It looks like you are correct in your assumption that serotonin evolved in the common ancestors of plants and animals, instead of being convergent evolution. [1

It looks like you are correct in your assumption that serotonin evolved in the common ancestors of plants and animals, instead of being convergent evolution. [1],[2] So it would seem as if the ctenophores lost the ability to produce these chemicals, and went on to develop a nervous system that didn't require them. But the article is really about the evolution of the nervous system, and so the pertinent question is whether (a) the evolution of the nervous system was more recent than 0.5B years ago, in which case the two nervous systems evolved independently, or whether (b) the nervous system evolved earlier than that, and then the evolution of the ctenophores adapted its nervous system to not require serotonin and dopamine. I think the article is suggesting (a). 1. https://www.sciencedirect.com/science/article/pii/S1569733910700692 https://www.sciencedirect.com/science/article/pii/S156973391... 2. https://en.wikipedia.org/wiki/Serotonin#Comparative_biology_and_evolution https://en.wikipedia.org/wiki/Serotonin#Comparative_biology_...