The Evolution of Death: Why We Are Living Longer

Chapter 3. Rethinking Lifecycles and Arrows

Chapter 3 Rethinking Lifecycles and Arrows There is no generally valid, orderly relationship between the average duration of life of the individuals composing a species and any other broad fact now known in their life history, or their structure, or their physiology. —Raymond Pearl, The Biology of Death He knew it was impossible for human physiology to change at less than glacial speed, but he suspected that some shocking transformation had nevertheless taken place in what was required to sustain human life. —Donna Leon, Willful Behaviour Even were one to concede that death is part of life, one would want to see how death is integrated into life’s other features before accepting death as evolving. Well, let us not mince words: one must reorient oneself entirely to life in order to integrate death. I cannot pretend that biologists share a single view of death’s integration into life. In fact, the two most widespread views—the cyclic view and the linear arrow view—portray life, and hence death, in entirely different ways. When life is viewed as a cycle, death is seen as the continuous ejection of corpses, while, when life is viewed linearly, death is seen as the endpoint for living things (bodies or somas, thalli, clones, and cells), branching off of an immortal germ stream. In cycling life, death plays an active role, and, thus, death has the potential to evolve. On the other hand, in linear life, death is excluded from an active role in shaping life and therefore the possibility of death’s evolution is barred. Thus death can evolve only if life is cyclic. 57 LIFE AS A CYCLE: LIFECYCLES CONNECT LIFE TO LIFE The notion of life cycling is really not that old, having been formalized as biogenesis by Thomas Huxley at the fin de siècle in contrast to the defunct notion of spontaneous generation, or life arising spontaneously from nonliving matter. Huxley’s doctrine was immediately distorted into a new version of William Harvey’s “all life comes from eggs,” and Rudolf Virchow’s “all cells come from cells” and corrupted into Ernst Haeckel’s “biogenic law,” that all life repeats its evolutionary history. Today, reinforced by molecular cladistics, biogenesis has returned to Huxley’s original doctrine that life is the source of life, and, hence, life cycles (open arrows in figure 3.1). Since the late nineteenth century, a “life cycle” or “life-cycle” has stood for life’s continuity.1 Here, “lifecycle” (the equivalent of “life’s cycle” but without a space, hyphen, or apostrophe) is employed with the intention of signifying continuity among life’s molecules, its cells, and organisms. This continuity also connotes an energetic relationship. In the ideal, Newtonian world, cycles are endless, and simple harmonic motion goes on forever. But, in the real, macroscopic world of living things, nothing moving fails to encounter resistance, and, hence, nothing moves forever without being pushed. At every level of complexity, from molecular to 58 HOW BIOLOGY MAKES SENSE OF DEATH FIGURE 3.1. A lifecycle, as proposed by Thomas Huxley. ecological and from cell to organism, at many points and in many ways, from inborn errors and outside pressures, smothering excess and asphyxiating absence, pollution and predators, accidents and hazards friction threatens life’s momentum. At the same time, the catabolism of resources obtained from the environment pushes lifecycles through bioenergetics, replication (DNA-dependent DNA synthesis), transcription (DNA-dependent RNA synthesis ) and translation (RNA-dependent protein synthesis), development and differentiation, growth and maintenance, anatomy and physiology, behavior and reproduction. Most of what one thinks of as life’s adaptations occurs on the power side of lifecycles, namely, the resources that give life more bounce to the ounce. Life’s devices for reducing friction, on the other hand, are less obvious. Thus, living things constantly dissipate energy and excess, waste and wreckage (the thin arrows in figure 3.1), all of which would otherwise drag down lifecycles and threaten life’s momentum. The environment absorbs many potential sources of drag, frequently recycling them back to life, but death does the lion’s share of removing drag. Death reduces life’s detritus to corpses that can be shed from lifecycles with a minimum of loss, disruption, and injury. In other words, death is the milieu that lubricates lifecycles. TYPES OF LIFECYCLES Lifecycles are not like other cycles in one way: lifecycles consist of two or more parts that...