Of handedness, homology, and hopeful monsters

Historically, cerebral hemispheric specialisation has been viewed as a uniquely human feature of brain organisation. But the last 20 years have seen the emergence of findings consistent with the idea that hemispheric specialisation is widespread among vertebrates. In The Evolution of Hemispheric Specialisation in Primates, edited by William D. Hopkins, frontline workers in this area argue for substantial evolutionary continuity between humans and other vertebrates in the expression of lateral asymmetries of behaviour, perception, and cerebral functional organisation. Although I am only incidentally involved in laterality research, I nevertheless enjoyed reading this volume*I learned a lot, and it seems clear that the field is in a transition to more nuanced considerations of cerebral specialisation, with exciting advances in theory, neuroimaging, genomics, richness of the behavioural records, and breadth of the species studied. It is one thing to argue that cerebral hemispheric specialisation is a widespread vertebrate (perhaps even invertebrate*see the chapter by Rogers) phenomenon, but altogether a different thing to argue for homology in specific human and nonhuman patterns of cerebral lateralisation, as many of the contributors to this volume do. This latter claim is tremendously controversial, and this is typically where the invectives start to fly (see, e.g., the spirited exchange between Crow, 2004, and Rogers, 2004). For decades, researchers have argued for and against the human species specificity of lateral asymmetries in behaviour, perception, and cerebral organisation (e.g., Balanov, Deglin, Kaufman, & Nikolaenko, 1984; Warren, 1980). For example, based on an extensive review of the literature, Warren (1980) concluded that ''the human pattern of handedness and cerebral laterality is species-unique'' (p. 357), whereas based on another extensive review of the literature, Balanov et al. (1984) concluded that there is a ''remarkable similarity between the functional asymmetry of the brain in human beings and animals'' (p. 431). This debate over (dis)continuity in brain function between species echoes a long-term debate over the very process of species formation. Almost immediately after the publication of Origin of Species (Darwin, 1859) debates arose over the mode and tempo of evolutionary change, with numerous authors arguing against Darwin's persistent commitment to gradualism in favour of more saltatory processes (e.g., Bateson, 1894; Huxley, 1863). Bateson (1894), for example, catalogued hundreds of examples of anatomical mutations in diverse taxa, representing profound changes in form in a single generation. Thus, aggregations of histologically distinct body parts are often duplicated beyond species-typical quantities: ''Meristic Variation in number of parts is often integral, and thus discontinuous'' (Bateson, 1894, p. 538). Bateson and others viewed mutation as a major source of discontinuous variation and concluded that evolutionary change could occur, in some instances, extremely rapidly. These considerations form the basis for more contemporary ''hopeful monster'' theories of evolutionary change (e.g., Eldredge & Gould, 1972; Goldschmidt,1940). For discontinuity theorists, macroevolutionary change is attributable to saltational evolutionary events, subsequently honed by natural selection. For continuity theorists, on the other hand, macroevolutionary change is a more gradual dialectic between phenotype and environment.