Probable evolutionary relationship of serotonin and the plant growth hormone indoleacetic acid/auxin

In D. W. Woolley, the Serotonin Hypothesis and the Genesis of Psychopharmacology, we learned that Woolley “believed serotonin played a role in brain development because of its resemblance to auxin (a plant hormone that stimulates cell growth and orients it toward sources of energy, such as light, in acts of photo- and other tropisms) and was the first to propose that serotonin dysfunctions interfered with learning and memory.”

Below is a digitized excerpt from my personal copy of Woolley’s 1962 book (gifted to me by a good friend), The Biochemical Bases of Psychoses – or the Serotonin Hypothesis about Mental Disease.

In these pages, taken from ‘Chapter 4 – Serotonin,’ Woolley documents a few structural and functional similarities between serotonin in animals and auxin in plants, and traces its evolutionary lineage back to our earliest ancestors.

Along with the work of more contemporary researchers, this is the foundation upon which I wrote Psychedelotropism.