Physical Development

Conceptual Foundations

For most of the 20th century, views of human development were conceptually driven by Cartesian splits, wherein proponents of one position or another jostle for supremacy. These either/or options of what drives human development include continuity/discontinuity, constancy/change, nature/nurture, and stability/instability. Of these, physical systems are most often discussed in terms of nature/nurture. In debating the primacy of nature (biology) versus nurture (environment), physical systems of development are ascribed to the nature end of the argument. Theoretical positions favoring a passive view of physical systems generally reflect a mechanistic viewpoint. Here, environment was viewed as the driving force behind human development, and the organism was seen as reactive, as evidenced in the work of Watson and Skinner. Organismic theorists—early Hall, Gessell, Piaget—suggested that physical systems played an active role in human development. Development was seen as epigenetic—an unfolding of human nature via maturation. At the extreme end of this argument, biological processes are primarily involved in activating the cascade of activities whose teleological end is “humanity,” an idea best expressed by the recapitulation that phylogeny (history of the species Homo sapiens sapiens) is seen in ontogeny (embryonic history of an individual).

Modern conceptions of human development assert a more contextual orientation, said to result from interactions among physical, cognitive, affective, and conative systems and the environment. Contrary to earlier perspectives, systems are seen to depend on other systems for expression. Such interdependence makes the isolated discussion of any element of human experience problematic. Equally important is recognition that systems exist temporally; development is a fluid process and “snapshot” observations are often misleading. Among research communities investigating physical development, two assumptions frame data interpretation: the laws of thermodynamics and theories of human evolution. Physicists remind us that the organization of all matter—including humans—follows the laws of thermodynamics. First, energy—such as in cells—cannot be destroyed, only transferred to another source. Next, all physical systems move toward states of entropy or maximum diffusion. Evolutionary theories suggest distal mechanisms for how Homo sapiens are structured and how these structures are passed from generation to generation among populations.

Though Darwin is universally linked with evolution, he was certainly not the first to consider it (i.e., Lamarck, Saint-Hilaire). He was, however, the first to express evolution as operating through the generation of population variance among traits and natural selection of those traits in a survival of the fittest. After 150 years of often visceral debate, the central tenants of evolution by natural selection have remained unchanged: Species are capable of overproducing offspring, and resources for support of offspring are limited. Therefore, a struggle for existence among individuals ensues. And, if individuals differ in traits (via adaptations, co-opted outcomes, or evolutionary noise) that enable them to survive and reproduce, and at least some variation in these traits is heritable, different populations will produce, to some degree, different offspring. The how of heritability took longer to fathom. Not until Hamilton's treatise on the evolution of social behavior did the explanatory power of genes enter the discussion of how traits are passed from one generation to the next. Prior to this, heredity was believed to operate at an individual or group level; species evolve because the fittest individuals or groups have a greater potential to reproduce. Instead, Hamilton's work demonstrated that, ultimately, evolution does not occur through individual or group survival. It occurs through survival of the fittest genes.

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