Smart evolution.
Question: Over the past few hundred thousand years, humans have greatly increased in intelligence and in the size of their brains. As intelligence appears to produce an advantage to survival, I wonder if all animals show a tendency to evolve greater intelligence over time. Or is the value of intelligence overrated?
ANSWER: The conventional answer is actually rather dull: animals are as intelligent as they need to be and no more. This is because brains are expensive to run-your brain uses up 19 per cent of your body鈥檚 total energy consumption-so any animal with a large brain has to justify the cost. For example, over the past few hundred thousand years, cheetahs and antelopes have been locked in a constant arms race but neither has chosen to evolve a bigger brain or, presumably, greater intelligence. Even though it is one option open to their evolution, and hence their ability to capture or evade the other, evolution has selected not to exercise it.
Yet there does appear to have been an increase in intelligence over the last few hundred million years. Mammals are more intelligent than fish, fish are more intelligent than crustaceans. Why is this?
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Bigger animals have bigger brains with more neurons. More neurons are needed to control their larger muscle mass and to collect data from their extra sensors. Among mammals, the equation is roughly: brain mass = 0.122 脳 (body mass)2/3, where masses are in grams. It seems that one side effect of having a bigger brain is to be more intelligent. Perhaps this is because the number of possible interconnections between neurons rises as the square of the number of neurons.
So it seems that evolution has led to increasing intelligence simply because it has led to increasing body size. Vertebrates can be bigger than insects and nematodes because, among other adaptations, they have internal skeletons and oxygen-carrying blood. Greater intelligence is a by-product. Yet, in spite of these considerations, it is clear that humans have a bigger brain than would be expected for a mammal of our size.
The traditional explanation is that our greatly increased intelligence gave humans a survival advantage, but there鈥檚 little evidence for that. If extra intelligence helped our ancestors survive, then we would expect evidence in the form of increasingly sophisticated tools. That doesn鈥檛 seem to have happened. Homo habilis鈥檚 tools stayed exactly the same for a million years, then Homo erectus鈥檚 tools stayed the same for another million years.
One theory says that unusually high intelligence aids our survival as much as a big tail aids a peacock鈥檚 survival. It costs a lot to maintain and so reduced our ancestors鈥� life expectancy. But like a peacock鈥檚 tail it made us sexier, and the sexiest is, of course, the most likely to reproduce. We chat each other up. We write sonnets. We tell jokes (scan any personal column to see the importance of GSOH).
Sexual selection is arbitrary: the object of desire can be bright scales, large horns or a big brain. And the positive feedback of sexual selection leads to fast evolution and great exaggeration of the chosen characteristic.
Peter Balch
Edinburgh
Although it is true that you need a large body to support a large brain, the two don鈥檛 necessarily go together-witness the dinosaurs, many of which had small brains and huge bodies. But, of course, they were very successful for much longer than humans are likely to be- Ed
Answer: Although many animals have evolved quite a high level of intelligence, it is by no means a desirable feature for all species to possess.
Animals have a limited supply of energy, so they have to choose how it is allocated. Two major uses of energy are brain power and reproduction. Humans and many other intelligent species have only a few offspring and look after them very carefully, which is partly thanks to intelligence. In contrast, insects, for example, tend to produce millions of eggs in the hope that a few will survive. This is very costly in terms of energy and any evolutionary trend towards having a larger brain would mean that they would have to reduce the number of eggs they produce.
So there are alternative ways of living-being intelligent with a low reproductive rate, having a simple brain and a high reproductive rate, or somewhere in between. Therefore it is simplistic and a little arrogant for humans to assume that intelligence is the be-all and end-all of evolution.
Andrew Pine
Cambridge
Dreamtime
Question: Do people who were born blind dream? And if they do, do they 鈥渟ee鈥� in their dreams?
Answer: People who are born blind certainly do dream. However, people who are born blind or become blind before the age of five do not 鈥渟ee鈥� in their dreams, though their dreams are as rich in narrative and detail as those of sighted people.
People who lose their sight after the age of seven can still have very vivid visual imagery in their dreams 20 to 30 years after the loss of vision. Between the ages of five and seven there is a bit of a grey area.
Rapid eye movements, or REMs-a series of jerky movements of the sleeper鈥檚 eyes-accompany dreaming sleep. The eyes move as if the sleeper is watching the scenes about which they are dreaming. However, people with congenital blindness, and those who lose their sight before the age of five, may lack REMs, or if they have them they are generally of low amplitude. Those who lose their sight after the age of seven continue to have REMs during dreaming.
As one might expect, congenitally blind dreamers report auditory imagery as being more vivid than sighted dreamers do. Interestingly, deaf people dream in sign language and report more vivid colours in their dreams than hearing individuals. Finally, blind people tend to dream more about familiar situations and less about novel ones than do sighted people.
Hugh Selsick
London
and
Fiona Baker
Wits Sleep Laboratory, Johannesburg
This week鈥檚 questions
Rainy days: Have you ever noticed that when it鈥檚 raining there is sometimes one 45掳 rivulet running from one corner of the window to the other, even if all the rest are running downwards and there is no wind. If a raindrop hits the rivulet, it flows with it. Why does this happen?
Jack Sedman (aged 12)
Darlington, County Durham
Fighting dirty: Why do most natural antibiotics come from soil fungi and bacteria? Why do they produce them?
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