Question: How has the brain evolved since life began?

Ben Brilot answered on 12 Mar 2013:

I agree completely with what James says: that’s a great question, but a really difficult one to answer.

So if you really want to start with “since life began”: in the beginning there was no brain, because life only existed in the form of single cells. You can only really start to develop a brain once you have lots of cells in your body, because some of those cells have to be specialised into doing the ‘thinking’ bit. Once an organism has lots of cells, then some of those cells can take on particular roles: some will form a protective outside layer (much like your skin), some will be very good at digesting food (like your stomach and gut cells) and others will be good at monitoring the outside world (maybe they respond to rapid changes in light or temperature). The last of those effectively become nerve cells: they respond to changes in the world (both outside of the body and inside the body) and pass on that information to other cells.

The most basic animals have networks of nerve cells spread across their body. So they can tell, for example, if another animal has just touched them on a particular part of their body and then those nerves send signals across the network that say ‘move away from this other animal’ for example. Things start to get interesting with what’s called ‘encephalization’. Some animals started to evolve a network where lots of nerves are concentrated in their front end: what you might call their ‘head’. Those cells can become specialised in responding not to outside information, but information from other nerve cells. They can start to become good at taking lots of different messages and combining them into a bigger picture of what the world is like. For instance, one ‘eye’ nerve cell might be able to tell that its little bit of the world just got darker, and the next one might then register the same thing a moment later. You need ‘brain’ cells to take those two bits of information and put them together to say: that’s a shadow moving across my eye, and it might well be a bird that’s flying down to eat me, I’d better do something about it.

So once you get to this level, bits of the brain specialise for taking information from different areas of the body and others for putting all this information together: so part of your brain receives information from the eye, and then passes it on to other parts of the brain that might need that information. Different animal brains then evolve completely differently according to what the animal needs to be able to do: if you rely on your eyesight a lot (like birds need to, think about flying fast through woodlands), that bit of the brain will evolve to be larger, since more nerve cells are needed to process lots of information. If you live underground, then the vision part of the brain isn’t much use to you, but touch might be really important, so the vision part gets smaller, the touch part gets bigger.
One of my favourite animals is a classic example of this: the star-nosed mole:
http://www.bbc.co.uk/nature/life/Star-nosed_Mole
http://scienceblogs.com/neurophilosophy/2009/08/26/the-star-nosed-moles-amazing-appendages/.

Anyway, coming finally to humans. We have pretty big brains compared to other animals, but in particular we have a very large part of the brain called the neocortex. This is the bit of the brain that seems to be involved with ‘complex’ thinking: things like language and reasoning. No one yet knows why this bit of our brain has become so big, but there are lots of different theories. One of them argues that we evolved in very complicated societies with lots of people, especially compared to other animals. So in order to keep track of who was who and what relationship you have with each person the neocortex evolved to be very large. As ever in science, not everyone agrees with this idea.

Anyway, that’s a very, very quick and rough idea. As James says, there’s an awful lot more to it than that, but I hope this bit was interesting.