The biological reasons for ageing: a major discovery of the late 20th century
After many decades of uncertainty and indecision, at the end of the twentieth century the biological reasons for ageing in most animals became very clear for the first time. All but the very simplest animals reproduce sexually, which means that they have a soma, or body, and a germ line that transmits their genes to the next generation. The resources available to any animal are allocated to three primary functions. The first is standard metabolism, which includes respiration, the synthesis of all essential metabolites and macromolecules, movement, feeding, digestion and excretion. The other two are all reproductive functions, and the many different mechanisms that maintain the integrity of the soma, at least for its reproductive lifetime. The resources allocated to maintenance and reproduction varies between mammalian species. Small rapidly breeding species invest more in reproduction, less in maintenance and consequently have short life spans, whereas slow breeding species, including man, invest more in maintenance and have long lifespans. There is now much evidence from comparative studies that the efficiency of maintenance mechanisms is directly correlated with life span. The main conclusion is that ageing is due to the eventual failure of the maintenance of the soma, and this also means that there are multiple causes of ageing. The evolved structure of the mammalian body is in fact incompatible with continuous survival, and it can be shown that the increased investment in resources that would be needed to maintain the soma indefinitely would reduce Darwinian fitness in a natural environment. The evolution of a limited lifespan is the optimal strategy for species survival, because in such environments populations are age-structured. Most animals die from predation, disease, starvation or drought, and very few, if any, will reach senescence.