There are a thousand ways in which humanity might destroy itself. What I'd like to discuss are the half-dozen or so ways I see humanity progressing- beyond homo sapiens.
1. The 'Star Trek' future: many futurists imagine that humanity will travel to other planets, and become a multi-terrestrial species. That is a kind of progress, I'll admit: to move from one planet (where humanity's existence could be easily wiped out by a single disaster- meteorite, virus or other) to many, provides some security for the continuance of the species. But in that vision of the future, humans don't evolve in any way. If you look at the characters on any of the Star Trek shows, they are really no different, physically, emotionally or most important, mentally, than you or me. But I don't think that's realistic. As our knowledge of DNA grows, the temptation to tinker with it will become irresistable. Especially when we could make ourselves smarter, stronger and healthier.
2. Supermen (and Superwomen): as imagined in films such as X-men, or T.V. shows such as Heroes, the humans of the future may have super abilities. However, I don't think they will include stopping time, walking through walls, etc. But these abilities could include the ability to breathe underwater, using gills like amphibians, or fly using wings grafted onto the human frame. More importantly, future humans might be smarter, with better memories. Unlike in Heroes or X-men, however, these abilities won't simply appear- they will be developed by scientists.
3. Humans adapted to space: instead of travelling in spaceships to other planets, humans could alter their bodies to allow themselves to live in space, without spacesuits or oxygen. How would this be accomplished? First, the human body would have to have an exoskeleton, or tougher skin, to protect it from the vacuum of space. Secondly, the body would have to recycle carbon dioxide into oxygen and vice-versa, and use sunlight for energy and propulsion (it would probably have large wings that it would use as solar sails). Obviously, such a person would not look exactly human. He or she would be as much plant as animal (plants use sunlight for energy, and convert carbon dioxide into oxygen).
4. Collective humanity: we have machines now (E.K.G., etc.) which can read minds, in a very crude fashion. If machinese were developed that could both read a mind and transmit that information to another mind directly, then telepathy might be possible. But telepathy is not just a better form of communication. It is a way for two minds to be joined. It is a way for a hundred minds to be joined, as well. Or a billion. Individual man or woman may become extinct.
5. Seperation of mind and body: the body is prone to accidents. It seems reckless that we let our brains (which are irreplacable) be carted around by our bodies (which can be replaced). Not if we can store our brains in safety (the part with the higher functions) while we control our body via remote link. That way, if your body gets hit by a truck, or falls off a cliff- it's not the end of your life. Just grow a new body. The best part is, although your brain is miles away from your body, it doesn't seem that way- it feels as if you and your body are still one.
6. Man and machine: remember what Obi One said about Darth Vader? "He is more machine than man, now." It seems likely that we will internalize more and more technology, blurring the line between human and hardware (although I think this will be a temporary stage in human development, before genetic technology catches up). This would include artificial organs and limbs, telephone implants, etc.
The 2oth Century was the Age of Technology. The 21st Century will be the Age of Biology.