The law of mortality revisited: interspecies comparisons of mortality.

In 1825 the British actuary Benjamin Gompertz believed he had discovered a consistency in the timing of death in people that was so important that he labelled his observation a 'law of mortality'. To Gompertz, this 'law' was equivalent in importance to Newton's law of gravity because he believed it would be observed in all living things. Gompertz's quest for the 'law' eventually failed, as did similar efforts by other scientists in the 19th and most of the 20th century. However, the search for the law of mortality was successfully resolved in 1996 when my colleagues and I discovered that the only way to 'see' Gompertz's law expressed as common age patterns of death across species was to partition total mortality into its intrinsic and extrinsic components, and examine mortality schedules on a uniform time scale. Scientists had been unable to reveal the law of mortality in the past not only because they could not partition total mortality, but also because of the previous anthropocentric perspective that forced some scientists to view duration of life along a single time scale - one based on human measurements of chronological time. The law of mortality is relevant today not only because it links the epidemiology of disease, ageing and death across species, but because it creates a window into the future for those who study disease epidemiology in animals that now live long enough in protected environments to experience the biological consequences of ageing. In this paper I summarize the historical search for the law of mortality, explain why the solution could only be found by linking two seemingly unconnected scientific disciplines (evolution biology and actuarial/demographics), explain why age patterns of disease and death in humans may be used to understand and predict disease epidemiology in other species, and describe how a new scientific discipline has arisen in the modern era as a result of this research.