Theories of ageing (modified from ref 22, 27, 36)
| Type of theory | Explanation |
| Genetic-molecular | Codon restriction: accuracy of DNA-mRNA impaired |
| Error theories: accuracy of mRNA-proteins impaired | |
| Gene regulation: postreproductive changes in gene expression | |
| Somatic mutation: radiation damage to DNA | |
| Cellular | Wear and tear: with use the body wears out and dies |
| Age pigments: lipofuscin deposit as a primary cause of ageing | |
| Free radicals: lead to cell damage and thus ageing | |
| Cross linking: increased cross linkage of DNA leading to irreversible damage to DNA and enzymes | |
| Evolutionary | Wide range of species-species variation in lifespan |
| Disposable soma | Balance between energy investment in reproduction and maintenance |
| System level | Neuroendocrine control: a “biological clock” controls development through neural and hormonal mechanisms |
| Immune control: thymus gland as an “immunological clock” leading to cell destruction due to failure to recognise self | |
| Age changes | Changes occurring throughout the whole lifespan, often involving deterioration of an anatomical/physiological nature |