God save us from an Earth in which all men are the same.
God save us from a colony where that is the goal, or assumes that for its norm.
Give me a thousand people speaking different tongues, worshiping different gods, and dreaming different dreams, and I will make of them a greater nation than you can make with ten thousand of your genengineered duplicates.
For mine will have the spark of greatness in them, while yours will live for conformity, worship mediocrity, and take their carefully modulated delight in predigested dreams.
-Reigning In Chaos: the founding of Guera Colony
This Alien Shore, by C.S. Friedman is a science fiction novel depicting a future in which Earth's people have scattered across both space and form.
Overall, the book is probably what I'd call "a good read", though honestly, I found the plot itself less interesting than the depictions of the story's various native societies and technologies. The protagonist is a young Terran woman named Jamisia who, despite an ostensibly standard appearance isn't exactly typical herself -- as the story progresses, she becomes more and more acquainted with the various "Others" who live in her brain, and finds herself having to work with these separate selves on a "teamwork" basis.
It would be giving away too much to explain why she is on the run, and what the peculiar circumstances (both experiential and technological) resulted in her internal plurality to begin with, but suffice to say that this is no cliche "multiple personalities" drama save for a few invoked stereotypes with regard to the individuals residing in Jamisia's brain. For one thing, Jamisia's storyline does not revolve around trying to "get rid of 'alters'" -- rather, at some point she comes to realize that regardless of how they got there, her brainmates are in her head to stay, and she's more likely to come to a state of healthy functionality if she learns to work with them and utilize their particular talents and tendencies as situations call for them.
Obviously this is a story and not meant to be prescriptive of "what to do if you find a whole bunch of other people living in your brain one day", but there are certainly some real-world folks who assert that integration is not the only viable response to multiplicity1.
This Alien Shore has a fair number of plot threads in addition to Jamisia's (though they all do overlap and eventually converge); there are storylines featuring hackers, a powerful and mysterious Net virus, trade monopolies, social tensions between Terrans and Variants, and a host of other intriguing turns of circumstance. This review would probably approach novel length if I attempted to comprehensively discuss every interesting point in the book here, so I'll just focus on a few things that stood out while I was reading.
Regardless of whether they are Terran or Variant, most characters in This Alien Shore are equipped with some form of technological augmentation.
On the physical side of things, "wellseeker" mechanisms maintain homeostatic conditions in the body in the face of stress, injury, or illness (this is something I'd actually really like to see developed in reality -- the book doesn't specifically mention what degree of increased longevity has been achieved in its featured societies, but considering how much of age-related disease is caused by ongoing wear-and-tear along with the escalation of small problems to pathologies, I can see how something like a "wellseeker" might be very useful in increasing the healthspan.
In addition to being able to perform tissue repairs in the event of wounding, the wellseekers can modulate things like heart rate, blood pressure, hormone levels, etc. (and people also have the option to manually override homeostasis-seeking actions, so if they want to, they can still let their hearts pound when they're watching scary movies and such).
On the cognitive/perceptual side of things, people can read, watch video, communicate, program, operate various devices, etc., using their bio-interfacing "brainware". In fact, the interfacing capabilities of brainware have become so intertwined with many predominant cultural elements that babies are implanted with neural computers at birth (so that their own wetware will grow in concert with the new dynamic circuitry).
Many (if not most) characters have some form of implanted brainware, though there's definitely a sort of fuzzy gradient along which internal technology shades into "external" interfacing technology -- headsets allowing particular kinds of data access are popular, and among Variants, devices that take into account particular aspects of a given Variation are commonplace.
The descendants of those who stayed on Earth are more or less standard-issue humans per today's standards, save for the fact that most things currently classed as "disabilities" and obvious functional differences have been eliminated.
The descendants of those who took to the stars, however, manifest in a tremendous diversity of shapes, sizes, functionalities, and cognitive styles -- not only are many of the various phenotypes known to us today represented, but the mutating effects of warp-drive technology have expanded the range of viable forms far beyond what you'd even see in the Star Wars cantina. There is no "standard form" among the Variants -- and for the most part, even variations considered "extreme" by the Terrans are simply addressed practically, in full recognition of the unique value of each individual (and his or her particular skill set).
