Immortality of the germ-line versus disposability of the soma.

The germ-line of species is immortal, at least in the sense that an unbroken continuity extends backwards in time to the origin of terrestrial life and forwards in time to an indeterminate future. This observation is a truism. However, when set against the fact that the bodies of higher animals are intrinsically mortal, yet composed of the same basic materials as their germ cells, the observation leads to the central puzzle of gerontology. Each new-born individual begins its life as young as did each of its ancestors but with the same certitude, if it does not meet with an earlier accident, that within a specific span of time it will arrive, through a process of steadily accelerating decrepitude, at death. The puzzle has two sides: why and how has the somatic part of higher animals come to be mortal, and how is the germ-line kept free of the progressive deterioration in the soma?

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