Scientists have evolved three main theories to explain why we grow old. The first concerns the loss of cells or of irreplaceable parts.
Brain cells undoubtedly die off in their hundreds of thousands and cannot be manufactured again after a very infantile stage in human life.
However, this cannot be the complete explanation because people who suffer heavy damage to brain and body do not necessarily show the effects of ageing.
Moreover, animals have totally different ageing rates, but suffer cell destruction at similar speeds. A second theory concerns mutations or alterations.
A dividing cell does not always divide correctly. All kinds of errors may creep in, aided by natural radiation.
Some times the mutated cells may be harmful or put out of commission, with powerful effects on other cells, such as the endocrine glands or constituents of the blood.
In the 1960s this theory was supported by the discovery that 10 percent of the cells of very old women had lost an X chromosome.
A third explanation, which is not now so widely believed, is concerned with the accumulation of unwanted chemicals.
It is suggested that some vital substances can only be replaced at cell division and that a general decline in the rate of cell division could lead either to a lack of needed substances or an excess of unwanted ones.