Wednesday, November 19, 2008

The Brain and Our Extended Childhood

Most humans reach their maximal height by age 18. Many other species, including mammals like mice and dogs, would have long died before reaching 18 years old. And yet we humans take so long to reach our full physical maturity. What, one might wonder, is the evolutionary payoff of such a prolonged childhood?

The answer is in our head, literally. It is our big brain, the most incredible of organs. Your brain weighs only around 3lbs, which might not sound that big. But when you compare the size of the human brain to that of much larger mammals, like the elephant and whale, you realize how large our brain is relative to our overall body size.

Growing large brains requires a lot of energy. Humans could not have grown such large brains without spending such a long time in childhood and adolescence. Rather than rush to attain our full adult height, we invest more of our energies into growing our complex brains first. The latest Science podcast has a fascinating discussion of the evolution of childhood, and the paper is here. Here is a sample from the podcast:

Interviewer– Robert Frederick
So, with all that evidence, over what time period did our species evolve – from one that lived fast and died young, to one that lived slow and died old?

Interviewee – Ann Gibbons
Well, they’re closing in on the time and place. It looks like Homo erectus was living a much slower life. It’s interesting, Homo erectus looks like us– it’s tall, strapping, beginning to get a bigger brain at 1.8 million years, 1.5 million years– but it’s growing up like a chimpanzee. So, that means it had a very different family and social structure. It was reproducing more around age twelve, thirteen, rather than around nineteen. So, Homo erectus was not fully in place in a modern way; it was beginning to transition to a longer childhood though. And, then, we know that by the time our species appeared, 200,000 years ago in Africa, that the fully modern pattern was in place 180,000 years ago. So, the window of time has been narrowed to sometime between about 1.2, 1.5 million years ago and 200,000 years ago – when this strategy evolved in our ancestors. So, researchers are teasing that out, not just with fossil teeth but also reconstructions of the brain size – that’s an indirect measure because a large brain, while it doesn’t drive the long childhood, there has to be a long childhood for it to get so big.

Interviewer – Robert Frederick
Interviewee – Ann Gibbons
Because the brain actually – it’s very interesting. This is kind of complex, but it’s fascinating. Brains grow up very rapidly; however, to be able to handle the energetic burden for that brain to get so big, the body is small – it stays small in humans until we have that adolescent growth spurt. We stay small much longer. Partly, the thinking is to be able to balance this whole energy budget – you can’t be growing everything up at the same time – it’s just too expensive for the mother, it requires too many calories – she’s not able to support that, nor is the baby able to get that any calories. So, the brain goes up first, then the body has to catch up – and then, of course, there’s integration of the forebrain and other kinds of complex stuff that’s happening in the brain, later. So, it seems that the brain took advantage of this life history strategy – that we evolved a long life history strategy first for reproductive reasons, and that just set a nice template upon which the brain was able to get larger.