Evolution of Developmental Timing as a Driving Force of Brain Diversity

The question of how complex traits originate and diversify has marveled naturalists for millennia. From the notion of development as a series of transformations beyond 'pre-formed' growth by Aristotle, to von Baer's recognition of phylogenetic differentiation that set the foundations of modern evo-devo thinking, a central theme has been the nature of biological change (and conservation) across temporal scales. Since the last universal common ancestors, recurrent series of ontogenies have negotiated conservation and change, thus generating the phylogenetic tree against the regularities of planetary rhythms (e.g., tides, days, seasons) as well as organismic dynamics (e.g., embryogenesis, metabolism, behavior). Accordingly, differences in the relative timing of developmental processes (i.e., heterochronies) have long been considered as a major source of evolutionary diversity. To further reflect upon the mechanisms by which changes in developmental timing have shaped brain evolution, the 32nd Annual Karger Workshop in Evolutionary Neuroscience included a diverse panel of speakers to address the topic of Heterochrony in Comparative Neurodevelopment. This Special Edition of Brain Behavior & Evolution is a collection of articles contributed by these speakers around this central theme. The contributed papers are quite diverse in their focus, their methods, and the insights they provide. However, a common thread in these reflections is the understanding of organisms as dynamic systems, embedded within an ecological context, which arise via an epigenetic process of developmental transformations, and consist of coherent yet dissociable modules. Evolution takes place by a differential tinkering of these developmental processes, generating innovations within the constraints of an organism's viability. Our understanding of how development affects evolution has moved beyond a simplistic dichotomy of genotype-phenotype to incorporate the many contexts in which variation can result in the conservation of new contingencies. In our view, this collection of articles builds upon these notions to highlight some of the mechanisms by which heterochrony, understood as a consequence of the evolution of developmental processes rather than a developmental process in and of itself, has contributed to the generation of diversity in complex brain features.