Our life span is restricted. Everyone knows this and everyone accepts this as 'biologically' obvious, though for most of us this life seems to be too short. 'othing lives forever.' However, in this statement we think of artificially produced, technical objects; products which are subjected to natural wear and tear during use. This wear and tear leads to the result that at some time or other the object stops working and becomes unusable ('death' in the biological sense). But are the wear and tear and loss of function of technical objects and the death of living organisms really comparable or even similar?

An organism possesses many mechanisms for repair. It is not in principle necessary that a biological system should age and die. Nevertheless a restricted life span, aging and then death are basic characteristics of life. The reason for this is easy to recognize: in nature the existent organisms are regularly replaced by new types. Because of changes in the genetic material (mutations) these organisms have new characteristics and in the course of their individual lives they are tested for optimal or even better adaptation to environmental conditions. Immortality would disturb this system – it needs room for new and better life. This is one basic problem of evolution. Thus death is a basic precondition for the frictionless and rapid development towards better adaptation to the dominant environmental conditions. The restriction of life by death is then sensibly not left to chance, such as disease or accident. It is thus evidently an inherent property of the system of the organism from the first moment of its development. Life span and death are thus programmed from the start of life. This is known as the hypothesis of genetically programmed aging, ending in death, and is not particularly controversial among scientists. The theory does not necessarily posit aging in the sense of slow loss of function before death. Many organisms even die at the zenith of their physiological abilities. For example, many sorts of plant die shortly after flowering and many insects, fish and worms and other animals immediately after reproduction. This is a particularly clear demonstration of the programmatic character of death. The rare 'Progeria', a hereditary human disease which leads to premature aging, is a further very clear demonstration of the genetic basis of the aging process.

If life span is a genetically determined biological characteristic it is logically necessary to propose the existence of an internal clock, which in some way measures and controls the aging process and which finally determines death as the last step in a fixed programme. This last step can of course consist of a long succession of different ontogenetic processes. It is of great interest to investigate the site and the function of the 'clock' for life span and on the question of the unit and the beat in which this clock 'ticks'. Are these ticks for example heart beats, breathing acts, metabolic rates or something else? There are a large number of theories dealing with these questions and on the control and the bases of the aging processes per se.