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. 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.
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.
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.
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.
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|