Many Variants are described in This Alien Shore, some only in passing and some more extensively (as some of the main characters are Variants of one stripe or another). Some have tentacles, some are scaled, some have more or less than four limbs. Some locomote by walking; others crawl, slither, prance, or make use of any of various devices (e.g., some move about in personal mecha-like "exoskeleton" machines). Some have the sort of standard sensory set most people would probably associate with "human"; others might not have eyes (in which case they might use a kind of "sensing grid" to perceive their surroundings), while still others have entirely new organs that pick up data in ways that Terrans can't even fathom.
Of all the Variant groups discussed in the book, I found the Guerans to be the most interesting. The Guerans represent a colony populated by those who hold neurological diversity as a core value -- many of their ancestors were affected by the Hausman warp drive in such a way that the genes configuring brain architecture were mutated. As a result, contemporary Guerans manifest what seems to be a considerable portion of the existing real range of human variations (one main character is autistic, others seem to have attributes indicating Tourette's, ADHD, and amplified versions of personality traits like competitiveness), along with other, new cognitive and perceptual stylings.
Rather than seeking to "normalize" themselves or their children, Guerans take a different approach -- their overall culture is flexible and accommodating of many different thinking, learning, and sensing styles. Central meetingplaces at which many different Variants gather are furnished and decorated so as not to overwhelm those with highly sensitive senses, and such that people who use various technological assistive devices are able to do so effectively. Guerans are perhaps less physically diverse than other Variants (e.g., no scales or insectoid carapaces), but mentally, they span a spectrum broader than anything imaginable in the rest of the populated galaxy.
In order to ease communication between different Gueran Variations, most people wear customized face paint -- intricate patterns of lines and angles that, in the Gueran cultural dialect, indicate each person's predominant neurological, cognitive, and personality tendencies. For example, the autistic (Iru) programmer Masada has patterns that signal to others that they shouldn't take things like atypical eye contact and inflection to be signs of rudeness or shiftiness, and that he prefers direct communication to more passive "hinting". This is something I found to be tremendously cool -- I mean, if only I'd had something like that in elementary school, maybe I wouldn't have been sent out in the hall for the crime of "answering rhetorical questions".
I also liked how, among the Guerans, what might be termed "neurotypicality" was considered a type (nantana) and not some kind of universal default that everyone was supposed to aspire toward.
Of course there was a bit of conflict between some iru and some nantana, not to mention a bit of mutual "why can't this person just be more like me?", but overall, the communications between those with these particular differing neurologies seemed uncommonly rational and respectful. For one thing, the nantana, while occasionally annoyed by iru interactive styles, did not go around pitying and lamenting the iru for their very existence. There was no underlying cultural push on Guera to eventually create an "iru-free world"; this idea probably would have struck even the most stodgy nantana as both ridiculous and offensive (rather like how an Earthling might feel if it were suggested to him that we ought to try and eliminate all trees, or all walruses, or all blonde-haired girls).
The fact that some Variations do present people with particular challenges was not ignored by the Guerans. People on Guera tend to be equipped with whatever technology lets them most capitalize on their strengths while addressing their weaknesses; this technology is always employed with the consent of the person using it, and people generally have the freedom to minimize and maximize particular attributes per situational requirements.
For instance, autistic Guerans seem to have devices that enable them to turn down certain sensory channels when input becomes overwhelming -- and this is completely expected and "normal" in their culture, much in the way that it's normal for people today on Earth to employ ear plugs or sunglasses when the need arises. Difference isn't pathologized or idealized; it is simply accepted as something that exists, and worked with on that basis.
There's a lot more to the book, and to the areas of it I've touched on, than I've described here. But if you're interested in reading a speculative description of how different cultures might choose to manage diversity and "disability"-related issues in the context of a cool sci-fi adventure story, I highly recommend you pick up This Alien Shore.
1 - As for whether multiplicity, functional or otherwise, "exists" or not, my opinion is that this is one of those phenomena that isn't presently empirically measurable or provable any more than the existence of one "self" is measurable or quantifiable.
Most people do actually maintain various different "personas" -- i.e., they might seem a very different person at work than they do when out with friends on a Friday night -- and I'd wager that this tendency expresses itself to different degrees in different people, and is also highly affected by cultural factors. Many writers, for instance, describe the phenomenon of having their characters "come alive" and "write themselves" in their minds.