Quote of the Week
(January 20, 2025)

The ordered developmental stream of a mammal includes generation of the hundreds of different cell types in its body. It is hard to understand how a single genomic “blueprint” — or any other way of construing a fixed genetic sequence — could by itself provide the definitive causal basis for these hundreds of radically distinct ways of living. If the blueprint is compatible with all of them, do we have compelling grounds for thinking that it fundamentally determines any one type of cell, or organ, let alone all of them together? One might reasonably expect that other, contextual factors direct the developmental process toward particular outcomes of such different sorts.

A more balanced understanding arises when we watch how every cell displays its character through its life as a whole. That character, in all its qualitative richness, somehow seems decisive. In the case of each cell type, DNA is caught up in a seamless and integral way of being. When we grasp this integral nature, we quickly realize that the idea of DNA as the crucial causal determinant of the whole is an impossible one. As a specific kind of liver cell passes through its developmental lineage, it must sustain its entire organization in a coherent and well-directed manner from one cell generation to the next — including, for example, the cytoskeletal and cell membrane organization described in Chapter 4 (”The Sensitive, Dynamic Cell”). It must also bring about and orchestrate the elaborate performances of its chromosomes we saw in Chapter 3 (”What Brings Our Genome Alive?”) — performances that are unique to each type of cell and that chromosomes themselves have no way to set in motion.

Every individual part, including DNA, is shaped by, and gives expression to, the character of a larger whole. Only when we recognize that genes participate in a living whole can we find an answer to Lillie’s challenge “to explain how an unchanging complex can direct the course of an ordered developmental stream”. The answer — so we will find — is that there is no unchanging complex. Genes, like all parts of a cell or organism, gain their identities and meanings only within the context of innumerable, interpenetrating, living narratives expressing diverse physiological characters.

(from Chapter 7, “Epigenetics: A Brief Introduction”, in Organisms and Their Evolution — Agency and Meaning in the Drama of Life)

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