It is interesting (at least I find it interesting) to speculate on how humans might adapt to a new, or very different, environment to the one that has produced us. As I wrote a few months ago, this is one of the challenges when creating believable aliens for science fiction, even if you don't want them to be able to talk to or work with human protagonists. I know I'm not the first author to deal with this as it is a very tricky area. If you think of some of the Star Wars aliens as an example, many would really be hard pressed to communicate with their human counterparts, and some are, quite frankly, not really believable. The aliens in Farscape were much more realistic, and the solution to the communication problem creative (I do enjoy the Babble Fish idea in Hitchhikers Guide to the Galaxy ...), but one problem with any kind of translation/translator is that a large part of communication is neither verbal nor even oral.
So, back to my original thought, how might humans evolve in a different environment?
Genetics play a huge role, and recent studies of the remains of Neanderthals and Denisovan DNA, suggest that modern humans did not just interact with these vanished human species, but mated with them, and the hybridisation is a large part of how modern Homo Sapiens Sapiens developed. We know too, that mass migrations in Europe, Asia and elsewhere, have brought an exchange of genes that have also played a large part in our evolution, a fact borne out by the fate of some isolated representatives of our species which include regression and even a stalling of further evolution. Scientists suggest that there are just seven definitive Haplotypes outside of Africa, and some forty-five in that continent. That suggests the future evolutionary path may well begin with a broadening of our genetic pool by the introduction of new genes from African groups. Where will that lead us? A good question, perhaps to a more heat tolerant, less water dependent human? It might be useful if the climate changes as dramatically as most scientists suggest.
And that's just if we stay on the planet.
We know that astronauts suffer a loss of bone density during a prolonged stay in space. They are also subject to exposure to some dangerous radiation. The latter can be at least partially screened, but the changes in our bones is more difficult if one is contemplating a lengthy stay in space. Then there is the fact that we need exposure to sunlight to remain healthy - some authorities say we need a minimum of twenty minutes a day - as sunlight triggers a process in our skin that results in the production of Vitamin D. This is essential to our mental health, and is believed to be related to a number of disabling genetic disorders such as Multiple Schlerosis, Muscular Dystrophy and one or two others. It can be addressed by including it as a supplement into our diet, but that is not the most efficient way to do so.
Exercise in a zero gravity environment helps maintain muscle strength, and slows the loss of bone density (Don't forget the bones are essential as our red blood cells are produced in the marrow), but doesn't entirely eliminate it. What all of this suggests is this; if we colonise space, and we do so without some form of artificial gravity or the simulation of it, we will very likely begin to evolve into something very different to what we are, and how we appear, now. If you think of the creatures that live in the oceans, their shapes, skeletal structures and even the distribution of their organs have been dictated by their environment. They are perfectly adapted to it. They swim rather than walk, they get their essential elements from it, along with their food. They don't need ears, so they sense movement and sound in ways we don't experience. Their eye structure differs from ours as well (though our eyes have evolved from a water dwelling creature originally) and living in an environment and medium which produces a form of zero gravity, limbs tend to be unnecessary.
Which raises several interesting possibility for future generations of humans living in a zero-gravity environment. Could we adapt? I think the jury is still arguing that one, my feeling is probably not, and certainly not rapidly enough for a viable population to develop. The alternative is for humanity to start genetically modifying selected people to develop a strain of Homo Sapiens adapted to live entirely in space. Many years ago a now forgotten SciFi author explored it in a series of YA books which postulated a future society in which children born on space stations were raised in a different atmospheric mix, and in the reduced gravity of the stations. This made them taller, increased thoracic volume and produced several other physiological changes. It also meant that if any of these space raised humans returned to Earth, they needed to wear a space suit as Earth's atmosphere contained too much oxygen for them, and the gravity was high enough to cause strain on their bones. Certainly the most recent studies I've read regarding prolonged space flight, suggest that concept may not be far from the mark.
So, assuming we do find a way to undertake prolonged journeys in space, and to reach very distant worlds, these are all considerations we will need to take into account when colonising them. It won't be as simple as the founding of colonies in the 15th to 20th Centuries, that is certain. The humans who settle them will begin to adapt and change. It won't be obvious for the first four or five generations I suspect, but then the differences between them and those "at home" will start to mount up. How great the divergence will become will be dictated by the adaptation necessary to their new environment and exposure to new viral and bacterial agents. On the one hand it will be an exciting and interesting experience. On the other it will take humanity into completely uncharted waters.
And what of those who remain on Earth? That will, I think, be most of us and our descendants. As I wrote earlier, we will also be adapting and changing as this planet continues on its own evolution of change. The continents will change, the oceans will change, we, along with all the other species on it, will need to adapt to whatever climactic changes come with those changes. Hopefully, our replacements will learn to work with the rest of nature and the environment to limit our own footprint while we become whatever we need to be in a future world.