That 10% of your Brain Claim

While I was watching TV the other day, a trailer for the film Lucy came on.  I'm a big Sci-Fi and Fantasy fan, so normally something like this would be right up my street, but the entire premise of this film annoys me.

One of the defining features of Science Fiction is that it is meant to be based on Science fact and extrapolated.  To quote Robert A. Heinlein: 

"a handy short definition of almost all science fiction might read: realistic speculation about possible future events, based solidly on adequate knowledge of the real world, past and present, and on a thorough understanding of the nature and significance of the scientific method."

 This science fiction film is based on a scientific myth, and more annoyingly from my perspective, a myth that is related to my field.

So in case you haven't seen the trailer yet, Lucy is based on the idea that humans only use 10% of their brains.  It's a myth (with sometimes varying levels of brain use) that's been going around for at least 100 years and there are various origin stories.  It's not entirely clear how the myth started, but what is clear is that it is a myth.  Neuroscientists have refuted it from a neuroscience perspective, but here I'd like to talk about why it's ridiculous from a evolutionary point.

There are plenty of occasions where there are parts of humans which are not very (or at all useful). Working on Transposons, I am well aware that much of our DNA, for example, is useless.  However, for something that is not useful to stick around, it means that it needs to be less (or the same level of) harmful to keep it, than it is to get rid of it.  In other words, we can have useless neutral traits, but if the trait is detrimental, organisms without it are likely to have a higher level of fitness than those with it.

Only using 10% of our brains is very unlikely to fall into this neutral category.  One of the main effects of human brain size is that we need to have a large skull to fit it in.  The size of the human skull versus the size of the human pelvis is a big reason for the short length of gestation time we have compared to other large mammals.  This means that our infants are helpless when born and need a lot more care and energy expended on looking after them.  The large skull size also leads to mothers and children dying in childbirth, sure we have c-sections now and a lot less deaths, but in evolutionary terms, that's so recent to not have an effect.

Another reason a 90% useless brain would be unlikely to develop is that it's a really expensive organ.  It uses huge amounts of energy and oxygen, which would be a massive waste of resources.  An individual with a smaller brain that was utilized more (giving the same amount of brain power) would almost definitely be able to out compete it. 

So I'm unlikely to go and see Lucy, because despite being an expert at suspension of disbelief, this is all just a step too far for me.