[A Second Look at the Origins of the Concept of Epigenetics'].

Even today, 'epigenetics' is a rather difficult field to define. The explosive growth of epigenetics over the last twenty years is sometimes seen as a revolutionary event in the life sciences, a paradigm shift that would devalue genetics or the standard view of the evolutionary synthesis. The aim of this paper is to place this controversial issue in its historical context. Building on the excellent work of David Haig, I will show that in the late 1950s, the modern concept of epigenetics emerged as an extension of the nascent theory of molecular biology. Given that genetic information was assimilated to the DNA sequence, and that each cell of an organism was supposed to possess a complete genome, it was thought that certain as yet undiscovered molecular mechanisms were necessary to regulate gene expression. These hypothetical "epigenetic systems", which would not modify the DNA sequence, should also have heritable effect on gene expression, which would explain the stability of cell differentiation during embryogenesis.