1. The brain homology hypothesis entails losses across the board, with more than 75 percent of existing animal phyla having quite literally lost their heads and exhibiting secondarily degenerated nervous systems. In contrast, the convergence scenario does not require any brain/head/eye losses in the bilaterian phyla that lack these features, because in that hypothesis these groups never had them in the first place. Instead, the heavy lifting for the convergene hypothesis comes through its remarkable postulation that brains originated from primeval nervous systems at least three or four times within Bilateria. Both the homology scenario. and the convergence scenario seem improbable, and yet one of them must be true.Given that both scenarios are consistent with the extant phylogenetic distribution of nervous systems, which hypothesis offers the better explanation of the observed data? [...] The scientific jury is still deliberating and far from a verdict. We are therefore venturing into the frontiers of scientific knowledge.
Contingency and Convergence: Toward a Cosmic Biology of Body and Mind
