Brain homology: Dohrn of a new era?

swims ‘upside down’, suggested – at least to him – not a vertebrate ancestor but, like the ascidian tadpole, an evolutionary simplified or ‘degenerate’ fish. Anton Dohrn was convinced that annelids were the ancestors of chordates. He suggested that structures could evolve through time leading to a complete change in their structural and functional identity. In his argument for annelid ancestry, Dohrn proposed that a common ancestor to the vertebrates might have had a ringlike brain around the mouth, as does an annelid, whose dorsal brain is connected to the ventral nerve cord and ganglia by connectives that extend around the stomodeum (the front part of the gut). To achieve transformation from an annelid to a vertebrate nervous system, there must evolve in the latter a brain that was not perforated by the stomodeum. To obtain this condition, the annelid mouth would have closed, and a new mouth opened above the brain. Dohrn suggested that this new mouth would have arisen by functional modification of gill slits that would have been present in the ancestral annelid. As a result of this transformation, the inversion would have been that of the vertebrate, to account for its ventral mouth. Dohrn’s monograph, which has been translated by Michael Ghiselin, with a commentary, enunciates Dohrn’s crucial ideas about ‘Funktionswechsel’ or ‘the principle succession of functions’, meaning the evoluIn 1875, the great embryologist Karl von Baer, then in his 82nd year, received a monograph dedicated to him proposing an annelid ancestry of the vertebrates [Ghiselin, 1994]. The monograph was written by Anton Dohrn, best known as the creator and tireless promoter of the Naples Zoological Station, now known as the ‘Stazione Zoologica Anton Dohrn’. A few years earlier, in 1867 and 1871, one of the greatest Russian biologists of the time, Alexandr Kowalevsky, who had worked in the Zoological Station, concluded that Amphioxus is a chordate, as are tunicates, and that these were the most logical candidates for vertebrate ancestry [Kowalevsky, 1867]. Kowalevsky’s colleague, Philip Owsjannikow, working in St. Petersburg, had determined in 1868 that Amphioxus possessed a dorsal nerve cord and what he interpreted as a small rostral brain [Owsjannikow, 1867]. Kowalevsky and Ilja Metschnikoff, his friend and colleague, had also published their discoveries of commonality of embryonic cell organization (‘germ layers’) shared across the Metazoa, a finding that underpinned Charles Darwin’s hypothesis of a common metazoan ancestor [Kowalevsky, 1871; Levit, 2007]. Whether those discoveries encouraged Anton Dohrn to consider an alternative ancestry of the vertebrates is not known. What is known, however, is that Dohrn’s own studies of the lancelet Amphioxus lanceolatus , which typically Published online: December 22, 2010