Introduction

At the close of Appendix A we emphasized that during the course of evolution, changes in organismal development from a fertilized egg to an adult result in altered anatomical features. Some changes during the development of an individual organism result from changes in behaviour, for example exercise alters the size and strength of muscles and can also affect skeletal structure. Although it is not widely believed that such changes are passed on to later generations, we cannot rule out the possibility altogether. Also, it is almost axiomatic that, as Aristotle said, ‘Nature is true to type.’ Cats beget cats, dogs beget dogs and humans beget humans. To explain this, some modern biologists use the metaphor of a computer programme, or algorithm, which is responsible for such consistency in development. In any case, during the course of evolution, consistency of development was periodically interrupted, and developmental pathways were altered. Otherwise, the most complex organisms would still be microscopic single cells, or small aggregates of several identical cells. Therefore, to understand how humans emerged from ape-like ancestors we must be as familiar as possible with normal developmental processes, and all of the factors that cause them to change, from mutations of DNA to environmental influences.

Epigenesis and epigenetics

Even prior to Darwin, it was clear to some evolutionists that the diversity of living forms had to have resulted largely from modifications that occurred during embryonic and juvenile development, a process known as ‘epigenesis’.