So, a race of sapient aliens I designed for a series of short stories owes its sapience in part to a symbiote which lives in their brains and produces hormones that induce the brain to grow faster and for longer.

It occurs to me that, since the brain signals the reproductive organs to begin puberty, causing it to develop faster would likely result in puberty beginning earlier that would otherwise be expected, but on the other hand their bodies may have evolved to account for this, so I must ask: How long of a timespan, evolutionarily, would a species have to have a symbiote such as the one described above in order for having it not to cause early puberty? Would evolutionary pressure even be strong enough to prevent puberty from being at least slightly earlier than one would expect? If puberty would, even after a few million years, be earlier than one would predict, how much so would it be?

  • $\begingroup$ Survival traits can influence evolution quite rapidly. Reproductive traits even more so... nothing is quite so direct as those. This sort of thing could settle any problems in 20 generations... or 1, if things go really badly. $\endgroup$
    – John O
    Commented Jan 28, 2022 at 18:30
  • $\begingroup$ The onset of puberty is triggered by a timer. I don't see how the size of the alarm clock is linked with the time of the alarm. (Anecdotically, elephants have brains about 4 times as heavy as human brains; but elephants become sexually mature at about the same age as humans.) $\endgroup$
    – AlexP
    Commented Jan 28, 2022 at 20:18
  • $\begingroup$ Actually, in humans, puberty is triggered by body weight... at about 40kg. That's why better nutrition has led to earlier puberty in humans. As for this alien species... it's up to the OP. $\endgroup$
    – Monty Wild
    Commented Jan 29, 2022 at 8:40

3 Answers 3


You're looking about it wrong. Early is subjective. When this parasite is normalized across the population everyone would mature at the same rate. Maturation would only be early relative to the the non-infected, non-sapient baseline.

If there are any complications related to a change in maturation rate, it is more likely that traits to mitigate the complications would evolve, rather than maturation rate regressing to pre-infected levels.


It occurs to me that, since the brain signals the reproductive organs to begin puberty, causing it to develop faster would likely result in puberty beginning earlier that would otherwise be expected

I do not think this is a good assumption, at least if your aliens are like humans. The brain mostly grows before the age of 2, as shown in the graph below. The brain does indeed control the onset of puberty, but it is via a complex hormonal control system that is not well understood rather than directly linked to brain growth. If your symbiont can control the growth process well enough to grow a bigger brain then there is not reason it could not trigger puberty at the optimum point for its purposes.

Brain growth curve

If you still want to estimate a time scale for these changes, I would look at the changes we see in fish exposed to overfishing. We recognise that overfishing drives commercial species to mature earlier and at a smaller weight. This change has happened over recent decades. To estimate a time period you would need to decide on the strength of the evolutionary pressure, ie. what is the relationship between age of entering puberty and reproductive fitness, and how genetically malleable that age is.


This being an alien species, by definition we can't know how long it must have associated with the symbiote in order for puberty to be delayed.

In theory, if a species related to these aliens was being parasitised, and the parasite evolved to delay puberty, and then in a zoonotic event, the parasite crossed over to the OP's alien species, the answer might be zero time... the parasite/symbiote may always have delayed puberty from the very first time that the two interacted.

Alternatively, if the symbiote/parasite didn't originally have the effect of delaying puberty from the first interaction, we can't know how long it might take for delayed puberty to evolve, because we don't know how much might have to change, we don't know how rapidly their genetics changes each generation, nor do we even know how long its generations take. If this alien uses neutronium alchemy, it might evolve in a few months or less. If it lives on a really cold planet, with generations in the thousands of years and a very low mutation rate, it might take billions of years. Since we don't know the rate of genetic drift per generation, we can't even say over how many generations it might evolve.


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