In a conversation I had, I was encouraged to contemplate on what life is. The easier question in my opinion is to study life and analyze what life does. My current theory is that life is like a machine which adapts and evolves in order to survive as long as possible. This machine or biological web (however you want to look at it), is ultimately all going to be wiped out when the universe faces heat death. In order words, the dark energy is expanding the universe so that the distances between the stars, between planets, between galaxies, eventually puts everything into isolation until complete darkness in all directions.
So life really only will have a mission until this heat death of the universe finally occurs. For a video on this heat death have a look:
This means life has a time constraint of 100 billion years. Humans have an estimated 10,000 to 100 million more years.