Well, according to the super-computer Deep Thought depicted in Douglas Adams’ The Hitchikers Guide to the Galaxy, the answer is ’42’.
It kind of depends what your belief system is, because given our free-will life is what we make if it. Ultimately, however, all the great diversity and complexity of life could simply be distilled to one key need, which is the need of your DNA, and specifically your genes, to survive. Your genes compete to exist, and continue existing, and the great complexity of life is the means by – and on the basis of which – evolution has worked to enable this.
Genes of course have no will, or desires. They are not ‘headed in a particular direction’. Genes that work well in a given environment get to survive, and those that don’t get lost, unless mutations occur that improve their fitness.
Of course, our genes have resulted in us, and part of being us are the dynamic relationships we share as families and friends, of working to provide food and keeping ourselves happy. These are part and parcel of what, in our case, results in the survival of our genes, or, by a curious quirk of human nature, the decision to not permit your own particular genes to survive.
I think Jim has got the biological answer nailed…. personally I go with Deep Thought – the answer to the question posed is 42, which means we must be asking the wrong question… so the only thing to do is keep on asking questions!