The Biology of Aging: Observations and Principles

By Robert Arking | Go to book overview

11
Systemic Theories of Senescence

11.1
If Aging Is Systemic, Why Is There
So Much Variability?

Systemic theories of biological function often invoke thoughts of an organized program designed to bring about the function in question. The word “program” means different things in different contexts, but it often is interpreted as meaning a set of coded instructions that a computer must follow to complete the task assigned to it. One implication of this concept is the idea that there is a very tight and precise linkage between the coded instructions and the operation of the computer; the computer can do nothing that is not accurately written down in the instructional sequence beforehand. Biologists use terms such as “genetic program” or “developmental program” as a convenient shorthand to describe the coordinated and sequential events that constitute organismic development. This adoption of a metaphor originally developed in cybernetics makes it easy to assume that the rigor and precision of the computer term is equally valid of the biological term. This is not correct.

Many of the processes upon which normal plant and animal development depend are not directly specified by the nucleotide sequence in the genome (Wilkins 1986). These essential processes are emergent properties, implicit perhaps in our definition of what we expect normal ectodermal or mesodermal cells, for example, to do, but not explicitly set down (Edelman 1988). During embryonic development, different cells predictably migrate to certain regions of the body and meet with another population of distinct cells originating elsewhere that have predictably migrated to the same area, and the two populations interact to form a new tissue type. To the best of our knowledge, no gene directs an ectodermal cell to migrate to a specific position and there interact in a certain way with a mesodermal cell type that bears complementary instructions. Because these are emergent properties that are not precisely spelled out in any single DNA strand, we should not be surprised if any two organisms show significant environmentally modulated variation (i.e., chance) in the timing and manner with which these higher-order interactions are expressed (see Kirkwood and Finch 2002).

As a result of “noise in the system,” the developmental program of humans, for example, gives rise after about 266 days to a baby. But as any obstetrician and most mothers know, very few infants are born after exactly 266 days of development. Most (about 75%) are born sometime within 252–278 days, and a substantial minority are born at even more extreme times. These differences amount to more than a 10% variation in normal developmental rate. Our own developmental program yields not a precise result, but a normal probability distribution of results. Emergent properties are more prone than determinant ones to vary at the macroscopic level as a result of random microscopic variations. Now, if aging itself is an emergent property, why should we be surprised when human adults— subjected to considerably more environmental modulation than any fetus—show comparable variation in the times at which individuals display

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