Something that very much intrigues me is how the realms each have different species that we might consider to look exactly like us (phrasing it this way 'cause "Humans look like Time Lords" and "From a certain point of view").

Often (though not always) in Sci Fi and Fantasy, other species tend to look at least a little different from us, such as different skin colours, ear shapes, and/or proportions at least. I mean, if a species came from another planet, it wouldn't make much sense for them to look exactly like us bipedal monkeys. Sometimes writers come up with explanations, such as those species actually being related to humans one way or another, convergent evolution, or common origins.

Now given how MK is a sort of fantasy-leaning kitchen sink setting where it's possible that the Elder Gods might have had a hand in ensuring that species' across the realms look a certain way, I'm willing to grant them looking like us. But given that these are different evolutionary lines, at the very least we might expect some internal differences (i.e. Time Lords having two hearts instead of one). And I personally suspect there may be a number of internal, less visible differences.

First, for the purposes of this thread, I'm going to refer to all these species' by their realm of origin if they have no specific name; so Earthrealmers, Outworlders, Chaosrealmers, Edenians, etc. I feel like when the writers mention that "Shokans are half human half dragon", then they're referring to Outworlders rather than Earthrealmers for the Human half. So this is to simplify.

Second, let's keep this specifically reigned in on the generically human species'; Shokans, Tarkatans, Kytinn, and Vampires are subjects for their own discussions.

My thoughts on what may be the case for each of the species' we could talk about here, based on what I have gather thus far from the games and the species' representative characters (speaking in relation to Earthrealmer biology):

General: The general case for human species' beyond Earthrealm seems to be that:

• They often live significantly longer, with their lifespans stretching into unknown thousands of years, potentially even to the point of biological immortality.
• They tend to be more magically gifted, with exceptions such as Shang Tsung being on-par with, or even surpassing many, so this one isn't really a hard-n-fast difference. (though this seems to have also led to them being significantly less technologically advanced than humans, with the seemingly medieval-tech Edenians and Seidans proving to be perhaps the most advanced of them all)

Chaosrealmers: It seems to be the case that Chaosrealmers are more resistant to pain, or at least have a higher tolerance for it, given that Havik ripped half his face off, and seemingly isn't noticing any pain (though it's also possible that he's in constant physical pain and just doesn't show it). Either way, it seems that Chaosrealmers are able to contort, stretch, and break their bodies apart without concern (many of Havik's moves include him twisting parts of his body, and he detaches his hand in Joker's MK11 ending as a gag)

All speculation from here:
Beyond this, I intuit that, due to the nature of Chaosrealm, they may also be much more prone to mutation (potentially even to the point that genetic science has no predictive power whatsoever, and evolution simply doesn't occur), and their internal anatomy may be shifting, and constantly changing and rearranging itself, thus making those otherwise unnatural contortions more possible. They may also be better at healing from injuries. This is mainly just something I'm extrapolating from his lack of concern from his hand coming off, and him having his hand back when mauling a seidan in Joker's MK11 ending. (or he just doesn't care anyway, and his hand was healed magically)
That being said, it's also entirely possible that every chaosrealmer has unique abilities, and that at least some of this is only applicable to Havik.

Edenians: From the outset, it seems like the traits I listed above in the General section might be the only differences, but from what I can gather, it's possible that Edenians, due to their gods being much more involved in their affairs and their gene-pool as evidenced by Argus, then they may have much more divine ancestry. This may manifest in them having unusually long lifespans even by other realm humans' standards (no way of telling without actual confirmation from NRS, and I refuse to make a twitter account, so for now I guess this is up in the air), and seems to also manifest in them being more innately gifted in specific types of magic (it's probable that the innate magic affinity is truly random, since Mileena is Kitana's clone, yet she seems to have a completely different magical affinity. Such phenomena can occur IRL, such as cat fur patterns being completely different even when they're cloned).
Examples to the latter would include Kitana being gifted in wind magic, Sindel in sound magic, and Mileena's teleportation magic. None of these characters are mages, yet they seem to be rather potent at these. Hell, Mileena was able to teleport around seemingly right out of the test tube.
Rain may be gifted with water magic simply through his Edenian biology, but his father's blood could have possibly augmented the connection, such as to the point of being able to reduce himself to a purely water form, just like Raiden being able to turn into lightning. Beyond that, not much else I can specifically call out.

Outworlders: We only have three main example characters to work with here (Bo' Rai Cho, Li Mei, and Skarlet), and all of them are really just limited to the general traits above along with any sort of magics they learn. They might be the most biologically similar to humans, with only really genetic unrelatedness due to being from another world. (if you were to try gene-sequencing, Outworlders and Earthrealmers might potentially appear 100% unrelated according to the read-out, though could also still be genetically compatible enough for medicine and reproduction to work both ways)

Osh-Tekks: Ohsh-Tekks are definitely easier to visually distinguish, being physically larger than Earthrealmers if Kotal is anything to go off of (and we will be going off of him since he's the only example we have. His soldiers are Outworlders who dress themselves in an Osh-Tekk manner to show their allegiance to their emperor). And obviously with greater size they would also be more massive and physically stronger.
They are also apparently empowered by sunlight, potentially far surpassing their regular physical limitations when directly exposed, which may be why they seem to wear so little. They can also apparently manipulate sunlight's intensity to teh point that they can actually weaponize it, with Kotal being able to flat-out incinerate his opponents with sunlight alone. However, this ability may be rather intensive, since he doesn't use it every time he fights someone.
Blood may also play some role in their biology that enhances them, however, I can't find anything to this effect. That being said, if there is a connection, it could alternatively be a pain response that the Osh-Tekks culturally associated with blood.

Seidans: It's hard to say with Seidans. Hotaru and Dairou both seem to have some sort of yellow magic effect in common, however, Darrius doesn't, so either he just doesn't like to use it, or it's a magic that the Seidan Guard teach their members. Really, I think that, besides the general stuff above, the rest may simply be more about basic biological stuff. They might have a lower chance of mutation (maybe even to the point that it's simply impossible for them, and thus evolution beyond stuff like genetic drift becomes impossible, which could be very dangerous should a sizable number of them die out, as they would have no way beyond either interbreeding with other realms' humans, and literal divine intervention, of restoring biodiversity), jitters and shakes could be unknown to them, and they may also have perfect physical symmetry in all members. One thing that's very interesting--even though it isn't necessarily biological--is that when Dairou assassinated Hotaru in his Deception ending, he felt compelled to reveal himself first, meanwhile I highly doubt that Darrius would be so courteous. It's possible that seidans may be more suggestible based on their upbringing. Like, they might be less prone to personality shifts and such.
Nothing else I can really identify about their biology.

Feel free to correct me on anything, and share anything you can find in other media.