My Contribution
The greatest thing someone can do in their lifetime is to nurture new life. I don’t mean that people should go out and have a million kids, because over indulgence in reproducing your own DNA can actually be a drain on the system… I think. I mean people should nurture life of all kinds. Plant a flower, save a kitten from the pound, let the spider in your room lay its eggs. Give things a chance to reproduce. The system works best when it has as many chances to try new things as possible. Since I am not that interested in having children of my own, my greatest contribution will probably be when I die. Who knows how many lifeforms will benefit from the material that I will leave in the dirt. But knowing the simple fact that my eventual death will help the system try out new configurations of life, gives me some piece of mind. Who knows, maybe it could even be the genesis of some new wonderful species.
** EDIT tho now that I think about it, the greatest thing people are doing for life on this planet is eating and pooping… Just the simple act of recycling the biomass into nutient rich waste contibutes greatly to the system… And I suppose thats the beauty of the system. Just the act of trying to survive makes the whole thing work. So next time you clog a toilet don’t be embarresed, you just made a large contribution to life… 
So next time somebody asks me what the meaning of life is, I will smile and say: The meaning of life is… Shit.