Biology Magazine

Researchers Have Been Underestimating Human Brain Size

Posted on the 28 April 2016 by Reprieve @EvoAnth

One of the most impressive things about humans is our brain size. And now our noggin is about to get even more impressive. It turns out we've been underestimating brain size for years.

Now, of course we haven't been wrong about how big the brain is in absolute terms. It would be silly to not know that at this point in human history. The issue is how big our is brain compared to our body. After all, whales have even larger brains but they also have ruddy large bodies, so we still get to feel special.

If an animal has a brain larger than it should have, given its body size, it's "encephalised". Humans are one of the most encephalised animals on the planet. And it was this we were underestimating.

But perhaps more interestingly, this work also revealed our early ancestors were more encephalised than we thought. This early growth could shed light on why our brains began encephalising in the first place.

Math is hard, lets go shopping

There is a very strong link between brain size and body size; with bigger bodies producing bigger brains. There's some debate over why this happens. Some suggest it's because a bigger body needs more brain power to control. Others argue that neurons get bigger in bigger animals.

Regardless of the ultimate explanation, it's clear that this strong relationship exists. In fact, this relationship is so strong that the evolution of one of these traits can drive the development of another one. Dr Grabowski, a researcher from the Natural History Museum in New York, recently discovered that this explains why humans have one of the largest bodies of all primates (second only to gorillas). It's simply a by-product of our super big brains.

However, although our brain forced us to develop big bodies; the body didn't catch up completely. Once it started to get too large, evolution drove the body to remain the same size whilst our brain kept growing. Which is a shame because I always wanted to be a giant. Thus, despite the growth of our body we still have an unusually large brain. But just how encephalised are we?

It turns out this is a much harder question to answer than you'd think. Brains obviously very between individuals. How to take this into account when calculating the encephalisation of a species? Different groups of animals also seem to have slightly different brain/body relationships. So how you divide up animals also plays into it.

All of this means that we have a brain 3 - 8 times larger than it should be; depending on how you do your calculations. Which is a frustratingly large margin of error. And it also seems to have frustrated Dr Grabowski. Using the data from his previous research on body and brain size in human evolution (along with a plethora of previous research on encephalisation in primates) he sought to provide a final answer on just how big the human brain is supposed to be.

It turns out it's ruddy big.

How big is the human brain?

So here it is, the moment you've all be waiting for. The real amount the human family (and chimps) are encephalised.

Humans clock in nicely at the top of the chart with a brain 7.61 times bigger than it should be. Much bigger than the 6.28 figure previously used. So we get to feel great about ourselves.

However, these results do more than give us an ego boost. They also provide some very interesting insights into human evolution.

  • Homo habilis has a bigger brain than the Homo erectus it supposedly evolved into. For years many have been doubting the link between the two species. This seems to be a big nail the coffin.
  • A massive amount of brain growth occurred in Homo erectus. They really represent the dawn of modern humans.
  • There's a greater difference between humans and Neanderthals than previous research has reported. Maybe that's why they had to steal our technology

But perhaps the most significant finding is that even the earliest hominin looked at had a fairly big brain. Australopithecus afarensis is Lucy's species, and is often categorised as very ape-like. But with an encephalisation quotiant almost a full point higher than chimps', it seems like we've been mis-underestimating her.

So . . .

What kicked off brain size growth in Lucy?

A lot of factors seem to have driven the growth of brain size in the human family. Things like increasing sociality or tool use ( but not cooking). But which of these would have been influencing human evolution millions of years before the main bulk of encephalisation?

When picking out the top discoveries of 2015 I shunned the famous Homo naledi in favour of the Lomekwian industry. This set of rocks from Africa took top place because - at 3.3 million years old - they represented the oldest stone tools ever found. Stone tools whose creation curiously coincides with this spike in encephalisation seen in another 3.3 million year old find: Lucy. Could it be that tool use is what kicked off the growth of the human brain?

Another, seemingly unrelated paper, came out around the same time as Dr Grabowski's work. It reveals that there is indeed a link between brain size and tool use in primates.

Well, not tool use specifically. Rather, how the "complexity of the manipulation involved in getting food." Or in slightly less pretentious terms: how complex was the hand movements involved in getting food. That - rather than the type of food itself - was correlated with brain size in primates. As an interesting aside, I find the lack of correlation with food type fascinating. It means an animal which cracks open a nut will wind up with a larger brain than one which can just swallow it whole.

Tool use can involve a lot of complex manipulation. Could the manufacture and use of the Lomekwian be what drove this early increase in brain size?

A bit more work is needed first. With only one Lomekwian site found its hard to say just how complex it really was. And the aforementioned research on manipulation didn't focus on tool use. Someone needs to check that the correlation still holds if the complexity is in the form manipulating tools. Still, many pieces are starting to slot into place. And I can feel justified for giving the Lomekwian such a high ranking in 2015.

All of this means that the most important moment in human history wasn't anything dramatic. It was a group of apes in Africa realising that broken rocks were sharp.

A seemingly insignificant discovery that paved the way for the most significant species of all.


How big a brain is, relative to the body, is something of evolutionary significance. It turns out we've been under-estimating the brain size of humans. Recalculating it reveals some interesting things about our ancestors. Like the fact brain growth began early in our evolution; perhaps due to tool use.


Grabowski, M., Costa, B., Rossoni, D., Marroig, G., DeSilva, J., Herculano-Houzel, S., Neubauer, S. and Grabowski, M., 2016. From Bigger Brains to Bigger Bodies: The Correlated Evolution of Human Brain and Body Size.Current Anthropology, 57(2), pp.000-000.

Grabowski, M., Voje, K.L. and Hansen, T.F., 2016. Evolutionary modeling and correcting for observation error support a 3/5 brain-body allometry for primates. Journal of Human Evolution, 94, pp.106-116.

Heldstab, S.A., Kosonen, Z.K., Koski, S.E., Burkart, J.M., van Schaik, C.P. and Isler, K., 2016. Manipulation complexity in primates coevolved with brain size and terrestriality.Scientific Reports, 6.

Photo by Ryan Somma
Researchers have been underestimating human brain size

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