"For decades, demographers and gerontologists noticed that late life human data did not fit [expected] models: there was a shortage of deaths," write Michael R. Rose, Casandra L. Rauser, and Laurence D. Mueller. "More specifically, the exponential increase in age-specific death rate seemed to slow down considerably, if not cease."
The sudden plateau in mortality rates after a certain age has long been observed with other organisms, but its presence in human populations has been dismissed as a result of the advent of nursing homes and modern medicine. However, close examination of demographic data supports a distinct third phase of life history known as "late life," characterized by the cessation of age-related deterioration.
"Late life is a unique and distinct phase of life very different from aging," write the authors. "Each phase evolves according to very different rules. Evolutionary biology has a new set of problems to solve."
The authors posit that late life arises after the forces of natural selection affecting both fertility and mortality cease to have an impact.
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Rose, Michael R., Casandra L. Rauser, Laurence D. Mueller. "Late Life: A New Frontier for Physiology." Physiological and Biochemical Zoology 78:6.