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In biology I've learned that cells rapidly divide and can grow and split undefintely, and that certain parts of the body have to grow and evolve before growing, but I am tied up on the fact that the body produces hormones to have only grow up to a few feet tall and stops growing at a certain age.

Why does it stop in terms of height or physical mass when it can still keep on growing? Why do the hormones tell the body to stop growing when it can continue?

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  • $\begingroup$ Welcome to Biology.SE. Can you please clarify certain parts of the body have to grow and evolve before growing $\endgroup$ – Remi.b May 29 '18 at 2:14
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    $\begingroup$ Is there any reason you phrased your question as being specific to humans? You seem to assume "bigger is better" for humans. Would you assume the same for other species as well? $\endgroup$ – Remi.b May 29 '18 at 2:15
  • $\begingroup$ “Can grow and split [indefinitely]”, this is incorrect for normal cells due to something called the Hayflick limit. This statement isn’t essential to your question, but I felt like it needed to be clarified. $\endgroup$ – Hawkeye May 29 '18 at 22:05
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The question is a bit vague but I will take it to mean the following

Why does it (the body) stop in terms of height or physical mass when it can still keep on growing?

The answer is physics, specifically the ability for a body of a specific shape and structure to support itself and move itself, followed by the energy requirement to feed all those cells.

Strength of a bone scales with its cross sectional area, similar to muscle. Mean while mass scales with its volume. As a result, mass increases faster than bone strength. So the bigger your are, the weaker your bones become if all proportions are maintained. As a result for a certain design of animal there is an upper limit of how big it can become before it unable to support its own weight or able to move.

Next comes energy requirement. In general the more living bio mass you have, the more energy you require to keep it alive. The scaling isn't proportional but there is a general trend. And the more energy required the more food is needed, and given the habitat an animal lives in, there is only so much food around. And this places a limit on how big an animal can grow. An animal living in its niche has an energy budget.

Associated with energy budget, we meet biology and natural selection. Energy placed into growing, isn't energy placed into reproducing. All animals eventually die, either by predator or bad climate (winter, drought, even bad luck) . So it becomes a balance, given the amount of energy available, how much energy should be put into growth to give a more robust structure to survive vs how much energy should be put into reproduction so that the organism can multiply.

Put too much energy into growing bigger, your probability of dying before reproducing increases as you take too much time growing before becoming sexually mature. Put too much energy into reproducing, upon sexual maturity your body is small and weak and your die before you spend much time reproducing.

The ultimate right answer is dependent on the animal's niche, the physical environment around it, its biology, and the predators around it.

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  • $\begingroup$ Your third reason seems to me more relevant. Arguably we should look for the first clue in reproductive success. After all we grow until... maturity. Bones could grow stronger, larger bodies can gather more food. Contra, we could carry on growing stronger to protect our progeny. Socially though, we reach maximum power when we have to pay for the tuition fees. $\endgroup$ – Alain Pannetier May 29 '18 at 4:32
  • $\begingroup$ I'm not sure this answers what the OP is asking. Granted that the question is far from clear, but I took it to be asking what mechanism the body uses to control size. For instance, in Africa we can find ethnic groups with an average (male) height of 1.45 m ("pygmys"), other groups, other groups that average 1.9 m. And of course we can breed dogs from chihuahua to Irish wolfhound, horses from minis to drafts, & likewise for most domesticated animals (and plants, e.g. miniature roses). So what are the biological pathways that regulate size? $\endgroup$ – jamesqf May 29 '18 at 5:04
  • $\begingroup$ Thank you for the answer, it's answered and clarified my question; but, going along the thought of limited resources/energy, say that someone lives in some sort of "utopia" where resources are abundant, there are no environmental threats, no need for sexual reproduction, and some sort of evolutionary design that can support the body regardless of how large it is. Although this thought may be very abstract and impossible to happen, if such could, should a person be able to grow "indefinitely" (or very large)? $\endgroup$ – Randy Huang May 29 '18 at 22:54
  • $\begingroup$ I would expect that organism to become very large and very old. I have listed two real world examples. These is a honey fungus colony that spanning 8.9 square km with an estimated weight of 605 tons, and about 2400 years old. There is also Pando a clonal colony of an individual male quaking aspen, about 6600 tons in mass and 10,000 year old, although some sections may be 1 million years old. en.wikipedia.org/wiki/Largest_organisms> $\endgroup$ – JayCkat May 31 '18 at 9:13

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