Naked Science Forum
Life Sciences => Physiology & Medicine => Topic started by: Eric A. Taylor on 29/03/2010 15:45:17
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Humans spend an awful long time unable to reproduce. If the point of an organism is to pass on it's genes then why do humans spend nearly 1/4 of their lives as sexually immature.
Girls don't usually star having periods until they reach the age of 11 or 12 and can't really safely bear a child until after 14 or 15 (I'm talking physical rather than emotional maturity here). Boys don't usually produce sperm until age 13 or 14. Add to that emotional maturity, most people don't get there until well after 20 years old.
Just looking at the physical side of maturity (around 15) this corresponds to over 20% of your life (if you live to 70) spent just growing up! 20 years is several times the entire life span of most other mammals. Humans take this long just to grow up. Does it take this long just for a child's brain to fully develop? If so how is this an evolutionary advantage? Really it seems to be a disadvantage. An organism that takes a long time to become capable of reproduction runs a much higher risk of being killed before they get there. Especially if that organism can't run very fast, isn't really very strong, has no sharp teeth or claws or hard shell to protect it.
Take away it's tools and humans don't do well with even fairly small predictors. Even a bobcat could take out a human if it was lucky, and send in a wolf pack and the human will be lunch for sure.
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The simple answer is that, yes, our brains take that long to mature, not least because they are not very big when we are born (a state of affairs for which our mothers are doubtless grateful).
As you say, we are not emotionally mature enough to raise kids until we are quite old and that's the reason.
On the other hand our massive brain power lest us address exactly the sort of problem you mention. A bobcat might take on a single human, but a single human is a bit of a rarity; we are social. Humans are generally within reach to help other humans and we are normally predisposed to help.
It's true that a single unarmed human would be a snack for a pack of wolves; but remember it's not humans who are listed as an endangered species. Humans are particularly good at being armed.
Incidentally, I understand that while humans are not the fastest runners or swimmers, we are the fastest runners that can swim well and the fastest swimmers that run well.
We are excellent generalists.
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I agree with BC. The only thing I would add is that we seem to have evolved in a way so that we are born with not many instinctive behaviours. (We even have some that disappear quite soon after we are born.)
This has the advantage that we can learn from our parents and family, and we, as a group, can change our behaviors to adapt to rapid changes in the environment. Our brains are less "hardwired" and more able to adapt. The cost, of course, is the size of our brains, and the time it takes to learn all the latest human survival information.
One example: Lots of young animals (deer for example) are able to get up and walk almost immediately from birth. They cannot be learning how to do it. It must be innate. Humans take about a year to learn how to walk. They are not physically capable of doing so at birth of course, but even if they were, I suspect it would still take them a long time to learn all the motor skills.
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I see your points. Humans are capable of running down any other animal in the world. The reason is simple. Normally when a prey animal sees a predator it will take off running as fast as it can for as long as it can. The predator will give chase also running as fast as it can. This is called anaerobic excessive (sprinting). Your body is working harder than it can maintain so fairly quickly the body runs out of oxygen and the muscles stop working. Usually this happens in the predator first so the predator needs to be more cunning in the way it approaches it's prey. The fastest land animals can run very fast but only for a few seconds. Humans can't run very fast at all but they can run in an aerobic fashion (jogging) which means they can keep running for hours. The prey runs away and gets to a spot where it can rest but very soon here comes the hunters again and it has to run away again before it's fully rested. Repeat this a few times and the prey becomes so tired you can walk up and have yourself a nice supper.
However humans haven't always been really smart. "Lucy" was not much smarter than a chimp yet she walked upright and would have been a slow runner as a result. How did she survive long enough to evolve into us?
Without the advanced intelligences to make weapons and plan hunts walking upright will get you killed. But without having your hands free it's hard to develop advanced intelligence.
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Well, I dunno! I'm not sure, but is it possible "Lucy" was part of a "dead end"?
