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In my setting I have a species of anthropomorphs (basically humans) that grow from baby to full-size in the shortest time period possible. The babies are the size of human babies. How short can that time period get, and what modifications should their body/genetic material have to grow at an incredibly fast pace?

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I could name a few factors.

Some species with short lifespans might be forced to procreate at a younger age and spread their genes as quick as possible before getting eaten by predators. Which means that females will be forced to give birth at a much younger age. Over a long period of time natural selection will cause males to pick females who are the most fertile. The females who become fertile the quickest will be able to pick the best genes from all the males.

This will cause gene mutations in both males and females of your species to reach puberty and adulthood at a much faster rate.

Other factors are hormones. Providing higher levels of Human growth hormone , estrogen and testosterone can accelerate the the growth rate of your antropomorphs/humans aswell.

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  • $\begingroup$ Thanks, that's extremely useful $\endgroup$
    – lollo259
    Mar 30, 2023 at 16:57
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    $\begingroup$ There are species of rodent that are sexually mature at birth even. Growth rates can just about how fast nutrition is recieved, sky is the limit. It's brain and experience that slow you down. $\endgroup$
    – John McD
    Mar 30, 2023 at 17:00
  • $\begingroup$ Could a humanoid that lives 20 years have enough reason to be forced to procreate at a much younger age (2-3 years) therefore making their bodies already mature, or it should live way less than 20 years? $\endgroup$
    – lollo259
    Mar 30, 2023 at 17:32
  • $\begingroup$ @lollo259 I don't think "living to age 20" is enough for that. Unlikely even; that would mean up to 10 generations by the time of death -- something humans don't even reach nowadays. Being able to procreate early is one thing, but to be successful, you also need to be able to raise your offspring (that is, keep them alive long enough they can survive and procreate without your help). $\endgroup$
    – Abigail
    Mar 31, 2023 at 14:41
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An example of another animal of similar body mass to humans is a cougar (puma, mountain lion). These reach maturity in two or three years.'

HOWEVER the puma's brain development is not comparable to that of humans. More similar animals, like chimpanzees and orangutans, take ten-plus years to mature because, more similarly to humans, their brains require significant maturation after birth. Not to say that a cougar's or wolf's brain doesn't change and mature after birth -- but they are less helpless when born and don't need to reach the level of mental maturity a human or great ape does.

Physical maturity, then, might be possible in less than five years, perhaps as little as three (learning to walk upright is more complex than learning to walk on all fours, for instance, and doing so requires more growth in the legs, spine, and related musculature), but mental maturity (given similar final mental condition) seems unlikely to be completed in less than fifteen to twenty years, as is the case now.

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    $\begingroup$ Well, except that the brain keeps growing for that extended time, so you can't have the skull sutures closing -- which pretty much means the long bones can't stop growing either, so nothing gained by growing too fast. $\endgroup$
    – Zeiss Ikon
    Mar 30, 2023 at 14:20
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    $\begingroup$ If the skull stops growing, it will limit the size of the brain and you'll wind up with a condition akin to microcephaly -- and the part of the brain most affected will be the cortex, where higher thought functions are believed to reside. If, on the other hand, the skull grows to adult size before the brain does, you'll have the brain "loose" in the skull, with risk of concussion or spinal injury from the exaggerated movement. We are evolved for our bones and muscles and brains to grow together; change that, and you'll quickly run into issues with weak, clumsy, stupid 8 year old "adults." $\endgroup$
    – Zeiss Ikon
    Mar 30, 2023 at 17:35
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    $\begingroup$ Human brains reach their full sizes around puberty, but they're hardly finished at that point. A lot of the later maturation is about changing and trimming down the wiring that already exists. I don't think it's too far-fetched to have an adult-sized humanoid at 2 years old with a full-sized brain, that then takes 2 decades to trim down and tune from a wasteful (in the sense of brain cells needed proportional to behavior complexity) instinct-executor into an efficient and (more) rational thinking machine. $\endgroup$
    – AI0867
    Mar 31, 2023 at 8:18
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    $\begingroup$ @EiríkrÚtlendi: I think you misread. AI0867 observed that human brains stop expanding physically larger around puberty, and that the changes after that point are internal. Therefore he suggests that It seems plausible for a creature to grow that same full-sized brain in 2-3 years (still as stupid as a normal human 3 year old), and then spend the next 17 on only-internal changes (just like a normal human brain matures without growing larger from 16-26) $\endgroup$ Mar 31, 2023 at 18:00
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    $\begingroup$ @EiríkrÚtlendi Blue Whales grow a vastly larger brain in drastically less time, so clearly physical size is not limited by time. We all agree that the shift of that mass to develop into something healthy and effective will take 20 years. I agree that after 3 years it's more of a human-shaped-animal, at best. $\endgroup$ Mar 31, 2023 at 22:39
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You can grow an adult body within a couple of years, compare to animals, but you cannot grow an adult brain that fast, your adults would be immature emotionally and intellectually.

