This means that the serotonin mechanism in our brain, that helps to regulate our mood and gives us feedback on our status in the social hierarchy, evolved before we diverged from lobsters, meaning that it is older than 300M years.

no, sorry, that’s not the case. If it were true, all descendants from the common ancestor would have that trait. Clearly that is not the case since butterflies and spiders (and foxes and bears) are equally evolved from that common ancestor and do not have dominance hierarchies.

Humans are descended from apes and that’s good enough to prove the point. We have dominance hierarchies. Chimpanzees are are nearest relatives, but we are NOT chimps. We can decide what we want to base our hierarchy on. It can be power or competence or beauty or ability to sing and dance. I kinda like that last one even if it would put me pretty low on the totem pole.