Growth Models of Developmental
Mabel L. Rice
University of Kansas
From cells to whole organisms, there is a time to grow and a time to proliferate; a time to keep silent and a time to express; a time to change and a time to refrain from transformation. But where are the cellular and organismal timepieces and how do they mark off time and keep the myriad physiological events in sync?
—Purnell (2003, p. 325)
The prior quote is from a recent issue of Science with a special section on the topic of developmental timing. The temporal events that guide development are key parts of the puzzle of contemporary molecular genetics. Genes are known to turn on at certain times in development; microRNAs show temporal- and tissue-specific patterns of gene expression (cf. Carrington & Ambros, 2003); and genes activate to trigger downstream developmental events removed in time from the triggering event (cf. Gehring & Ikeo, 1999; Marcus & Fisher, 2003). Timing mechanisms are fundamental to the way that inherited elements direct an individual's growth, whether at the cellular or organism level.
The fact that language acquisition is a developmental phenomenon that emerges in the early childhood period is a commonplace observation that is known to parents and scholars alike. The literature is full of debates about the extent to which growth in language early on is attributable to inborn, language-specialized mechanisms; inborn, generalized learning mechanisms that are heavily influenced by environmental input; or fundamental learning algorithms that are largely acquired by experience. As