I'm sure the whole process didn't happen quickly. For instance, chimpanzees exhibit similar learning skills to those of humans, and their babies take quite a while to develop, so they may be descendants of a common ancestor with humans, but they took a slightly different evolutionary path. That does not seem to be contradicted by genetic research.
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I have two suggestions to your question.
i read that our ancestors the homo erectus were also maturing slowly - and the proposed reason is the time it takes to learn to be independent. Complex society requires a long time to learn and you learn better if your teacher is larger than you - physically - it is easier to 'correct' the behavior of a student who is smaller than you.
The second point is about the age/size when organisms start to reproduce. In fish it was observed that is safe environment without much pressure of predation fish matured sexually at later age and larger body. If predation was introduced, very swiftly the age age and size of reproductive fish decreased. As humans are relatively safe - living in a clan/tribe with fire and weapons, there is no rush to start and reproduce.
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Well the question asked by you that why human get more time to become mature is good but I think that we have no answer for this question that why is this happens? You are right that every life other than human will become mature very early but as I think that we live more than them so that according to that time we become mature a little later.
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Good question.
Some humans mature early (c.f. precocious puberty), which puts them closer to the timescale of Chimpanzees. They usually have a smaller mature size.
So perhaps there is selective pressure to be bigger for either sexual selection or survival reasons, and the late sexual maturity just reflects how evolution solved the "make me bigger" problem.
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I think I have a good answer to this question.
The size of the human brain (or body) does not adequately explain the duration of human development. Blue whales, the largest animals that have ever inhabited the earth, take 5 years to reach sexual maturity (hunter-gatherer humans BTW actually take longer than moderns to reach sexual maturity, around 16 years). While whales don't have encephalization quotients as high as humans, their brains are much more massive than ours. That is to say that there is no fundamental reason in terms of tissue growth rates why our brains could not grow to mature size in a year or less.
It is certainly a disadvantage to have offspring remain helpless and dependent long after birth. So there must be a very good evolutionary reason why we develop so slowly.
My theory is similar to Yair Doza's. Our large brains exist for a purpose. That is to enable our species to utilize and transmit a very complex cultural legacy.
A Wildebeest's survival strategy is almost entirely instinctive. So it makes sense for a Wildebeest to be born on the run ready to engage in many of the same behaviors as an adult. The instinctive programming is already there. The Wildebeest's problem is merely one of calories. They need to consume X calories in order to scale their body to adult size. Period.
To be a productive member of a human group complex culturally determined behaviors are essential. The archeological and historical records clearly indicate that human groups have, for thousands of generations, had unique means of acquiring food, providing themselves with shelter, regulating mate selection, interacting with competitors, etc. Not to mention that human languages are unique to specific groups. The effect of this is that humans are necessarily born lacking most of the software they need in order to function. So the behavioral incompetence of human offspring is an inevitable product of our dependence on acquired culture rather than instinct.
Given this physical incompetence, it is easy to see the evolutionary justification for our delayed physical maturity. Cultural acquisition, unlike tissue growth, involves rate limiting factors on the right time-scale to be binding constraints on the possible speed of human development. If a human child grew to maturity in a couple of years, like, say, a water buffalo, they would still just be getting a grip on their native language, a sophisticated understanding of which is necessary for them to even begin to assimilate their broader cultural environment.
This would add a member (a big baby man-child) who, though contributing little or nothing to the industry of the group, would require an adult calorie budget. Clearly an inefficient arrangement. Instead a human infant is essentially a brain with eyes and ears that you can carry around in a sack while it learns the language. Around the time language begins to come online the child develops the muscular strength and coordination needed to use their miniature prototype body to practice the physical skills that will be required to implement the cultural programming they are now able to receive.
As, over the course of a decade, they slowly master their group's physical culture their body matures in step. The demands they put on the group are gradually surpassed by the contributions they are able to make. Only now, after years devoted to cultural acquisition, does it make sense for the individual to attain adult size. The hardware is deployed in sync with the requisite software.
What do you think?