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.