Abstract
Dean's reflection on the evolution of the central nervous system cholinergic system and its possible contribution to schizophrenia is fascinating [1]. In discussing epigenetics, however, he perhaps inadvertently perpetuates a common misunderstanding, and thus goes too far in talking of a fusing of Darwinian and Lamarkian evolution. He states, “…while epigenetic mechanisms were predicted not to be heritable, trans-generational epigenetic effects have now been identified”, implying, surely, that they are heritable. It is a partial truth: Youngson and Whitlaw, the authors he cites, indeed provide some examples of multigen-eration gametic transmission of an original environmental effect, but explicitly emphasize, even in the abstract, that this is the exception not the rule [2], It is difficult for changes in the soma to get into the germ line if those lines separate early in development. They make the further point that epigenetic processes are most useful in fine tuning an organism to environmental differences that vary over short periods, a generation or two, but preserve intact the genome that has evolved over millennia for a broader, ‘average’ environment.
Our evolved predisposition to psychiatric disorder may have been advantageous in its evolutionary environment, or a price paid for another adaptation, but in most of us remains quiescent. Certainly it would be useful to discover the epigenetic switches so we could prevent expression of disorder. They may be similar to those intrauterine experiences that ‘tune’ the foetus's metabolism (epigenetics) to an anticipated extra-uterine environment, sometimes erroneously. They are one generation effects.
We seem to have an evolved (genetic) propensity for ‘attachment’ without which we wouldn't survive at all. Parenting draws out a particular style (reflected in epigenetic changes to the genome) that works in that specific social environment, but may cause problems elsewhere, and may itself change with time. The behavioural manifestations (in the grown-up offspring) are then experienced by the next generation, whose genome is epigenetically tuned in turn. Multigeneration, but not heritable or ‘gametic’.