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(First of all, perhaps this is a duplicate of this question, but I'll give it a go since I'd like to focus on something different. Or the answer is there but I couldn't quite catch it. Oh boy... *facepalms*)

A human has been kidnapped by aliens; after years and years of capture and travel, he meets a fellow human, almost identical to him (not just humanoid in shape, with a head, a torso, two arms and two legs: a human) - the difference is that the new one doesn't come from Earth and has never been there: in fact, he comes from a planet in another galaxy, with a different tech level than Earth's (irrelevant for the issue). How (and if) can this be possible?

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    $\begingroup$ Happens in David Brin's Uplift series. $\endgroup$ – A E Nov 24 '14 at 15:22
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    $\begingroup$ This is going to sound a little nitpicky, but can you define "human?" Modern science doesn't even have a meaning for that word which can keep up in the challenging scenario you chose. It has Homo sapiens, but the mere definition of species is not sufficiently exact to declare if an ET is of our same species or not. $\endgroup$ – Cort Ammon - Reinstate Monica Nov 24 '14 at 16:17
  • $\begingroup$ @CortAmmon, as human, I mean "something that looks almost exactly like us", an alien or a race that could blend in with the inhabitants of planet Earth without any problem. What may or may not differ from us is their insides, their systems (what they eat, how many hearts they have, where they have them, shape of lungs) - up so far, that issue is in the grey area. $\endgroup$ – Rodolfo Dias Nov 25 '14 at 7:44
  • $\begingroup$ The Doctor can pass for a human but is not one as are a few other 'aliens' in various TV series. There are to many possible explanations for this to be a good question. Make it specific please. $\endgroup$ – user3042 Nov 25 '14 at 11:58
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    $\begingroup$ @AE where? Can't remember --- and I think I have read all of them. $\endgroup$ – Rmano Nov 25 '14 at 16:32
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If you want human and not just humanoid, then the advice about convergent evolution in that other question isn't really applicable - getting the basic shape is certainly plausible, but a near-perfect match starts stacking up too many coincedences. You essentially have three options:

1) It's not a coincedence

There's some kind of link between the Earth human and the alien 'human'. Maybe the 'alien' is the descendant of previous kidnap subjects who were transplanted to this other planet, maybe both Earth and the other planet were seeded with life as part of some alien terraforming process, maybe the aliens were deliberately shaped to resemble humans for some reason (or earth humans were shaped to resemble aliens!), maybe there's some cosmic... something... that pushes intelligent life towards evolving into 'human' form.

Whatever it is, the alien somehow looks like a human for a reason, despite the distance between Earth and the alien planet.

2) The universe is a REALLY big place

Getting (almost) exactly the same result in two places independently isn't impossible, just very unlikely. If you roll the dice enough times, even the most unlikely coincedences have a chance of coming up eventually. Note, however, that for this to make sense on its own you need incredibly large numbers of alien races - enough that it would be impossible for a human to meet more than a tiny fraction of them in one lifetime. This shifts the question from 'why does this species exist' to 'why did he happen meet one', but fortunately that's a little easier to justify. Introducing the alien to someone who looks like him is an understandable impulse.

3) Probability? What's that?

Yes, it's freakishly unlikely. It happened anyway, and you don't feel like discussing the matter further. (This is not a recommended option, but as noted above 'impossible' and 'unlikely' - even 'very very very unlikely' - aren't at all the same thing.)

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  • $\begingroup$ I like your 2) :p $\endgroup$ – Rodolfo Dias Nov 24 '14 at 16:11
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    $\begingroup$ #1 is how Star Trek did it, I don't know where they said it, but at some point it is established that all the humanoid planets were seeded by some ancient alien race. $\endgroup$ – TaylorAllred Oct 31 '16 at 22:18
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To answer this question you can forget convergent evolution or anything like that. Even if you seeded the planet with earth-like bacteria the chance of them evolving to something biologically human are so small that even in an infinite universe it just won't happen. A bipedal shape with a head and arms may well emerge, but it would not be biologically compatible with humans.

This leaves only two options:

Transplantation

A seed population of biologically modern humans (so probably within the past 50 thousand years) was taken from our planet and placed on the other one. The motivations and technologies used for performing the transplantation could vary widely but the results would be what you desire.

