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I'm starting a world where humans have traveled to many planets and exhibit as much diversity as dog breeds would. Some changes are merely cosmetic like coloration while others are more drastic like changes in biology. In other worlds the universe is populated my many human subspecies. Now, before I go into detail about each race I would like to know exactly how far I can alter their biology before the inhabitants stop being human.

The common definition of species is a group of living organisms consisting of similar individuals capable of exchanging genes or interbreeding. However this doesn't really help since in my world interbreeding is made possible by the human genes each species shares. For storytelling purposes two very different species can make a normal human, because the human genes are still present.

So with this in mind, what's the limit? How drastic can the changes be until on the species can no longer be labeled "human"?

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    $\begingroup$ This is known as the "Ship of Theseus" paradox - if you take something, and gradually replace it piece-by-piece, at what point does it cease to be the original thing? $\endgroup$ – F1Krazy Feb 2 '20 at 12:38
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    $\begingroup$ The limit is opinion-based. Here are three opinions as examples: Some people have thought that folks of a slightly different skin color or ancestry weren't human. Humans today carry some DNA from closely-related species. We call werewolves nonhuman even though they spend only 1/56 of their time in different form. $\endgroup$ – user535733 Feb 2 '20 at 12:38
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    $\begingroup$ This is a language question, not a biology question. Our definition of what a species is or any other biological classification stops being useful when you change the rules of biology, it's just a word now. what is the purpose of labeling organisms as human or non-human in your world? Are there any implications if we cross the line to non-human? language and words are a tool without deeper meaning behind it. If you cannot longer communicate your intentions with the word human, that's the point. So think of cases where it's useful to categorize and then ask yourself the question again. $\endgroup$ – Raditz_35 Feb 2 '20 at 13:42
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    $\begingroup$ @LiveInAmbeR I think this is a great question to ask philosophically, but not a great question to ask here because there is no "right" or "best" answer. All philosophical arguments on this subject have equal merit so we can't provide the "most useful" solution, just a discussion. As long as your subspecies can interbreed, they're all biologically human. As for what's considered socially human - that's an excellent and profound theme to explore through your story. $\endgroup$ – Zxyrra Feb 2 '20 at 17:04
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    $\begingroup$ There are numerous stories written on this premise. One prominent one is by Isaac Asimov (the name of the story escapes me) where robots commit genocide against humans that had apparently strayed too far and now weren't considered "human" according to the Three Laws. But there are many more angles - can you be a vampire or a werewolf and a human? How much cybernetics it takes to stop being human? Does "uploading the brain" also produce a human? We can write numerous more stories, essays, articles, works and we won't reach an agreement for what is human. $\endgroup$ – VLAZ Feb 2 '20 at 17:59
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It is up to you, but note that the definition of 'human' has been getting broader over time

I think once you enter a technologically advanced world such as in the question, the definition of human would only really be 'a thinking, conscious being', and even that is pushing it.

Basically, 'humans' as we take it now would be the exception rather than the norm.

Although DNA may have a part to play, we can manipulate it easily and create our own form of 'breeding' artificially if necessary, and perhaps by the time divergence becomes physically noticeable we would likely be placing emphasis instead on thinking and ideas. Physical attributes and characteristics may tend to become less important in this future - instead we will be more attentive to intellectual pursuits, ideas and culture to define who 'we' are.

The term 'human' in the past was quite constrained, excluding people whom we now include in the definition, and the definition has been widening as time progresses - and this would only continue especially with genetic knowledge and manipulation. You may find that AI and robots, virtual humans, people that look like jellyfish, would be classified as 'human' in the future if you continue the trend.

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  • $\begingroup$ The phrase that I see shows up most commonly in science fiction these days is “baseline human”... that’s the term we seem to have collectively settled on for a non-cybernetic, no-artificial-parts, all-genes-were-in-homo-sapiens-in-20th-century human. But from there spring all the Ghost-in-the-Shell-and-more variations. $\endgroup$ – SRM Feb 2 '20 at 14:15
  • $\begingroup$ I think we can narrow it down to 'a thinking, conscious being which has a consciousness modeled after the function of a biological human brain.' I wouldn't consider a conscious supercomputer as human unless it's simulating human consciousness. $\endgroup$ – Dragongeek Feb 2 '20 at 16:32
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Simply DNA. Scientists examine remains from our history and if possible, have created a DNA sequence for those that look human. Through this they have determined that the line known as homosapiens at some time in that history mated with neanderthals. The more evidence we find the clearer this picture becomes.

In your model, you know who went where and what modifications came about. Your DNA lineage is clearer and without ambiguity. The bigger question is which of those humans mated with other species in the galaxy and what is the effect on the human DNA parts. Star Trek did a lot of this speculation.

The comparison to dog breeds is an insult to man's loyal companions. The greater challenge in your human breeding program is what twists can you introduce that make man better than he currently is... and vice versa, what changes make him worse. What morality does your future society have on all these things? Does it make one modification less acceptable in that society and why?

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  • $\begingroup$ welcome to StackExchange. Your answer is interesting, but how is the comparison to dog breeds an insult to man's best friend? $\endgroup$ – user71341 Feb 2 '20 at 13:13
  • $\begingroup$ Thank you. If humans had the morality of dogs, you would understand. $\endgroup$ – user72081 Feb 2 '20 at 13:44
  • $\begingroup$ Dogs have no morality. They lack sufficient self-awareness to choose good or evil. They are unthinkingly loyal to their pack. For a dog, “I was just following orders” is an acceptable defense. Primates, dolphins, elephants... there are species that show higher level choice capacity. Dogs cannot be evil. Without the ability to choose, there is no morality, only programming. $\endgroup$ – SRM Feb 2 '20 at 14:22
  • $\begingroup$ @SRM, you know we are talking about the ranking order of humans and dogs... $\endgroup$ – user72081 Feb 2 '20 at 15:07
  • $\begingroup$ @castlewrks And I'm saying dogs don't have a ranking... they aren't on the scale. You cannot compare humans to dogs, and saying that humans would be better if they had the "morals" of dogs is equivalent to saying humans would be better if they had no intelligence. (Dogs are intelligent, but not the same kind.) $\endgroup$ – SRM Feb 2 '20 at 19:52