A Developmental Approach to an Evolutionary Problem

We humans take some fifteen to twenty years to make the developmental journey from conception to our final adult form. A major feature of this journey is that it involves an enormous increase in organismic size – from a single cell to many trillions of cells – and a corresponding increase in complexity, leading to the adult complement of more than 200 different cell types and a wide range of organs and structures. This increase in organismic complexity is, however, far from linear. Indeed, the vast majority of the developmental period is dominated by allometric growth of already-formed parts, leading only to a change in relative bodily proportions. This is true not only of postnatal growth but also of around three-quarters of the time spent in utero. The basic body plan, including all of the major organ systems, is established within the first couple of months after fertilization.

This picture of an early ‘creative‘ or morphogenetic phase followed by a much longer phase of allometric growth has long been recognized by embryologists, and is applicable to a wide range of animal taxa. Working outwards from our human starting point, it is certainly true of other mammals, and of birds. It also appears to hold for many invertebrate phyla.

The morphogenetic/allometric distinction is more difficult to apply to groups characterized by complex life histories, such as amphibians and insects.