(A) For many years, it was assumed that porifera (sponges) were the earliest animals, but some researchers now argue that instead, ctenophores (comb jellies) evolved first. Yet, the details of how important aspects of the nervous system evolved remain unclear. Ramos-Vicente et al. propose that the sub-families of ionotropic glutamate receptors (Epsilon: orange; Lambda: grey; AKDF: blue; NMDA: red) were present in the last common ancestor (LCA) of all animals, with certain sub-families being lost (indicated by a cross) one or more times during evolution. (B) An alternative scheme, which we favor, proposes that a precursor of Epsilon receptors was the only family present in the last common ancestor. Gene duplication would have led to the evolution of the AKDF sub-family in the ancestor of placozoans, cnidaria and bilaterians, with the Lambda sub-family appearing only in sponges. Finally, another gene duplication event would have given rise to NMDA receptors in cnidarians and bilaterians.
Image credit: Vignettes from phylopic.org. Ctenophores: Mali'o Kodis, photograph by Aqua-Photos; Porifera: Mali'o Kodis, photograph by Derek Keats (CC BY 3.0); Placozoans: Mali'o Kodis, photograph from Wikimedia (CC BY 3.0); Cnidaria: Qiang Ou (CC BY 3.0); Bilateria: Human (CC0), Hemichordata: Mali'o Kodis, drawing by Manvir Singh (CC BY 3.0).