Afterword Universality and Brain Mechanisms

In the mid-1980s, when the study of molecular mechanisms in the nervous system first emerged, comparisons between vertebrates and invertebrates began to appear. A fly on the wall of a molecular neurobiology meeting at that time would have heard much talk of “higher” and “lower” organisms. He would have concluded that his cousins, the fruit flies, had evolved from nematodes, and similarly, that frogs had evolved from fruit flies, and likewise mice from frogs. These formulations recalled the Great Chain of Being (Fig. 1), an idea that had strong historical roots dating back to Plato and Aristotle. At one particular neurobiology meeting, a developmental biologist with a strong evolutionary background was asked to give a short summary of phylogeny for the assembled group. He described the two major branches of metazoan evolution, protostomes and deuterostomes (Fig. 1), and tried his best to undo the concepts of “higher” versus “lower,” as well as of a single, continuous line of descent. Over the next two days, it was clear that his discourse was taken as meaning that there was not a single Great Chain of Being; instead, there were actually two. All extant species are certainly not, in fact, evolved directly from each other, but instead represent the currently living products of many different lineage branches. If this is so, then what kinds of meaningful comparisons can we make and what can they tell us? Homologies have traditionally been the goal of evolutionary comparisons. Originally, this meant morphological homology of a structure...