How our brains didn’t get big

ResearchBlogging.orgThe social brain hypothesis suggests that our brains, specifically our neocortex, grew larger because it allowed us to remember more relationships and thus live in larger groups which gave us an evolutionary advantage. The evidence for this is rather strong, allowing us to put one piece of the puzzle firmly in place.

But alas, one piece is not a complete picture and so scientists – being the glutton for mystery they are – began to ponder how our brain got so big. And from this hard-core thinking the expensive tissue hypothesis was born.

Hypothesis birth is the antithesis to child birth: your hypothesis could actually make you money and nobody cares if you kill it with a crossbow

The expensive tissue hypothesis seeks to explain how our brains got so big in light of the fact that they’re very energetically expensive to maintain (see what they did there?) yet we don’t use any more energy whilst resting than chimps do.

So, they reasoned, perhaps there is a trade-off. Maybe one part of our body decreased its energy consumption costs in order to allow our brain to gobble up more of our energy.  After a bit of further research, they concluded that it was the digestive system which had fallen on this evolutionary grenade to allow our brains to grow big.

This idea makes sense, particularly if one remembers that when people started preparing food they wouldn’t need such large guts as some of their job was already being done by cooking etc.

So the idea that there was an evolutionary trade-off between our guts and our brains became widely accepted in the EvoAnth community. This support was bolstered by research which suggested that the gut reduction/brain growth also happened in other animals.

But this is not the end of our story. Not content to leave well enough alone, some researchers did a more thorough study of whether the expensive tissue trade-off was present in other species.

"People" in this context means "those who accept the expensive tissue hypothesis"

They analysed the size of various body organs in over 100 mammal species, including non-human apes, and compared them to the size of those animals’ brains. Their results showed that, contrary to what the expensive tissue hypothesis predicts, there is no correlation between brain size and the size of other organs.

This, the scientists argue, would suggest that the hypothesised trade-off does not occur and the expensive tissue hypothesis should be dropped as an explanation for how our brains got big.

Of course, their argument is predicated on the validity of the data they’ve gathered. And it does seem to be rather valid. They gathered results from a large range of animals – enough to ensure their results were statistically significant – yet were still picky about the animals they measured, not bothering with specimens which had been stored in alcohol or other liquids that might influence organ size.

Most importantly, they also controlled for fat-deposits, which introduce a lot of variation between individuals of the same species. Thus they could be sure when they controlled for body mass, they were controlling for the size of the species and not the size of a particularly obese individual.

Lines between organs means there's a correlation. A + or - next to the line means it was positive or negative. The thicker the line, the better the correlation.

So, the research is valid which is pretty strong evidence the expensive tissue hypothesis is not. Unless, of course, our lineage is the only one to have made the trade-off, but then that’s a bit too close to special pleading for my taste.

But the story still doesn’t end there.

No, that would be too simple. The research did find that there was a negative correlation between adipose (fat) deposits and brain size (which is highlighted handily in red on the crazy line chart above). So, perhaps there is a trade-off after all, but instead of being between gut size and brain size, it is between brain size and fat deposit size.

There's a lot of funny colours, shapes and words on this chart. All you have to know is there's a line and that means there's a correlation

Given the strength of the research, it seems likely the adipose/brain correlation is valid but whether that means there is an evolutionary trade off going on is another question. Since they observe that modern humans don’t have such a correlation then it seems that it might not be the case, but there is something here worth investigating.

So like all good refutations, it closes one door but opens another.

Navarrete, A., van Schaik, C., & Isler, K. (2011). Energetics and the evolution of human brain size Nature, 480 (7375), 91-93 DOI: 10.1038/nature10629

6 thoughts on “How our brains didn’t get big

  1. Ok so I have two questions:

    1 – when you say they have the same energy expenditure, was that whole body expenditure or just nervous expenditure?

    2 – Is the human brain more folded than that of other primates?

    As far as I know there are a lot of arguments going around as to why our brains are more capable and they usually amount to size, surface area or weight of the brain in relation to the body mass, nervous energy expenditure or the networking within the brain itself. I haven’t seen many clear cut answers though yet, this is probably the closest I have come!

  2. 1: “relative whole-body energy consumption rates of individuals at rest are about equal in the two species” or, if you want it said non-sciencey “chimp bodies and human bodies consume the same amount of energy when they are resting (after consumption rates are adjusted for body size).”

    2. It does appear to be, although I do not know if this is simply an artefact of larger brains (a bigger brain will naturally have more/deeper folds even if they are relatively the same size as a smaller brain).

    3. I have another post on the issue 😉

  3. I always have difficulty reading statements that explain why/how a certain trait evolved. Take for instance your first sentence, “The social brain hypothesis suggests that our brains, specifically our neocortex, grew larger because it allowed us to remember more relationships and thus live in larger groups which gave us an evolutionary advantage.”

    Is it that our brains grew larger to allow for more complex relationships or was there a pressure that favored remembering relationships and living in larger groups that selected for larger brains?

    • For the longest time I also had difficulty comprehending which way round it went. Basically:

      Living in larger groups is advantageous.

      Ape x has a mutation that allows him to live in larger groups (in this case a larger brain).

      Thus Ape x’s mutation is beneficial and will become fixed in the population.

  4. Pingback: Earth’s orbit and human evolution | EvoAnth

  5. Pingback: Our big brains evolved to make tools | EvoAnth

You evolved too. Have a say.

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s