As worldbuilders, we routinely tweak our unwitting human victims to new environments, new abilities, and new dangers. So far, we've accelerated the natural healing rate, opened new doorways, and given the ability to manipulate anything.

We've tweaked the human species so it can survive in heavier, bloodier, and thinner atmospheres, among countless others.

Despite all these changes, we still consider these tweaked humans to be humans. But are they really? How many or how drastic of a change can we introduce to a subset of the human species before the lay-human no longer identifies the victims of our imaginations as members of their own species?

Is there a defining limit that, on one side, declares the subset to still be human and, on the other side, a whole new species? If so, what is it?

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    $\begingroup$ There is no defining limit, just as there is no defining limit for "life" within the confines of science. This may be too broad to be discussed. Perhaps you can direct the question towards one facet of tweaking. For example, I personally define human far more by the mind than I do by the body. Another example along those lines is the Bene Gesserit from Frank Herbert's Dune, who are seeking genetic traits affecting the mind that they declare essential for being "human." $\endgroup$ – Cort Ammon Jul 28 '15 at 16:18
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    $\begingroup$ Do you mean human as in possessing a quality of 'humanity' or human as in a member of the species Homo sapiens? $\endgroup$ – Scott Downey Jul 28 '15 at 16:27
  • $\begingroup$ @CortAmmon This is really a two-part question: how the lay-person perceives humanity, and how humanity is defined. My concern for the latter is on a purely species basis, not a psychological one. Elves and humans can have the same psychology, but they are different species. $\endgroup$ – Frostfyre Jul 28 '15 at 16:28
  • $\begingroup$ @ScottDowney For the defining limit: purely on a species basis. $\endgroup$ – Frostfyre Jul 28 '15 at 16:29
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    $\begingroup$ @Frostfyre okay, now for the really painful bit: science doesn't have a defining line between species. Believe it or not, something just that important to the study of biology is a fuzzy subject! $\endgroup$ – Cort Ammon Jul 28 '15 at 17:56

From a purely biological perspective, an offshoot of H. Sapiens is a distinct species when the two groups can no longer interbreed. A species can also be defined as the largest possible gene pool under a given set of circumstances.

From a psychological perspective, it's a bit fuzzier. "What does it mean to be human" is certainly one of the oldest questions that humans have asked and it continues on today. However, I think that psychologically a lay-human can identify another human based on certain characteristics of physical form or life experience. If a living thing fails to meet these criteria, it may be classified as "inhuman" or a creepy "something else".

Note that these norms are based on fairly open and accepting definition of what makes something human. There have been plenty of times in history where a particular group of people is defined as not-human then oppressed/slaughtered, for example, the German classification of the Jews as not human and therefore worthy of slaughter.

Physical form

The body needs to look something like this or the female equivalent along the path from baby to elderly. Missing limbs is okay because a human being can still live if it's missing an arm. However, missing a head while maintaining the ability to move is distinctly inhuman.

Generic Human

Physical weakness

An individual who does not behave in the way that humans do, in terms of endurance, strength or (in)destructability can be classified as not human. The further outside the bounds of normal characteristics an individual goes, the less human they become. Being very weak does not preclude classification as human since there are many very weak humans.

Mental weakness

It's a common thing to hear "...all too human..." when a character in a book makes a huge mistake or succumbs to a typical human frailty such as despair, madness, anger, or fear. Something that looks like a human but does not display any of these these emotional/mental characteristics doesn't usually qualify. Example: Super soldiers who fear nothing and feel no pain at slaughtering other humans could qualify as "not human".

Comparable Intelligence

Society is pretty accepting over very low intelligence to normal to even high intelligence but extremely high intelligence has an alienating quality that precludes definition as human.


In Bicentennial Man, a robot slowly gains more and more human traits over the course of his 200 year life. All his organs and body become biologically based with the exception of his positronic brain. For the better part of his life he lobbies to be accepted as human but the global society continually denies him that designation until he permits himself to die. With that act of mortality he places himself within the bounds of human mortality.

If you can't die, you aren't human.

Progression through Life Stages

An individual who doesn't have a childhood because they were created in adult form could be considered kind-of human but kind-of not. It would depend on the specifics of the story to determine how "kind-of not" the individual may be. The movie, "The Curious Case of Benjamin Button" where Benjamin Button is born as an old man and ages backwards might satisfy the "not human" definition because his life is so radically different than anyone else's. Terminating a life before reaching a particular life stage is not enough to preclude defining an individual as human.

If there are substantial deviations from the normal characteristics in any of these categories, an individual or group could easily be classified as "not human".

  • $\begingroup$ If inmortality (regarding age) is developed and is aplicable to most humans then being inmortal likely will not be considered as inhuman but rather a sign of superior status. Much as having a Porsche is a sign of higher status than having a Ford. $\endgroup$ – Anonymous Coward Jul 28 '15 at 23:37
  • $\begingroup$ @JoseAntonioDuraOlmos true. That same logic could apply to every other category as well. $\endgroup$ – Green Jul 29 '15 at 0:05
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    $\begingroup$ You have to have 4 arms and 4 legs to count as human? Damn... $\endgroup$ – Miniman Jul 29 '15 at 3:04
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    $\begingroup$ @SeanR Ah, perhaps but that is a different question. $\endgroup$ – Green Jul 29 '15 at 11:19
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    $\begingroup$ @Panzercrisis, that's true but we are now departing from "what is human" into "what is a species" and both questions are equally complicated. Good point though :) $\endgroup$ – Green Jul 29 '15 at 13:40

What is considered Human is a scientific or philosophical question. I am going to argue that in the context of WorldBuilding, the philosophical answer is going to have more influence for the world than the scientific answer.

