Science fiction is full of humans interbreeding with intelligent aliens from all over the galaxy.
In the original Star Trek series, Captain Kirk mates with a wide variety of aliens, although he never fathers any children with them that we know of. Spock is half Human, half Vulcan. There are several characters in Star Trek: The Next Generation who are the result of alien-human mixing, as well. Worf is 100% Klingon, but his wife, K’Ehleyr, is half Klingon and half Human. Their son, Alexander, is therefore one quarter Human and three quarters Klingon. Deanna Troi is half Human, half Betazoid. The list goes on and is certainly not limited to Star Trek.
I hope I’m not ruining anyone’s fantasy by pointing out up front that this is not even remotely plausible.
Imagine a human trying to produce offspring with an insect or a tree. Once you’ve stopped laughing, imagine trying to do the same with a bacterium, which is as distantly related from humans as you can get on Earth. Now consider that reproducing with a life form from another planet is far less likely to work out than trying to reproduce with a bacterium from Earth.
For two different species to be reproductively compatible, they cannot have very much evolutionary distance between them. Zebras (Equus zebra), horses (Equus ferus) and donkeys (Equus africanus) can all reproduce with one another. Lions (Panthera leo) and tigers (Panthera tigris) can sometimes produce offspring. The common chimpanzee (Pan troglodytes) and the bonobo (Pan paniscus) can produce offspring. (As is typical of cross-species hybrids, the offspring of these pairs are not fertile themselves.) I hope you noticed a commonality among all of these matches: they are all in the same genus. Lions and tigers are both in the same genus (Panthera) and they can sometimes reproduce together, but neither the lion nor the tiger can reproduce with a chimpanzee or a zebra because they are not closely related. This is not a hard and fast rule, though. Many species that are in the same genus cannot reproduce together, and occasionally species that are not in the same genus can reproduce together.
Life on Earth began somewhere between 3 and 4 billion years ago. Since then, life has evolved and diversified to produce the wide range of species that we see today. Another planet with life would have had its own origin and its own evolution to produce whatever diversity that planet has in whatever amount of time it took. An intelligent species on another planet would be unable to reproduce with most of the other species on its planet, just as most species on Earth cannot reproduce with one another. Humans are not able to reproduce with any other species on our planet. Even chimpanzees and bonobos, which are our closest evolutionary relatives, cannot produce offspring with humans (though it is not for a lack of trying). We share almost 99% of our DNA with chimpanzees, and are physically capable of mating, and yet we cannot reproduce with them. So what are the chances of being able to reproduce with a species that evolved from a completely separate origin of life? In this particular case, the likelihood is roughly equal to the percent of our DNA that we share: 0%.
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