There is individual selection for people to believe that they are of value because it improves their ability with respect to medium to long range projects. The improved ability could obviously have a positive impact on their fitness, that is, their viability or fertility, in any environment. Since there is selection for it directly, it does not matter whether it is true. Biologically, truth is just a proxy for advantageousness and there is no need for a proxy here.
There is individual selection for us to acknowledge some other believers as being of value as well. The reason is that it is advantageous to co-operate with others who believe that they are of value on medium to long-range projects.
Once we acknowledge some others, we are logically committed to acknowledge all other believers as being of equal value. We do not believe that our value depends on any other properties we have. (This issue will be discussed in more detail later.) Therefore, there can be no justification for acknowledging some believers but denying the same acknowledgment to others. There is no legitimate difference to which one can point to justify discrimination.
For the same reason, there is no way to maintain that some are intrinsically more valuable than others. Some can be more instrumentally valuable than others in some circumstances, however, in the way that physicians are more useful than philosophers at accident scenes.
There is a breaker that switches off our belief. The belief stays on unless it would commit us to sacrificing our own lives or health to benefit relatives, friends, or strangers. In addition, it would stay on unless it would commit us to sacrifice the lives or health of our kin to benefit friends or strangers. And it would stay on unless it committed us to sacrifice the lives or health of our friends to benefit strangers. There is individual selection for the first limit on commitment, kin selection for the second, and the third would be the result of reciprocal altruism. We can prefer ourselves to our kin, our kin to our friends, and our friends to strangers, in that order. This hierarchy of preferences is biologically advantageous. As always, the gains must outweigh the losses.
Normativity enters the physical world through the concept of value contained in the belief that we are of value. Moral facts are constituted by the relationship between the belief and true descriptions of our actions.