The Evolution of Death: Why We Are Living Longer

Chapter 6. Neoteny and Longevity

Chapter 6 Neoteny and Longevity The young of the human race show some anthropoid features, whereas the young of the chimpanzee approach more nearly to the human than the adult chimpanzee does. That seems to show that . . . our ancestors were more Simian than we are. —Agatha Christie, The Man in the Brown Suit Obviously, a transformation or metamorphosis is necessary in order that the adult organism may function (except that the neoteinic type of organism functions and grows as a larva until it attains maturity). —N. J. Berrill, Growth, Development, and Pattern The bagpipe model of life extension illustrated in chapter 2 suggests that we are living longer because our juvenile stage of development is percolating into our adult stages. Chapter 6 examines this suggestion, beginning with a discussion of the phenomenon of juvenilization known in the evolutionary literature as neoteny, from the Greek meaning stretching (“extending” or “holding onto”) the new or youthful.1 The chapter goes on to suggest that increased numbers of self-renewing (SR) stemcells acquired during development might slow down the rate of aging in adults.2 The additional SR cells might be gleaned from stocks of primordial germ cells (PGCs) that would otherwise have become germ cells. Were that the case, the current trend toward reduced fecundity may be linked to the current trend toward increased longevity. 133 THE TIME IS OUT OF JOINT Neoteny belongs to the class of evolutionary mechanisms known as heterochronies or age-related deviations of development.3 Heterochrony is diagnosed when deviations in timing and/or rate of development lead to asynchronies among processes or to disparities in morphology, for example, when parasites exhibit hypersexual development and morphological reduction.4 Caleb Finch followed the path of heterochrony to aging.5 Taking his lead from Gavin de Beer and Stephen Jay Gould, Finch called juvenilization “paedogenesis ” and attributed it to either of two evolutionary processes: progenesis , or paedomorphosis, and neoteny, or fetalization.6 Progenesis and neoteny may be thought of as running in opposite directions—one accelerating, one slowing, and one affecting larvae or juveniles, one affecting adults. In progenesis , sexual maturity is pushed back into the juvenile stage, whereas in neoteny, development slows and sexually mature adults retain juvenile morphology . For example, aphagous dipterans, aphids, and mayflies that form oocytes before hatching are progenic, as is the tiny, pedomorphic vertebrate, the infantfish, Schindleria brevipinguis, whose lifetime is over at two months.7 In contrast, Finch defines neoteny as “sexual maturation at the usual age, but with retarded development of the other somatic tissues.”8 The facultative neotenic Mexican axolotl, Ambystoma mexicanum, and the tiger salamander, Ambystoma tigrinum, which are sexually mature but morphologically larval, are the classic examples of neoteny. Adult development is suppressed by comparatively low levels of thyroid hormone, even though thyroid hormone receptors are present and capable of binding exogenous hormone.9 Neoteny is epitomized by asynchronous, slow development of larval characteristics coupled with the maturation of gonads. While becoming sexual, the plastic juvenile morphology remains dominant. That is, individuals mature sexually while retaining the characteristics of youthfulness. Historically, Albert von Kölliker is credited with proposing that larvae might acquire sexuality, while Alexandr Onufrievich Kovalevskii (Alexander Kowalevsky) first suggested “that the larval ascidian might be the actual ancestor of the vertebrates.”10 Walter Garstang then completed the loop by suggesting that the development of sexual maturity in an overgrown, swimming ascidian larva resulted in the loss of the original adult ascidian.11 Contrary to Ernst Haeckel’s notion of recapitulation that would have locked developmental stages in an irreversible sequence, Garstang’s “neoteny” uncoupled adult morphology from sexual maturity and allowed the latter to work with larval morphology. Neoteny has long figured into many evolutionary schemes. The celebrated zoologist Libbie Hyman, among others, suggested that the bilateria, which is to say most animals, originated via neoteny from larval radiates 134 HOW DEATH EVOLVES AND WHERE IT IS HEADING resembling cnidarian’s planula larvae.12 Indeed, fish (ice goby), amphibians (the obligate neotenic mud puppy Necturus maculosus and Proteus anguinus), birds (the flightless ratites), and altricial mammals, notably ourselves, seem to have taken the route of neoteny and adopted new evolutionary directions out of old evolutionary patterns. Human beings seem to be experiencing evolution both by progenic (selected, accelerated growth), and neotenous (generalized slow and prolonged development) mechanisms. Progenesis is also suggested by the “[a]cceleration of the rates of maturation during the past...