Answer by Guy Gamzu:
The ultimate reason most likely will be gradual distinction.
Our human species – Homo Sapiens – is in fact already at the end of its 300,000 years journey. Within a matter of several hundreds of years (or less), humans will cease to exist. The scientific revolution 500 years ago has initiated this process. There are currently three main catalysts driving us to this ultimate change:
1. Genetic manipulation
Genetic manipulation will enable all sorts of improvements on future generations; their health, intelligence and traits. It will also enable better maintenance of our body parts and endless extension of life (read immortality).
Cyborgs – bionic body parts are already developed to an amazing level, but together with organic parts – it will become even more sophisticated and powerful.
Singularity – the ultimate unification of the brain with computers will redefine the definition of our existence. Who knows what's the meaning of physical presence when you can download the entire brain of an individual. We currently use our brain and organs to 'pull' information. But technology is already capable of reading some of our intentions (e.g. move body parts). The next phase is to be able to both read & transmit directly to the brain.
The result of those three vectors working in tandem is something we may call today a 'superhuman'.
The startling fact is that from a biologic perspective, this superhuman will have less in common with us humans living today, than we have with our Apes ancestors dating a couple of million years back!
It may sound to many as science fiction, but it isn't.
Is it good? is that what we really want?
That is the topic for another question.