As humans, we find it difficult to imagine a world in which we don't exist. We are so used to the idea of mankind as the world's dominant species that we overlook the fact that planet Earth was around long before us, and will likely still be here long after we're gone.
Of course, there is no way of predicting when and how the human race will become extinct, but there are already some developments, both natural and man-made, that could present a threat to our survival in the long term.
Check out this gallery to learn which are the greatest long-term threats facing mankind.