I don't mean our private lives, our personal choices and hopes, the plans we make along the years. I imagine that each and every one of us believes our lives do have a purpose, or many. What I mean is life as a natural phenomenon, this strange assembly of matter endowed with autonomy, capable of absorbing energy from the environment and preserving itself through reproduction.
All life forms have one essential purpose: survival. This is even more important than reproduction. After all, babies and grannies are alive but don't reproduce. To be alive is more than passing genes along. To be alive is to want to remain alive. This is an essential difference between living creatures and other forms of material organization, such as stars or rocks.
These forms simply exist, passively allowing the unfolding of the physical processes that define their interactions with themselves and their surroundings. For rocks, it's a give and take with erosion; for stars, they withstand their gravitational imploding while there is enough fuel in their cores. There is no energy architecture, no planning to prolong what is inevitable.
The essential difference between the living and the non-living is the urge for preservation. Life is a form of material organization that strives to perpetuate itself.
The confusion with respect to the purpose of life shows up when we consider the amazing diversity of life forms. Given such richness and creativity, it's hard to accept that all of this is just the result of a purposeless accident, without any intention of creating ever-more-complex creatures. Things get worse when we learn that the history of life on Earth shows an increasing complexity.
Life has been around planet Earth for at least 3.5 billion years. During the first 2.5 billion years there were only unicellular bacteria. Only some 600 million years ago did diversity take off. After the Cambrian explosion, at about 550 million years ago, we see the multicellular complexity we associate with higher life forms. From there to here life took over the oceans, land and air with amazing speed and resilience.
No wonder so many people think that life has a purpose, that of increasing its complexity. Of course, the apex of this process would be us, intelligent humans.
This conclusion, however, is false. There is no "plan" to make life more complex so that it can finally generate intelligent beings. (The eminent biologist Ernst Mayr makes a powerful argument against this kind of teleology.) Take the dinosaurs, for example: they were here for some 150 million years and were pretty stupid. We don't see velociraptors using radio telescopes or iPads. Life wants to preserve itself. As long as it is well adapted to its environment it will remain as is, with the possibility of the occasional beneficial mutation.
If the environment changes drastically, life will respond. Either by dying or, for those species that survive, through mutations that may drive radical changes in short periods, as in the punctuated equilibrium hypothesis of Stephen Jay Gould and Niles Eldridge that — even if somewhat controversial — seems to contain a germ of truth.
If we changed one or more dramatic events in Earth's history, say, the fall of the asteroid that helped eliminate the dinosaurs 65 million years ago, life's history would also change. It's quite possible that we wouldn't even be here.
The lesson from life is simple: in Nature, creation and destruction dance together. But in this choreography there is no choreographer.