Asking for a scientific definition of when a fetus becomes self-aware, or even if it deserves to be regarded as a living being before that, is an exercise in futility.
Well, a philosophical one, maybe. "Let's consider it human when it can form thoughts in its brain", for example.
it all started as a utilitarian decision
It started when the first cell ate another cell. Killing for nutrient absorption is an old constant in evolution. We are also social, so we form social circles that include humans and every being that we consider part of ours. It is not about friendship but beyond that, a sense of identity.
But as you say, all of this is mostly conventions, traditions and unchanged beliefs carried from the past into a present when, even if we think about it, we won't be able to change the way humanity as a whole sees anything.