Why Do Humans Grow Up So Slowly? Blame the Brain 128
sciencehabit (1205606) writes Humans are late bloomers when compared with other primates — they spend almost twice as long in childhood and adolescence as chimps, gibbons, or macaques do. But why? One widely accepted but hard-to-test theory is that children's brains consume so much energy that they divert glucose from the rest of the body, slowing growth. Now, a clever study of glucose uptake and body growth in children confirms this 'expensive tissue' hypothesis.
Re:not so fast (Score:5, Interesting)
Yup, and what makes it even more rubbish is the idea that simply feeding someone more food is enough to change their biochemistry, metabolism, and energy distribution budget towards diverting more energy towards growth and less towards the brain, and that blood glucose levels are determined by dietary carbohydrates.
But I do enjoy reading the pseudo-intellectual armchair philosophizing that we see so often.
Re:not so fast (Score:4, Interesting)
Nobody should be allowed to comment on genetics or evolution until they've read The Selfish Gene. While some small parts of it are arguably out-dated, it really helps orient one's mindset regarding evolutionary genetics. The Selfish Gene will survive as an extant and useful work much longer than Darwin's On the Origin of Species.
Even many biologists should read it. Too many biologists lack rigor when they hypothesize about evolutionary behavior. The Selfish Gene really lays out not only what has been effectively proven about evolutionary genetics, but provides examples of the complex but elegant mechanisms that _new_ evolutionary processes (e.g. group selection) will probably also look like if they can ever be proven.
Re:not so fast (Score:5, Interesting)
Not necessarily. The major threat to children in primitive hunter gatherer societies is not predators but hunger. By staying smaller during their formative years, they reduce the amount of calories need to survive.
This. Also, it takes time to learn the vast amount of information that it takes for a human being to really be smart enough to manipulate its environment... which evolution has obviously selected for. Chimps, for example, often actually outpace human learning for up to 2 years, but then humans continue to learn while the chimp rapidly levels off. Keeping resource use to a low level during this long learning phase is likely a long-term survival trait.
Also it should be noted that another factor in humans' slow growth is already known: humans can only have babies with brains so big, before birth becomes a very big problem. So a longer period is needed for the human brain to grow to its adult size.
But the selection pressures are different on boys and girls. Girls are generally able to procreate as soon as they reach puberty. But boys need to wait till they are older, and have built up social status. So it makes sense for girls to mature faster, and that is what happens. Look at a group of kids in 4th or 5th grade, and the girls are several inches taller than the boys.
It is more accurate to say that boys and girls mature at different rates.
If you adjust for the probable influence of estrogen mimics in our current environment, human females start to mature sexually before males do, but actually finish their sexual maturation later. You are referring more to social factors than genetic: often males need to be older to establish themselves in order to semi-permanently mate, but that is not the same things as physical sexual maturity needed to procreate.
Re:not so fast (Score:2, Interesting)
RE "Correlation does not imply causation!"
You are wrong. Correlation does indeed imply causation.
If you had said "Correlation does not prove causation!", then I would say you are correct.
I'm really not buying it (Score:4, Interesting)
For most species, childhood is all risk, no benefit (where benefit = breeding), and so it is to be got through as fast as possible (or at least in time for next breeding season). If glucose shortage was the only reason for doubling the length of our childhood, there would be a huge evolutionary pressure towards kids who could metabolize much more food and reach adulthood in half the time.
There is an obvious reason why humans have such a long childhood - it is because we have so very much to learn. Little bodies can learn as well as big bodies, and cost less to maintain.
Re:not so fast (Score:4, Interesting)
You are referring more to social factors than genetic
These are not separate factors. Genes influence social behavior, and social behavior influences which genes are selected. In ALL human societies, men prefer women younger than themselves that are physically attractive, which correlates with fertility. In ALL human societies, women prefer men with high social status, and greater resources. It is unlikely that such universally pervasive preferences are purely "social" rather than genetically innate. Chimpanzee males have no preference for younger females, and when given a choice of mates, will prefer older females. Female chimps do not have the same decline in fertility with age that women have, and more mature and experienced females have a greater chance of successfully rearing offspring.
Humankind and eusociality (Score:5, Interesting)
humans are not an eusocial species.
I decided to fact check this claim. Eusociality, according to Wikipedia and the references it cites, is defined as three aspects of the behavior of a species:
I agree that humans are not as close to the eusocial ideal as bees and mole rats, but we're closer than a lot of other species.
Re:Sperm to frogs (Score:5, Interesting)
Lifespan among other mammals is usually short because the required training to make it is not that complicated, unlike in a complex human or even great ape society. By the way the life expectancy of most cavemen was less than 40 years, and compared to horses and elephants, it's not that long. Only in recent times through agricultural and technological advances and good life has life expectancy increased. So this ultra life expectancy of 80 years may not be long because the brain requires it or demands it, but more like the brain allows it, so why not? Having great-grandmothers, grandmothers mothers and daughters together in a village, usually makes for a more successful village where members proliferate marrying into other villages taking their customs of sticking together through the long generations, and having long generations, compared to short lifespan mother-child only structures usually found in the wild, where the grandmother and great grandmother don't participate, and don't make a difference whether they still exist or not.
Re:not so fast (Score:4, Interesting)
You'll never be able to convince people that toasters don't cause suicidal tendencies in teenagers.
Depends on the toaster, wouldn't you agree? I have had toasters that made me want to kill whoever sold it to me.
I think, if we take away the hype and the misunderstandins on the part of the article, that what we have here is an interesting observation that does support the theory that brain-growth may be one of the factors determining when we become adults. I don't think it is true, though; it seems to me that the biggest evolutionary advantage we have is, in fact, the prolonged period of brain development and plasticity and the evolution of the family unit that supports a long childhood; this, incidentally, includes the fact that we, as the only species I know of, also live long after reproduction. Having grand-parents who can pass their experience on to the youngest, seems like a huge advantage to me.