Science fiction, the good stuff anyway, goes to great lengths to reason about what will happen to our culture and civilization when stunning scientific advances allow us to alter our environment in profound ways. However, I've noticed that there isn't much science fiction which alters what it means to be human.
The problem is that most or all scientific advances in s.f. fall into the category of tools. Hyper drives, teleporters, cyber eyes, livers which metabolize alcohol or other poisons in 3 seconds or less, wormholes, space ships, are just tools to be used by what usually appears to be normal human beings. Sometimes the human has something like telepathy, but even that is often portrayed as a magical cell phone or action at a distance tool.
So again, why is there no science fiction which alters the definition of humanness?
One answer maybe that once a human consciousness ceases to be human, and by definition becomes alien, we as humans--who experience these characters, can never scry the intentions of the alien characters. This disconnects them from us and we don't care about their actions since they cannot be put into pattern or predicted by us. As an example, in the s.f. show Babylon 5, the only true aliens were the Vorlons and the Shadows. All the other alien's motivations and responses were decidedly human-like and could be easily categorized as something a human could have done. However, the Vorlons and Shadows, to my disappointment in the end of the series, had reasoning and motivations that were understandable by humans. It was a let down precisely because the intrinsic alienness--which generated much of their horror, of those two races vanished when human consciousness laws applied to them. They just became every day humans, who just happened to have a lot of power.
So, why am I talking about this stuff at all? I see some trends in our civilization which will provide us with the best tool that could ever exist. A tool which may alter our own implementation.
It will take its form as a set of tools. One of them will be the genome sequencing of all life. Another will be understanding of protein function and folding. Another will be unraveling of the information processing equations of the mammalian brain (you should see the work at IBM modeling a rat brain cell by cell, and Paul Allen's mouse brain project). Another will be nanotechnology at the atomic level for self assembly of structures which have interesting and strange properties allowed by the laws of nature, but rarely seen in nature, like negative refractive lenses for example. Another will be enough synchronized computing power to bootstrap the first of the new forms of life.
Once these technological fields are mature, humanity will begin, by instinct, to tamper with its own implementation. It will be slow, since the affront to the Identity of what it means to be Human will cause a deep fundamental resistance. At first it will be very small modifications to ourselves, things that can be viewed as tools and only tools--an organ replacement, cancer treatment, hearing replacement. Then it will move on to performance enhancement, longevity, increased efficiency of our body, wider selection of energy acquisition methods.
Finally, we will move to the brain itself. This will be the last, final, and most altering frontier of modification. One day, we will decide that having better mathematical ability is en vogue, craft a dozen grams of new neocortex with a nourishing and cooling blood supply, and graft it, made out of our own cells, into our head. The bone will regrow naturally around it as if it was always there and unimaginably complex mathematics will be available to us with enough training and time. Laws will be set up governing certain mathematical rules about how our consciousness is implemented as to keep some sort of normalcy in the self interaction with our own species. We will endlessly tinker with ourselves until our consciousnesses are linked by high speed, high bandwidth electronic devices and our many brains function in a truly parallelized fashion.
We will become a collective, since that is the natural extension of group behavior, culture, and language which has allowed the Idea of us to evolve and prosper throughout the countless millenia of the past. It will provide us with low latency planetary computational ability and great waves of information integration will ride across the surface of the earth, passing each new being for its little bit of processing. The intelligence, now even beyond the concept of decision making by my reckoning, simply designs whatever body and consciousness it needs to leave this planet as it now has the information processing capability to understand what really needs to happen.
The thoughts, desires, and emotions, of that hyper consciousness, and whether or not those concepts even have meaning anymore at that time to someone like me, call to me like the Siren's song.
End of Line.