Introduction

Interest in theories of aging has undergone a revival during the last three decades or so. This is due partly to the relevant experiments being conducted on a molecular level and partly to studies of nematodes that suggest that life spans of species, that is, a putative maximal number of years alive, need not be fixed. Uncomprehending journalists love to seize on such nuggets of undigested information and to speculate on human longevity, suggesting that life may be extended to 150 years or more. However, most researchers have been taught not to argue from one species to another, so when considering the eyes, the author considers it prudent to confine discussion to the human species, however great the temptation to do otherwise.

A question that keeps cropping up concerns the existence of one or more genes for longevity. It would appear to be universally true that evolution responds to biological pressures, and it is hard to see what pressure could be brought to bear on living organisms to prolong their lives well beyond their reproductive capacity. There is little doubt that there exist genes for promoting cellular repairs, and it is easy to understand that they could develop in efficiency if the survival of a species would demand this. However, this is a concept very different from one postulating the ad hoc existence of genes for longevity. Hence, where applicable, the hypotheses underlying the considerations in this chapter rest on cellular repair, not vital prolongation.