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    $\begingroup$ So I should look at the animals that mature in a couple of years, and see what makes them mature in that short time $\endgroup$
    – lollo259
    Mar 30, 2023 at 14:08
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    $\begingroup$ @lollo259 Those animals don't need to do much more than eat, sleep, and reproduce. A key differentiator is the complexity of food acquiring behavior. The more complex, the longer it takes to mature. Complex hunting behavior needs many years to teach the next generation. $\endgroup$
    – David R
    Mar 30, 2023 at 14:33
  • $\begingroup$ @DavidR yes but, for the body in itself I just have to look at those animals $\endgroup$
    – lollo259
    Mar 30, 2023 at 14:36
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    $\begingroup$ @lollo259, if all you want it is a human-shaped animal, sure. But, if you want humans, with human culture and behavior, you're just not going to be able to do that with a three-year growth period, not with anything resembling real-world biological processes. Since this is fiction, you could add in some other handwavey mechanism that automagically adds the human into these immature and too-small brains. $\endgroup$ Mar 31, 2023 at 17:10
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    $\begingroup$ @lollo259, there are limits to genetic memory. How to get around on four limbs? Possibly -- look at various other animals, able to walk shortly after birth. Bipedal locomotion is more complicated, and relies on muscles that humans just don't have developed at birth. But how to talk? How to control fire? How to knap flint into useful edges? How to build and operate a lathe? These are all beyond the possibilities of genetic memory. So, basically, anything resembling culture requires learning, which requires time. $\endgroup$ Mar 31, 2023 at 18:36
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Adult is kind of iffy here, because it is possible to grow to full size significantly before reaching mental maturity. This happens in a lot of animals, though obviously the most familiar example is dogs 🐕.

I am going to assume you meant full-size from context.

Humans grow unusually (and kind of pointlessly) slowly. For comparison a beef cattle 🐄 only live 2-3 years.

Humans would almost certainly reduce full-size time down to 3 years or younger eventually with enough survival, just because it gives a massive inherent advantage in education 🏫 and training, making life easier and increasing effective intelligence. So anthropomorphs could have just been around for a long time.

As for further increases in speed, the easiest way is to just grow cells outside the body and inject them in. This could be their own, or you could make them feeder cells grown. Maybe in a plant-esq thing. This is far quicker than digestion.

As for other measures, maybe having a cartilage proto skeleton before the bone one.

Kind of hard to predict those effects, as no animal has that growth strategy, mostly because civilisation to support it has not been around long enough to develop it.


So, in all, 3 years or lower is easy naturally, if other measures are employed, it will be a lot faster, but very hard to predict a limit.

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    $\begingroup$ I'll edit adult into full-size then. Unfortunately in this setting there exists only "aborigenal" type of technology, so the children's growth needs to be somewhat in their genetic code. $\endgroup$
    – lollo259
    Mar 30, 2023 at 19:34
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    $\begingroup$ Baby blue whales grow ~200lbs/day, and ~4 feet/month, so we don't need cellular injection to get bigger, merely calories. A human-sized two-year old "human" would have a lot of difficulty learning to walk though $\endgroup$ Mar 31, 2023 at 18:07
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There is substantial variation within existing humans on the age vs height for children. I have shown only the first graph from here.

Note that the lines on this graph may be misleading. A baby born "on" one of these lines does not necessarily stay there. These are the percentile lines so, by definition, they go up as they do. A child might move from one line to another, growing faster or slower than their cohort.

You could get a substantial start by determining the factors that produce this range. There will very likely be genetic factors. There will also very likely be factors of diet.

So, if there were some factor that required babies to grow quickly, a species might rather quickly evolve to match. Maybe there is a predator that only takes prey under a certain size. Or maybe there is a harsh winter such that smaller people tend to freeze. Those with genetics to grow quickly, and who live in cultures where the right kind of food is provided for them to grow, will suddenly be tall. The ones that don't will tend to die out.

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