Our brain is kind of a big deal, so we’ve been trying to figure out how it got so big for ages. But what if we’ve been looking at it wrong by focusing on the brain? Perhaps its size is simply a side effect of other changes, like the fact that our ancestors became hairless.
The common ancestor of body parts
On the surface (pun intended) our skin and brain might sound totally unconnected. But roll the clock back and they share a common ancestor.
See, early human embryos are made up of 3 layers of cells. The outermost of these is called ectoderm, but memorising the name isn’t too important unless you want to sound smart at parties. What is important is that this layer goes on to become both the brain and the skin.
This connection led Denda et al.1 to wonder if there might actually be a link in their evolution as well. Could changes to the skin impact the brain? And if so, what change might have had a big impact?
Well, there’s been one fairly big change to our skin: it became hairless. Or, to be more technical, our hair became so thin and fine (on some of us more so than others) that we’re essentially hairless.
Denda et al. speculate that this exposed our skin to a whole bunch of new stimuli. In response, it had to get “smarter”, developing new nerve receptors to deal with all this input. This, in turn, might have driven brain growth. Partly to help deal with all this new information, and also perhaps partly because of their common ancestry in development. Changes in one part of the ectoderm might have had an impact elsewhere.
The bulk of their evidence for this hypothesis comes from the fact that our skin does have a lot of nerves and sensory stuff. But that isn’t their only line of evidence. They also point to the fact that other hairless animals are very smart1.
Many of these examples come from marine mammals, given that hair loss is a common adaptation to an aquatic existence. Whales, dolphins, and other cetaceans are all famously smart and famously hairless1. However, is there any indication that this is causation, not just correlation?
There are more clear-cut examples from other marine species. The elephantfish generates an electric field to map out its environment, even when the water is cloudy. Much of this sensing is done with the skin and accordingly, they have one of the bigger brains in the fish world to help deal with this extra stimuli. Similarly, octopi do all sorts of crazy things with their skin and have 8 “mini brains”, along with one fancy main brain, as a result1.
I am disappointed that we don’t have either of those abilities. Being able to sense my way to the toilet in the night with just electrical skin would be pretty great. But the fact we don’t have such exciting skin does raise questions about whether these marine examples are actually useful comparisons. If the skin has to be super crazy to drive brain development, then the fact ours is so bland blows this hypothesis out the water.
Still, this is an under-investigated aspect of our brain development. Probably because skin doesn’t fossilise. So investigations into this hypothesis may well yield some interesting results. But I’m not holding my breath that this will overturn everything we know about human brain evolution.
- Denda, M., Menon, G.K. and Elias, P.M., 2018. Did Hairlessness Stimulate an Increase in Hominin Brain Size. Insights from the Cutaneous Neurosensory Interface and Comparative Vertebrate Morphology. Anthropol, 6(199), pp.2332-0915.