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In a story I am writing, I want to have a creature that is physically very similar to humans, yet has a different brain (e.g. smaller size and smoother). How many generations would it take for something like this to realistically happen? Could this theoretically happen within 10,000 - 20,000 years?

Note This would be the time it took for another species to branch off

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    $\begingroup$ Though I'm very thankful for the green check, I'd just like to hint that it's customary (and a good tactic to get good answers) to wait a bit before accepting an answer. Waiting 24 hours would allow people from around the world to see your question, and be encouraged to provide more answers. You can then choose the best one :) $\endgroup$
    – EdvinW
    Mar 25, 2022 at 20:04

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Your suggested time frame is long enough to have allowed wolves to evolve into a very wide range of dogs.

I don't see why humans should be any different. While it might not be enough time to give them feathers or gills, you could get away with most adjustments that could be described as changing the size or degree of something. You could easily make them furrier, short limbed, more muscular or change the colour of their hair or skin to pretty much any naturally occurring hues. A good starting point would be to look at the various mutations people are born with: if they allow the barer to survive to the point were they can procreate, that's likely possible to have in an entire breading population!

Humans are sometimes born with small heads, a condition known as microcephaly, and the linked Wikipedia page suggests they can live long lives and (to differen degrees) function in society.

But how do we know the small heads are not necessarily one-offs, which cannot be kept through generations throughout a population?

In fact, you could look at any domesticated animal. Dogs are just the most eye catching since their wild relatives are still around and well known. Decreased brain size, as it so happens, is one of the traits many domesticated animals share!

According to this site "Domestic pigs have brains 35% smaller than wild boars, while domesticated cats have 30% less brain mass than their wild counterparts." I would take this as proof you could stably and consistently reduce human brain size by at least 35% over 10 000 years of evolution, likely much more since our brains are so over-sized to begin with. You just need some environment where your sub-population could stay alive without a very competent brain, and preferably some selection favouring either smaller heads or the behavioural changes that come with a smaller brain.

If you would "domesticate" humans like farm animals, to use them for meat or for mindless labour, you might actually favour quite mentally challenged ones, as they are less likely to plot rebellions. You just need some keepers to keep them fed, and perhaps to bathe them once in a while.

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    $\begingroup$ "some selection favouring either smaller heads" - I foresee our increased dependency on C-sections leading to genes for narrower hips spreading, then after a major societal collapse because of disease/war etc. only the ones with smaller heads who can easily fit out the traditional way would live to repopulate - to an extent at least. $\endgroup$ Mar 25, 2022 at 21:50
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Lissencephaly

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https://www.newscientist.com/article/mg21929340-100-is-this-the-most-extraordinary-human-brain-ever-seen/

This can happen now. The depicted brain was from an adult resident of a state mental hospital; this person grew up into an adult despite the very smooth brain. Congenital viral infections are one of the main causes of lissencephaly and the degree of developmental disability varies widely from individual to individuals.

I could imagine a world in which persons who had congenital viral infections had some selective advantage. For example these persons infected in utero might then be immune to the virus that disabled them, while persons who catch the virus later in life often die - though some must go on to become carriers and then have infected babies.

Such a virus would cause selection for persons who were immune but also not so developmentally disabled that they could not fend for themselves or eventually raise their own children. That might take only a few generations.

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