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As far as I know there is no reason for the makeup of a given hormone to be universal as it merely serves as a carrier from one part of the brain to another. So why are the above hormones seen across the entirety of the animal kingdom, and used in more or less the same way?

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Evolution.

This is not special to hormones: receptors and ligands of all types are highly conserved across not only the animal kingdom, but also to single-celled organisms. The degree of conservation is typically dependent on the closeness of evolutionary relationship: more related organisms share more homology.

Because species evolve from common ancestors, they tend to share a lot of the same 'tools.' Changing the purpose of a hormone would likely require some sort of selective pressure, some new reason to use that molecule in a different way, and if the old and new roles conflict then there needs to be a replacement. This can happen, but it's complex and requires just the right conditions. More often, minor changes occur: among the steroid hormones, for example, you will find many small biochemical tweaks that make new molecules that have different functions.

Certainly the functions of some hormones are not entirely conserved, and if you look at the nervous systems of, for example, insects versus mammals, you will find some major differences, but the closer in evolutionary time you look the more similarity you will find.

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  • $\begingroup$ Thank you for the answer, I assumed it was something like that, I just had trouble with the idea of a shared ancestor between all these animals that still retained the complexity of hormones and neurotransmitters, do we have any idea what animal that is? P.S. I'm new here, is there anything I need to do to close the question or am I fine? $\endgroup$ – Shmuel Newmark Sep 7 '18 at 9:55
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    $\begingroup$ @ShmuelNewmark If an answer is helpful you should upvote it with the up arrow next to the answer; if a question has been solved by an answer you should accept it with the check mark. Sponges are the simplest extant animals, though they may or may not be in precisely the same lineage as the other animals, but even they have some rudimentary use of molecules like serotonin and dopamine - the purposes are almost certain different, but these things are very very old. C elegans is a roundworm and common model organism - you could start there. The actual common ancestor is long gone. $\endgroup$ – Bryan Krause Sep 7 '18 at 15:44

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