As an atheist, I have always wrestled with the problem of when a human life begins. I have concluded that it begins when the fertilized embryo attaches to the uterus. Until that time the embryo has the “potential” for life, but will never develop into a human being!
My morality is derived from the millions of years that social animals have been evolving. Baboons know its wrong to kill other baboons, chickens know that it is wrong to kill other chickens. Every society, be it human, ape, or chicken, follows social rules in order for their species to function as a social species. No mythological intervention is necessary!