advantage, the practice should have spread rapidly through the contiguous habitat. It may have required big brains to develop this habit, but it does not look like it holds enough advantage to drive the evolution of expensive big brains.
        Big brained AMH moved into Europe with their archaic behavior and competed evenly with the Neanderthal for 10,000 years. And the Neanderthal displayed only modest advantage over earlier hominins in their competing habitats. The first really pronounced ‘advantage’ came with ‘modern behavior’, not bigger brains.
        I’m sticking with sexual selection – chicks dig smart-alecky young men, and the more clever chicks can entice the most desirable mates. I credit women for driving the evolution of our intelligence.

        • However, the less elaborate forms of inter-group sharing do appear to act as risk reduction strategies. One of the more famous examples being the hxaro system found in Africa, although that is by no means the only one with many groups meeting up with others (and putting in extra effort to do so) in order to reinforce risk mitigation networks.

          In short, there’s plenty of evidence for this idea but crucially there’s none which indicates that it is the only factor involved in the evolution of encephalisation. There may have been other causes (I’ve mentioned elsewhere here that the “environmental complexity thesis” seems to hold water), even the influence of sexual selection. Assuming one has the evidence for these additional influences.

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