No purpose means any purpose

I don't believe that life has a purpose. I'm talking here about humans and all the living creatures that exist on this planet. You cannot assign a purpose to something that occurred from arbitrary events happening billions of years ago. I'm not going to ask myself if the Universe has a purpose, because it's beyond our ability to understand/determine that, but since we are contained in it, we can say that if it does have a purpose, maybe we share a small fraction of it.
In order to define a purpose for something, you actually need to answer the questions, Why it exists?, What function does it do?, What problem does it help solving and who benefits from it? In this case we can say that we are pure reproductive instruments in the game of evolution, our gene's way to measure their performance and determining the winners, which will be rewarded by passing into the next round (generation).
The collateral effect of this game is our life! The time spent between our birth and our death is called life, and what we do during this period of time, is something we can control to some degree. But there is no greater purpose to it since our existence as individuals is purely accidental. Our only function is to enable the passing of our genes to the next generation. That's it! So what then?
It seems that we've been given this opportunity to spend a certain amount of time in this form on this planet. What we do with it is entirely our choice, because nobody else cares.
And another thing, even if we do have a purpose, even if we inherited something from the purpose of the universe, nobody bothered to tell us, and more importantly didn't restrict us to do that one thing that is our purpose.
So does this mean we can do whatever we please? Well, yes! Somethings we can do only once and then die and others do repeatedly and enjoy.It's up to us.
Therefore, we can choose between passing our time aimlessly, or assigning ourselves a purpose which will give us meaning and make our life more pleasurable.
I'd sure say that the second is a more adequate choice, which leads me to the question,
If i were to choose a purpose for myself what would that be?, even more correct, What would be the best purpose i could choose for myself?
Well, if it's worthwhile to create a purpose it sure is worthwhile to help you feel better not miserable.
So the whole thing reduces to the search of a decent purpose for oneself based on a criteria which should help maximize the "feel good" component for that purpose.
So, i guess, a good purpose is one that is not beyond our physical / psychological abilities, but also not one that is easily achievable.
It should be a purpose that will always make us get the best of ourselves and become better trying to achieve it.
I've noticed that, generally, following a good purpose has as a result contribution to others. The fact that we're contributing with something to the humankind makes us feel better, and i suspect that maybe this is also part of our base function. This makes sense, because, although our genes are competing with each other inside the species genome they would not like for sure to become extinct. So they must be also aware about the good of the whole species while competing among them.
As time goes by, the evolution game is getting more complex, which would explain why traits as learning and thinking occurred.
We are after all machines capable of learning and reasoning, and i do believe a good purpose for a human is one that would use these traits to the full capacity, otherwise we would not be better then the animals which have a much narrower set of tools to use, like exposing their physical aptitudes either by beauty or by force.
So, to conclude, i think that when choosing a purpose for ourselves we should make sure that it employs constant learning and thinking as means to achieve it, and promotes contribution to others as a result.
This seems valid to me because it has the potential to make us feel whole beings using both our brain hemispheres, left (rational) and right (creative) and giving us the chance to express as individuals and also contribute to the good of all humankind.

A good article about the fact that life has no predefined purpose is this : Meaning of Life.