Your argument amounts to an appeal to nature, and appeals to nature are suspect for two (immediate) reasons:

* Humans are capable of rational activity, and have no need to appeal to our development for moral justification.

* What happened in our development (that which was) has absolutely no bearing on what we ought to do based on what currently is. We have strong archaeological and anthropological evidence that early humans cannibalized and murdered each other, yet we intuitively understand that those acts do not justify contemporary ones.