Why were the first hormones probably neurosecretions?

Answer 1

"Hormone" is a word that encompasses many different molecules. It includes any chemical that moves from one cell to another, producing a chemical change in the latter cell. This is a very broad definition, one reason that the evolution of hormones is so ambiguous. By this definition, neurotransmitters -- essential to the functioning of a nervous system -- are hormones.

Some evidence against the first hormones being neurosecretions is the presence of hormones in plants. If we assume that phytohormones in plants are derived from the same evolutionary precursor as animal hormones, then they must not have come from neurosecretions, because the earliest common ancestor between plants and animals existed before nervous systems.

On the other, hand, it could be possible that plant hormones are a type of molecules distinct from animal proteins, and therefore unrelated genetically. If we assume this, then there is reason to think that neurosecretions were the first hormones.

The evidence comes from comparing different animal phyla. Sponges, the most primitive phylum, seem to have relatively little in the way of recognizable hormones. Along came Cnidarians (sea jellies, corals) and with them came nervous systems. In modern cnidarians, some of the neuropeptides involved in the nervous system also stimulate certain reproductive functions. Not quite the same as hormone function in vertebrates, but it fits teh bill as described above. Those neuropeptides were by defnition neurosecretions (chemicals secreted by nervous cells).

As you move from one phylum to another, tracing the evolutionary path as closely as possible, you see more neurosecretions that have effects not just on neighboring nerve cells (i.e. the effect of a neurotransmitter) but also on cells further away in the animal's body. The chemicals being produced by nerve cells are beginning to act more and more like the hormones we recognize in complex vertebrate endocrine systems. And even the vertebrate endocrine system, with its complicated system of glands and ducts, produces many hormones that are chemically identical to certain vertebrate neurosecretions.

Taken all together, the evidence paints a picture of neurosecretions evolving to meet the needs of a nervous system and then being modified into the hormones we recognize in modern vertebrates.

Answer 2

I do not know the exact answer, but logically this could be the probable answer:

In any multicellular, multifunctional organisms like animals, all the body activities are controlled by the nervous system(s) (One central nervous system and/or one secondary nervous system).
This could be the closest reasoning for the first hormones probably being neurosecretions?