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A staple of fantasy involving time travel is the hero (or heroine) zipping off into the future and then meeting a future lover with whom they go on to have wacky time-based adventures before presumably settling down to make some babies.

Eventually, though, this won't work. Even if one of the future people falls in love with our time traveler, they won't be genetically compatible because humans will have evolved to much in the interim. How far into the future would someone need to travel before running into a species barrier between themselves and the locals? I'm assuming that there haven't been any major evolutionary bottlenecks due to some sort of apocalypse, and that people haven't utilized technology to artificially speciate themselves or stabilize their genome and actively prevent it.

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  • $\begingroup$ Not a duplicate, but similar and might be the best answer you're going to get: worldbuilding.stackexchange.com/questions/13773/… $\endgroup$
    – Dan Smolinske
    Commented Apr 22, 2015 at 16:44
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    $\begingroup$ You're also assuming technology isn't used to artificially stabilize the human genome, which may be the more likely future than speciation. $\endgroup$
    – Frostfyre
    Commented Apr 22, 2015 at 17:00
  • $\begingroup$ @Frostfyre Good point. I added that to the question. $\endgroup$
    – ckersch
    Commented Apr 22, 2015 at 17:06
  • $\begingroup$ What about dying before reproduction is possible? Possibly from VD caused by common future bacteria that past humans had no natural biological defense from. Even though they may genetically be the same species, functionally they would be distinct... $\endgroup$
    – Isaac Kotlicky
    Commented Apr 23, 2015 at 9:19

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The half million years result is true only if we postulate that nothing of any great import happens between now and then.

A portion of the human species isolated from the rest due to a cataclysm (natural or man made) for any significant amount of time will change much more quickly, although we are still talking several thousand years of isolation. And of course since we are talking about time travel, whoever is from the future will most likely be the recipient or descendant of some sort of genetic engineering program, which will most likely result in the inability to breed with baseline humans.

The other issue to look at is if humans have changed so much, why would your hero/heroine even be sexually attracted to these neo or post humans? Many of the cues for sexual attraction are subtle and very species specific (lions and tigers would not mate in the wild, looking at another answer here, and various species of even closely related birds won't mate outside of their species.) Humans may be less driven by such clues (we do after all carry a tiny percentage of Neanderthal and Denisovan DNA in our genes), but even then the evidence for outbreeding suggests that it was both rare and generally unsuccessful.

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As my answer to the question Dan Smolinske linked in the comments indicates, half a million years may be the lower bound for human speciation. This, of course, assumes all conditions on Earth remain the same (i.e., no apocalyptic event forces rapid adaptation).

Of course, in general, if two individuals are capable of interbreeding and producing an offspring in nature (the liger doesn't count because of this), then the two are said to be the same species. In the specific, this definition is less clear, as Berkeley points out.

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    $\begingroup$ It's worth noting that larger populations experience evolution more slowly than do smaller populations. There are more humans about than there are of most individual fish species, so you would likely need to adjust the number upwards. Additionally, the number of years required is probably less of a constant between species than the number of generations required. $\endgroup$
    – Saidoro
    Commented Apr 22, 2015 at 17:47
  • $\begingroup$ @Saidoro My number was a pessimist's consideration. The number is probably higher than I indicated, but there's no real research on the topic that I've been able to find. $\endgroup$
    – Frostfyre
    Commented Apr 22, 2015 at 17:58

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