A matter of timing
EvoDevo emerged as a new research field that is interested in how changes in organismal development relate to evolutionary changes that span over many generations (Hall, 2012; Hendrikse et al ., 2007). For example, EvoDevo scientists aim at identifying and explaining the specific developmental events that lead to the evolution of new traits and body plans generating morphological diversity (Wilkins, 2002). Focusing on developmental processes of whole organisms rather than just biochemical and cellular processes, scientists scrutinize reductionist views such as the one-to-one correlations between genotype and phenotype and the concept of a gene-”for ”-a-particular-trait. Going beyond mechanistic, genetic explanations for complex traits and behaviors by solely breaking them down into their constituent parts, scientists now look at processes on the level of the whole organism. One of the most surprising insights from EvoDevo is observation that many fundamental aspects of development are quite conserved across diverse organisms. Many key developmental pathways and mechanisms are shared among distantly related species, from simple organisms like fruit flies and nematodes to more complex organisms like vertebrates and plants. For example, the genetic toolkit responsible for body plan formation is highly conserved, indicating ancient origins and widespread utility across different taxa. Genes involved in building specific body parts and organs in the fruit fly have functional equivalents in humans (Carroll, 2017). Based on more and more findings supporting the idea of highly conserved key regulators of development, researchers postulated that important differences among organisms are not due to the presence and absence of genes. “Instead, much of the phenotypic diversity we see is due to changes in the regulation of expression in time and space of these highly conserved genes” (Smith, 2003, p. 616). Such processes include alterations in the timing or duration of developmental processes that can produce phenotypic differences. Along these lines, Barbara McClintock famously stated, “[i]f I could control the time of gene action, I could cause a fertilized snail egg to develop into an elephant. Their biochemistries are not all that different; it’s simply a matter of timing” (cited after West-Eberhard, 2003).
Temporal alterations concern changes in the timing of regulatory pathways or the timing of gene activation during development. The expression of traits may even be shifted from one life stage to another, a phenomenon that has been termed heterochrony (Smith, 2003; McKinney & McNamara, 1991; Reilly et al ., 1997). Some researchers go as far as saying that “all developmental events occur along a time line, any significant change is likely to result in a heterochrony at some level” (Raff, 1992, p. 211). The umbrella term of heterochrony comprises acceleration (precocious development) or retardation (delayed development) of specific developmental processes, relative to the ancestral condition (McKinney & McNamara, 1991) or relative to homologous processes in other taxa (Smith, 2003). A regulatory shift in the timing or duration of developmental processes can produce significant phenotypic differences between individuals or species that allow for the emergence of novel phenotypes and contribute to evolutionary diversification (Shubin et al ., 1997; West-Eberhard, 2003). Advances in EvoDevo were significantly enriched by philosophers who gave critical assessments of the conceptual, theoretical, and methodological foundations of the field. By employing philosophical analyses, biologists have come to a richer understanding of the complexities of development and evolution. The scrutinized definition of what a gene is helped in conceptualizing what genes can and cannot “do” in an organism (Moss, 2004) as well as which roles they play in development and evolution. Furthermore, the shift in focus from individual genes to development as a system that integrates genetic, environmental, and epigenetic factors (Oyama, 2000) contributed to the formulation of an extended evolutionary theory, i.e., the Extended Evolutionary Synthesis (Müller & Pigliucci, 2010).
Drawing on experiences from the interdisciplinary endeavors in EvoDevo, the question arises whether neuroscience can also benefit from philosophical analyses, in a similar manner as EvoDevo did. I will argue in favor of using a philosophical approach of conceptual inquiry to be adopted by neuroscience. I will substantiate this claim by means of two examples: first the expansion of the neocortex in hominid evolution and second how schizophrenia can be reconceptualized as a disorder of sensitive periods enabled by heterochronic shifts in brain development.