Directed Evolution

Perhaps a species was facing extinction (lets say their sun was going nova) and they had not yet worked out a way to perform faster than light travel or survive the trip to another star. Instead they create robot probes and send them out, these probes have archives of their culture and nature on-board and are programmed to work with any suitable lifeforms they find and gradually guide their DNA until they match the alien's form. Once that is done the cultural archives would then be made available.

Other scenarios could be imagined, but the key point is that we need an alien intelligence or mechanism that for some reason favors the human form. The machine when it arrives at a planet wipes out any existing dominant species (bye-bye dinosaurs) and then starts guiding the evolution of the suitable lifeforms towards the target form. With each generation it makes modest changes to the DNA that steer them in the desired direction while not causing them to be incompatible with the parent species.

As the process advances it starts pruning the evolutionary tree where it is not moving in the desired direction (bye-bye Neanderthals). Over millions of years eventually the modified species becomes genetically compatible with the original intelligence that built the device.

At that point the device makes contact and provides its archive, welcoming the latest members of the Alien's species to the vast galactic commonwealth that copies of the machine are creating all over the galaxy.

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    $\begingroup$ So THAT was the big strike we experienced on our planet! Poor little dinosaurs :) $\endgroup$ – Pavel Janicek Nov 24 '14 at 13:36
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As in several my other answers, I will try to list your options from the most plausible to least plausible:

1) Make apes win (again). I am not referring to the Planet of the Apes but rather on similar evolution process on different planet. In other words if this humanoid will be from planet of size of Earth, which is about the same distance from star of same qualities as Sun has, then it is really plausible to assume that ape-like organism will be dominant and the most intelligent ape will develop into something human-like looking

And yes, to me, there was only one scientifically plausible way of achieving it. So, health warning, the rest of my ideas is common sci-fi and magic trope, where the scientific plausibility drops by order of magnitude in each point:

2) One "Master race": It has been done in Star Trek so, why not repeat it again? In other words, the explanation to this could be that long long time ago, in galaxy far far away (pun intended) there was one master race of very powerful species which decided to populate our galaxy by their genome.

3) Space-time travelling paradox: Einstein is hard to understand. And you can assume, that since almost everyone knows E = m.c2, almost no one knows the correct time paradoxes of near speed of light travel. You could assume this and use wrong understanding of Theory of relativity to use it for time paradox purposes. Long story short: It was the Earth the whole time, only different time. (Did I tell you that I really love Planet of Apes?)

4) Alien experiment went wrong: Look, you already assume, that your main hero was captured for some time already. What actually prevent the kidnappers to play around with DNA of our hero?

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  • $\begingroup$ I'd just like to add my two cents to this answer. For point #1, the make-up of our star and distance to the sun didn't really cause apes to become dominant, rather it was a long series of happenstance that allowed mammals to thrive. For example, Mammals, and thus humans, were only able to get to the point they are today due to the Cretaceous–Paleogene extinction event. Before that, mammals existed as small "rats", and only survived by being small enough to avoid encounters with dinosaurs. $\endgroup$ – user3334871 Nov 24 '14 at 17:24
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If the alien "human" is able to meet the kidnapped human face to face despite being from a galaxy far, far away, then obviously intergalactic travel is a thing. There are various ways in which this might be the case, with the most obvious ones being a wormhole and a blue police telephone box. Whatever the means of transport was, there's no reason why it shouldn't be bidirectional, so that the alien's ancestors were able to reach and populate their planet by passing through a wormhole or whatever from Earth. Or it could be the other way around, that Earth humans originated on the alien planet, or both groups originated from some third location via alien seeding, but then there's a fossil record which becomes difficult to account for.

There are obvious parallels here with Doctor Who (and also with Interstellar but I won't go into that as I have't seen it, I've only read the reviews and seen the trailers). If we add time travel into the mix then the parallels become stronger. Technically the Doctor is a Gallifreyan, not a human, but I strongly suspect that the Gallifreyans are descended from humans, and the differences (e.g. having two hearts) are a genetically-engineered adaptation to the conditions on Gallifrey.

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