How does the human body know when to start puberty? Or when to stop it?
What triggers the body to all of a sudden start producing more hormones and start what seems to be a very different sequence of chemical reactions than it had been doing?
For that matter, what triggers cell specialization in a growing fetus? Once an egg is fertilized and start dividing, the first groups of cells must all be identical. What changes in order for specialization go begin?
Just a guess, but: millions of years of evolution?
That’s an answer? 
When I was in school for education, we were told that this is mainly a mystery. However, weight seems to play a role. Just over 100 lbs in girls tended to kick it in, while more like 120 lbs. seemed to kick it in for boys.
Of course, I’ve seen some really fat little kids who must weigh this much and they were not in puberty. I’ve also seen some very light women who are under 100 lbs and have gone through puberty.
I think it really remains a partial mystery.
Yes, evolution selected those whose genes were such that they go through life stages. But what triggers the pituitary gland to ramp up production when a person reaches 11 or 12 years of age? Are the genes keeping count of the years?
To me, saying “evolution did it” is no better than saying “God did it”, which is hogwash (the God part).
How would the testes know how much someone weighs?
Weight might explain puberty, but can it explain cell specialization? After a certain number of divisions the cell then specializes?
It’s simplistic, but that’s what the scientific answers will boil down to. Your body responds to millions of years of genetic code selection and mutations. How else would you explain sexual maturation and drive?
OK, but can we have a real answer please? Isn’t DNA surrounded by ring-like structures which twist like the wheels on a combination lock, and it’s the position of these rings at any given time which determines genetic expression?
Evolution might explain the reason the human species developed this mechanism. It doesn’t explain what the mechanism is, that’s what the OP is after. I agree that evolution is ducking the real question.
Last time I checked, the answer was ‘Nobody knows for sure’. As knowing how and why cells specialize would mean all kinds of advances in stem cell research, we can be sure that many scientists are working on finding a better answer.
BTW
Whatever it is that starts puberty does sometimes go wrong. Precocious puberty has been documented in infants.
I really meant in the average person. I know systems can and will go very wrong and cause rapid aging or other issues.
Is there really no understanding of what triggers the onset of things such as puberty or menopause? Interesting. Obviously we all have an internal “clock” of some sort. I thought perhaps some cells have only so many times they can divide or maybe there is just a slow buildup of some hormone in the body which triggers something else once it reaches a critical level, kind of like how high blood sugar will trigger a release of insulin and low blood sugar triggers glycogen release. But insulin release requires an obvious change in internal chemistry. Puberty seems to require only time for normal people.
Welcome to the wonderful world of the Pituitary Gland!
cue applause
The pituitary is often called the “Master Gland”, because it either makes or triggers the production of a huge range of human hormones. In particular, things like Luteinizing hormone and Follicle stimulating hormone which trigger ovulation and menstruation in women, and Thyroid Stimulating hormone and Growth hormone, which regulate skeletal growth and metabolism. The pituitary also releases melanocyte simulating hormones, which affect sex drive and appetite, as well as skin color. Um…what’d I leave out? Oh, testosterone, of course - while it’s mostly produced in the testes, it’s triggered by hormones released by the pituitary and the hypothalamus.
So what triggers the pituitary to do its thing and shift the hormonal cocktail at certain ages to create certain effects? Er…um…well…that’s the part we’re not so sure about.
One theory (not proven, but a theory) of *female *reproductive maturity is that a certain level, or perhaps ratio, of estrogen must be present in her body before her pituitary starts pumping out “puberty now!” hormones. To anthropomorphize a bit: Nature can “tell” when a girl is healthy and large enough to start having babies when she’s got some fat on her bones. Estrogen is stored in fat. So if she’s got enough estrogen, she’s probably fat enough to start having babies without dying.
Obviously, there’s something else missing here, because most obese toddlers don’t start puberty. And some kids who do have early puberty don’t have excessive body fat. But it’s a theory, and it hasn’t been discarded, and it may help to explain why our (well-fed) daughters are hitting puberty a bit sooner than their great-grandmothers did.