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Let's say we drop a few hundred thousand people on several different planets. We'll have intentionally picked the planets to be similar in physical attributes (temperature, gravity, etc.), have terraformed them to have similar atmospheres, and have constructed similar synthetic ecosystems on each one. These groups of people however have no contact (physical or informational) with each other for a million years.

Then, a group comes in contact with these planets. What is the chance that the people from each of these planets are still the same species? And just to sidestep the definition of species debate, I'm asking what are the chances they can still produce fertile offspring with people from one of the other planets.

Research so far:

The human species seems to have taken a few million years to shake out. I'm not sure how much cross-breeding between partially differentiated species would slow down or speed up differentiation.

This question is related, but it's asking what conditions could make the species shift happen faster. Here, I'm looking for answers about the expected timeline of species differentiation.

Vogon Poet
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Zags
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  • They can have children if the story needs them to have children. They cannot have children if the story needs them to be unable to have children. If the story does not have a specific need, then there is no need to decide whether they can or cannot have children. That's all there is. Voting to close as asking about a story not about a world. – AlexP Sep 09 '23 at 06:36
  • @AlexP I appreciate that I can do whatever I want as the author, but I prefer to limit my probabilisticly implausible steps. I've already used my "freak coincidence budget" on other things. – Zags Sep 09 '23 at 12:40
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    @AlexP your argument holds absolutely no water, as it can be used against literally every single worldbuilding question on this site. – M S Sep 09 '23 at 12:51
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    @MS: I actually mean it for this question. One million years is about 40,000 generations. Literally, it may equally be the case that the populations have drifted away sufficiently for them to have lost interfertility, or it may be the case that they did not. The author is at complete liberty to choose the situation they want. (In real paleoanthropology, anatomically modern humans and Neanderthal humans had been isolated for 300,000 to 700,000 years, depending on whom you ask, and when they met again they were still interfertile, although probably not completely.) – AlexP Sep 09 '23 at 14:24
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    I’m voting to close this question because this seems like a simple philosophical debate rather than asking to solve any problem. If your humans have been isolated for 1 million years your story decides if they are compatible or not as your story needs. A virus makes them incompatible if you want; or nothing happens and they’re compatible. – Vogon Poet Sep 09 '23 at 20:17
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    @AlexP Then the best answer is "yes, it can happen... or not. This means the world isn't preventing you from choosing what happens". You're making a prejudice on the question because you have its (best?) answer, not because of the question being asked. Moreover, the off-topic, story-based closure reason should be used only for when people ask "what a character or organization should do" . Please let's stop using that reason without checking its page. – Tortliena - inactive Sep 10 '23 at 02:29

1 Answers1

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Unless there was a major change like dropping a chromosome or blood sugar then 1 million years would still be able to interbreed.

Mutations are random and few and far between. Homo sapiens could and did interbreed with wildly divergent humans such as Neanderthals and Denisovans and probably homo erectus and others, who also interbred amongst themselves.

Lions and Tigers have had several millions of years divergence but can still interbreed and have fertile offspring.

Evolution might be fast-tracked on different planets though as your humans would be adapting to different ecosystems. So you make it happen however you want in terms of a story

Kilisi
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  • Given the recent publishings about there being a bottleneck for humans about 0.9MYA that also corresponded to a chromosome change (2a+2b->2), your argument loses some value, namely that mutations are "scarce". You also did not account for different environmental factors, namely something minor would definitely develop on each of those planets within that 1MY, that could force local populace to develop additional genes or eliminate some traits that ended up not helping survival. – Vesper Sep 09 '23 at 05:51
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    @Vesper: If that bottleneck actually happened (and opinions of informed people are far from settled), what we are looking at is genetic drift due to the founder effect. The question specifies "a few hundred thousand people", which is two orders of magnitude more than what this bottleneck hypothesis postulates. – AlexP Sep 09 '23 at 06:41
  • @AlexP even if it did not happen, there are chances that some local early cataclysm on one of those planets had separated a founder population of ca 1000 humans from the rest, and they evolved in different conditions causing enough genetic drift in Y chromosome to not produce fertile offspring with the others on the same planet when they would finally meet each other again. Especially if the humans would have troubles rebuilding the civilization due to some local reason like scarcity of iron or coal, for example. – Vesper Sep 09 '23 at 07:13