Life Span: A Conceptual Overview

perspectives on life span Life span should be considered within a larger biological framework concerned with life duration—a model where the end points of life for an individual do not necessarily correspond to the conventional events of birth PDR 29 Supp Carey.sp 7/6/06, 3:16 PM 1 2 L I F E S P A N : A C O N C E P T U A L O V E R V I E W and death. The concept of life span does not apply to bacteria that reproduce by binary fission, to plant species that reproduce by cloning, or to modular organisms with iterated growth such as coral or honeybee colonies. The state of existence for an individual refers to one of several possible states of living such as “normal state,” metabolic reduction (e.g., hibernation), or arrest (e.g., frozen embryo; dormancy); its time of existence refers to the sum total of all stages of the individual’s life up to its extinction. What constitutes an individual is conditional on the level of individualism. For example, the individual cell within a multicellular organism and the individual worker bee in a colony of honeybees are individuals on one level but are part of a greater whole on another. Death of a component part (e.g., through apoptosis) may or may not determine the death of the whole, although in each case the components (cells; worker bees) relinquish both their evolutionary heritage and their autonomy in favor of the higher organisms (or in the case of honey bees, the colony as “super-organism”) of which they are a part (Michod 1999). The distinction between mortal macroorganisms and immortal microorganisms is unclear with certain organisms. For example, slime molds such as Dictostelium discoideum feed most of the time as millions of individual, potentially immortal single-celled amoeba in the forest soil and litter. Under certain conditions, however, these millions form aggregate “swarms” in dense, coordinated, visible, mortal masses that look slug-like. These aggregates then sprout into a “flower” to create and disseminate spores (Bonner 1967; Murchie 1967). A different life span issue concerns the concept of replacement of constituent parts (i.e., cells). The hydra, Hydra littoralis, replaces all of its cells over the course of a few weeks and thus can potentially live forever (Campbell 1967). When a single reproductive event that occurs at the end of the life course results in the death of the individual, then life span is linked deterministically to the species’ natural history. This occurs with the seed set of annual plants (grasses), in drone (male) honeybees as a consequence of the mechanical damage caused by mating, in many mayfly species when a female’s abdomen ruptures to release her eggs after she drops into a lake or stream, and in anadromous salmon that die shortly after spawning. Life span can be considered indeterminate for species (including humans) that are capable of repeated (iteroparous) reproduction. This concept is consistent with everything that is known about the lack of physiological cutoff points in biology: all evidence suggests that species do not have an internal clock for terminating life. Death and extinction Death is the unique and singular event that ends life. Dying is a graded process in virtually all organisms starting with substages of infirmity (weakPDR 29 Supp Carey.sp 7/6/06, 3:16 PM 2 J A M E S R . C A R E Y 3 ness) and morbidity (sickliness) and progressing to coma, breath stoppage, heart stoppage, brain death, cessation of metabolism, tissue degradation, and loss of DNA integrity. Because of this progression of death by stages, it is useful to distinguish between the death of the organism as a whole, which refers to somatic or organismic death (the organism as an integrated functional unit), and the death of the whole organism, which refers to death of the various tissues and cells (Kass 1983). Whereas the death of the organism as a whole is usually an event—an abrupt crossing-over from the live to the dead state—the death of different parts of the whole organism is a protracted process of degradation. In broader biological contexts, the ways in which individuals can become extinct (as opposed to extinction by dying) as singular entities are through fission in bacteria where one individual becomes two or through fragmentation as in fungi where the filaments of the original fungal growth break into many pieces, each of which is essentially a seed for new growth. A remarkable example is splitting in flatworms where the original individual can be divided into parts, each being capable of growing into a new worm. In each of these cases, the original individual no longer exists; no part or division has precedence over the other. End points and perpetuity Mortal refers to individuals destined to die, immortal refers to individuals living forever (however, it also is used in the context of immortal germ lines), finitude (or finite) refers to limits, infinitude (or infinity) to anything without limits, determinate refers to a fixed process or program, and indeterminate refers to an open-ended process or rate. The term immortalization describes cell cultures that can be propagated generation after generation with no loss of viability (Blackburn 2000) such as the HeLa cell line (the first human cells to live indefinitely outside the body), which was derived from and named after cancer victim Henrietta Lack’s cells in the 1940s (Murchie 1967; Skloot 2001), and immortalized B lymphocytes, which are used to produce virtually unlimited quantities of identical antibody molecules (Köhler and Milstein 1975). The term preservation refers to maintenance in the existing state. The bacteria trapped in 25–30-million-year-old amber (Cano and Borucki 1995) or in a 250-million-yearold primary salt crystal (Vreeland et al. 2000) and the mitochondrial DNA isolated from a 29,000-year-old Neanderthal (Ovchinnkov et al. 2000) are all examples of preservation. Cryopreservation is a special type of preservation using ultra-low temperatures (–196o C) to preserve human sperm and embryos (Silver 1998; Trounson 1986). Dormancy describes any one of a number of evolved types of arrested growth ranging from torpor to hibernation (Carson and Stalker 1947; Hairston and Caceres 1996; Mansingh 1971; Masaki 1980). PDR 29 Supp Carey.sp 7/6/06, 3:16 PM 3 4 L I F E S P A N : A C O N C E P T U A L O V E R V I E W