Biosciences, i.e. the branches of natural science dealing with the structure and behaviours of livingorganism were known variously as 'Natural History', natural philosophy, and natural theology. Aging haslong since been ascribed to the gradual accumulation of DNA mutations in the genome of somatic cells.However, it is only recently that the necessary sophisticated technology has been developed to begintesting this theory and its consequences. It is increasingly important for physicians who treat olderpersons, and for researchers on aging, to understand how molecular biology informs clinicalexpressions of aging and age-related conditions such as osteoporosis, diabetes, osteoarthritis, frailty,cancer, and dementia.The ageing process is deleterious for fitness, but can nonetheless evolve as aconsequence of the declining force of natural selection at later ages, attributable to extrinsic hazards tosurvival: ageing can then occur as a side-effect of accumulation of mutations that lower fitness at laterages, or of natural selection in favour of mutations that increase fitness of the young but at the cost of ahigher subsequent rate of ageing.