Why Do Humans Grow Up So Slowly? Blame the Brain

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.

Comments

not so fast

[arth1]

That there is an inverse correlation between brain glucose use and body growth does not imply that the brain's use of glucose stymies the growth until later.
If that were the case, kids who are overfed carbohydrates would be smarter and taller, not fatter and dumber.

My guess is that slow growth is selected for because children who look like children enjoy special care and protection by adults. Growing to adult size by age 7 might be detrimental to survival.

Re:not so fast

[Beck_Neard]

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

[Xiver]

You can scream this at the top of your lungs until you are blue in the face, but it probably won't make a difference. Explaining it carefully might convince a few people, but not about things they already believe in. Its been said so much that you wouldn't even post it under your username, because you know it will get adversely moderated. People have read it so much they cringe when they see it.

You'll never be able to convince people that toasters don't cause suicidal tendencies in teenagers.

Re:not so fast

[Dahamma]

The evolutionary reality is even simpler that that (though the achievement of those is clearly not). The three main factors are:

1. be able to reproduce
2. be able to attract/acquire a mate
3. be able to care for/protect offspring long enough for them to reach #1

Clearly if it was just up #1 we would still be living alongside the rest of the primates. #2 can be a fairly complex social interaction - but insects are just as capable of it as humans. #3 is where the whole thing explodes, and is the key to investing all of those resources into the brain (and is what made it more evolutionarily advantageous to extend the time to #1 and #2).

Though of course in modern human society, social and technological advancement in #3 has so outpaced the first two that they barely seem to matter, and is why we are basically blowing past any "natural" population control. Our brains are letting us find clever ways of surviving and stripping the planet of resources, but unless we figure out a way to expand beyond the planet or stop using its finite resources we'll go through the same collapse seen in any other species going through a population explosion...

Re:not so fast

[Anonymous Coward]

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

[ShanghaiBill]

growing up fast is what should have been selected.

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. 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.

Re:not so fast

[Jane Q. Public]

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.

Critical Path

[Livius]

I would speculate that it's simply that, for humans in their eusocial foraging societies, brain development was the priority and there was no point in reaching sexual maturity and adulthood before the brain had developed and the individual had learned enough to be a full member of the community. The brain and the rest of the body are not competing for glucose, the brain is simply the critical path and the rest of the body has no need to develop faster.

Sperm to frogs

[tepples]

You have to clean out the pipes every once in a while before the sperm, well, mature.

Re:Sperm to frogs

[sillybilly]

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.

Humankind and eusociality

[tepples]

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:

• "cooperative brood care (including brood care of offspring from other individuals)": Daycare is a thing.
• "overlapping generations within a colony of adults": Grandparents are a thing.
• "a division of labor into reproductive and non-reproductive groups": Humankind appears to be moving in the direction of breeder vs. thinker classes. More affluent classes already tend to produce fewer children, and the public has become more accepting of a gay lifestyle. Furthermore, I've seen plenty of contempt for "breeders" and other childfree-by-choice advocacy on Slashdot.

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.