Why do you think we need one ?
We are but one of many animal species on this planet, and the simple purpose of all species is to pass on our genes to the next generation. Nothing more and nothing less.
However, having said that there should be a higher degree of natural selection going on. IE The female of the species should only be interested in selecting the strongest, fitest and smartist male to breed with and thus ensure those traits are inherited by her offspring.
As you can see this process has somewhat broken down in the human species with all males being allowed to breed and the main mass of females not caring about who or what the breed with.
So you end up with a gross over populated species made up mainly of weak willed and largely useless examples who will undoutedly go on to breed the next generation.