The Adaptive Significance of Retarded Development

Our paedomorphic morphology is a consequence of retarded development; in this sense, we are neotenous. I have tried to avoid the tradition of discussing human heterochrony by enumerating paedomorphic features, for I believe that retardation itself is the primary phenomenon—both because is serves as the ground for paedomorphic morphology and because it constitutes a central feature of human adaptation in its own right.

Retardation gains much of its adaptive significance in human evolution as the ground of paedomorphic morphology. To repeat an argument advanced throughout this chapter: our paedomorphic features are a set of adaptations coordinated by their common efficient cause of retarded development. We are not neotenous only because we possess an impressive set of paedomorphic characters; we are neotenous because these characters develop within a matrix of retarded development that coordinates their common appearance in human adults.

The early stages of ontogeny are a reservoir of potential adaptation. Juvenile features may be adaptations in their own right or simple topological consequences of morphogenetic development from simplicity to complexity. DuBrul and Laskin, for example, tie many fetal features to requirements of "close packing" in egg or womb: "The fetal head of the rat is rounded as in most mammals; evidently it fits the largest mass—the brain—into the smallest space—a sphere— within the constricting confines of the uterus. The face is diminutive, tucked in below and in front of the brain case. The skull need but 'unbend' during growth to make the long rectangular shape of the adult" (1961, p. 122). The same features are adaptive in adult humans for different reasons: the cranial flexure for upright posture, the small ventral face for balance and as a response to reduced dentition, the spherical shape of the cranial vault for economy in ossification (Abbie, 1947, p. 25). The availability of these features as transient stages in juvenile ancestors and the existence of a mechanism (retarded development) for their transfer to adult descendants establishes their preadaptive value.¹⁸ As Bolk (1915) noted in an early paper: "Bipedal walk found in the primitive, fetal, central position of the foramen magnum a lucky condition, sympathetic to its trend."

In some cases, we can make a stronger claim: as size increased in human evolution, retardation provided the only "escape" from an ancestral allometry and the only path to a favored adaptation. In an ingenious argument, marred by poor statistical definition and a confusing mixture of ontogenetic and static data, Hemmer (1969) plotted braincase length versus facial length for Australopithecus and Homo sa-