Thesis #33 – When late-adult plateaus in survival and reproduction occur, members of biological cohorts that reach such plateaus will show stabilization of some but not necessarily all functional characters.
Evolutionary theory makes a simple prediction about how the key characters of our life history evolve at very late adult ages: they should tend to plateau. These plateaus aren’t necessarily precisely flat, but they are expected to be very different from the rapid deterioration shown during the aging phase.
During the aging phase, sooner or later our functions deteriorate. Some of these functions deteriorate glacially in humans, with our relatively slow pattern of aging overall. But other functions decline with remarkable speed. This is shown most publicly in the short careers of track-and-field athletes and gymnasts, who may be essentially non-competitive at the world level by the time they reach their early thirties.
After aging comes to a stop with respect to survival and reproduction, what of these subordinate functional characters? There are two broadly conceivable possibilities. One is that all these functional characters will show slowing rates of decline, gliding toward to a reduced but steady level of useful function. The other is heterogeneity among these functions, with some continuing to decline, while others stabilize or even improve.
Early data collected by my graduate student, Parvin Shahrestani, suggests that the latter is the case: after aging stops in aggregate life history, individual functional characters show some variation in their pattern of change. Some stabilize, and perhaps may rise slightly. Others continue to decline, sometimes at a slower rate. Finally, and amazingly, some functional characters actually decline at a faster rate after mortality rates have stabilized.
There are some hints that this is the human case as well. For example, there is no sign that wrinkling abates with age. Indeed, it appears to be the case that the tissue matrix of our skin accumulate wrinkles largely as a function of the movement of our skin and its exposure to sunlight, desiccating wind, etc. But consider memory. One of the characteristic symptoms of human aging is a declining ability to acquire new names for people, animals, and places, terms that have no reliable semantic pattern. On the other hand, some very old people develop a remarkable ability to recall events of their early lives, often in considerable detail. What may be occurring is that some functions may interfere with each other, such that the decline of some may lead to the reinvigoration of others. Thus, perhaps, as we lose our ability to acquire new “random” terms and facts, our access to more meaningful memories is enhanced?
Evidently, there is much to be learned of the patterns of functional change in the post-aging phase of life, both in experimental animals and in clinical patients. But the idea that late adult life is always a period of universal functional stability is not correct.