My answer: To be considered Human, the life-form in question must be considered "one of us" at a societal or group level, where "the group" is the group of humans making the categorization.

If that sounds extremely vague, that's because it is. It is the only definition which is always accurate. Right now, if an alien arrived on Earth looking like a completely normal homo sapien - would most people instantly classify it as human? - In my opinion, I think it's most likely that it would have to be studied before it was accepted as "human".

This seems to indicate that our world currently would first look at the scientific question of humanity - which other answers have covered well. I believe this is because of our past - of excluding groups because of beliefs - which was undeniably overcome primarily because of science.

But in a world of our creation, the humans in our world are the "group", and thus, they decide what is "Human". We are able to suspend our belief somewhat on behalf of this other group - allowing us to imagine humans with gills, wings or whatever which science would rule out as being "human", but there comes a point where our suspension breaks. Usually this is at a point where we are no longer able to relate to that group at all. In other words, that other "group" is not one which we could imagine ourselves being a part of as humans.

Take for example, the X-Men. In these comics/movies, there are humans who have evolved to have special abilities. There are four different groups, with two different viewpoints:

  • "Normal" humans - with the viewpoint that "special" humans are mutants, therefore not human, and a danger to real humans. They would like mutants to be excluded from normal society, possibly even all killed.
  • "Normal" humans - with the viewpoint that "special" humans are still human.
  • "Special" humans - with the viewpoint that they are even better than "normal" humans. They exclude themselves from the normal "human" group. In a way, they consider themselves more human than the regular humans, because they are the future of humans.
  • "Special" humans - with the viewpoint that they are humans just like any normal person. This is the group that is followed as the "heroes", so it is interesting that it is the viewpoint that most readers/watchers would agree with, since we don't see a lot of examples of outspoken "normal" humans on their side.

The problem with the term human is that it currently represents a broad class of individuals that have a large number of traits in common. If I ask you to describe a human, you might say it’s a bipedal, intelligent, aerobic mammal with a certain body configuration. There are a million traits you could pick from that all humans share. But if you take away any of those traits, is what you have really no longer a human? If someone has no limbs are they no longer human? If someone is intellectually disabled are they no longer human? If someone is covered in hair are they no longer human? Of course not, all 3 of those cases exist. But if I point at a limbless, thoughtless, hairy blob on the ground, and I ask if that is a human, how do you answer?

Our definition of human works currently because the entire spectrum of possible organisms does not exist. There aren’t any edge cases of human-chimpanzee hybrids that anyone has to decide whether or not they are human. Once you consider the entire spectrum of possible creations it becomes obvious that no single attribute is capable of distinguishing between human and non-human. Of course that doesn’t mean people won’t still have opinions on what is human and what is not human, they just aren’t going to follow a formal definition, nor are people likely to agree upon it. The definition of human in a world of genetic engineering will be an entirely subjective one.

So the answer is simply that it depends on the society and the individuals. Today’s society would likely be split on whether a chimpanzee with enhanced intelligence is human. Hundreds of years ago such a creature would likely be considered non-human and hundreds of years from now such a creature may be considered more human than most. Every society and every individual within those societies will have different ideas about what is human and what isn’t. There's no single answer.


This has a close resemblance to Theseus' paradox or the Ship of Theseus. https://en.wikipedia.org/wiki/Ship_of_Theseus

As most other answers have pointed out, the point at which an object is defined as something entirely new is fuzzy. More so in the case of what it means to be human. From a psychological standpoint the line is extremely fuzzy. However, you have asked for the defining limit on a species basis i.e. from a biological standpoint.

Let's first look at this from another angle.

1) We have tinkered with the DNA of a number of organisms (plant and animal) and created completely different entities. https://en.wikipedia.org/wiki/Genetically_modified_organism

2) Through artificial selection new breeds of plants and animals have been created (See Artificial Speciation)

So the questions now are: do we consider Genetically Modified Organisms (GMOs) to be new species and do we consider new breeds created through artificial selection to be new species? I've looked around a bit for the answer and most new GMOs are considered to be 'strains' while artificial speciation is in fact a method of creating new species. (Example: Domestic sheep [Ovis aries] no longer produce viable offspring with Ovis orientalis, one species from which they are descended)

If science can somehow get to the point of creating this reproductive isolation through genetic modification, that's when you have your answer. To be technical though, that's when you'll get a species different from Homo Sapiens and belonging to the genus Homo.


The rule of thumb is that as long as it can mate with another (definitely) human being, and produce a reproductive offspring, it belongs to the same species. It might look repealing, or even be physically incompatible; think of the diversity of dog breeds - they are still the same species.


The line is blurry as any line can be. Science will provide one definition (or perhaps 5 or 10, if there is disagreement). Each religion will provide one. Many nations will provide their own. And on top of all of that, each individual person will have to come to grips with their own definition of "human."

Of these, the most agreed upon seems to be the "human spirit," which I define very loosely as "the deep down desire to do 'good,' where the definition of a 'deep down desire to do good' is specified by the observee." In our case with world building, the observer is the reader or watcher of our work. So thus the definition of the edge of humanity would be the point where our reader no longer feels the character has a deep down desire to do what they believe to be "good."

One powerful aspect of this is certainly "life." If you have a concept of life, and the humanoid demonstrates a desire to respect and help "life," even if it hurts things in the process, we tend to accept it as part of the human condition. Any creature which no longer values nor respects "life" is rapidly assigned to the bucket of "not-human" by the readers (although there is no particular reason why a character couldn't argue otherwise... its always interesting when human characters argue for the humanity of another)


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