1.2.1 Mortality curves
In contrast to their young counterparts, old people are at higher risk of dying, irrespective of the cause. This simple fact of observation received the attention of Alex Comfort. While discussing a way to measure the ādecrease in viability and an increase in vulnerabilityā, which he referred as the properties of senescence (i.e. ageing), he defined this process as a āprogressive increase throughout life, or after a given stadium, in the likelihood that a given individual will die, during the next succeeding unit of time, from randomly distributed causesā (Comfort, 1956; the sentence was maintained in the following editions of the book).
Such a strict definition of ageing is in fact a description of the mortality rate, as a function of the final event of the ageing process but not a description of the conditions underlying this rate. However, the elucidation and quantitative assessment of the progress of these conditions are critical.
To measure and analyze the mortality rate, one should have access to a cohort of people and verify the lifespan of each cohort member; the ensuing mathematical analysis would then produce a theoretical model for the interpretation of the data. The first record of such analysis, perhaps the first scientific assessment related to ageing, was made in the 19th century by the English actuary Benjamin Gompertz. He found that, after a considerable risk of death during early infancy, there was a reduction that extended through to young adulthood; from then onwards, the risk increased progressively, doubling every 8 years. This finding, later adapted to become known as the GompertzāMakeham law, is thus a measurement of the risk of death and is usually depicted as a curve, where the mortality rates, or probabilities of death, are plotted against age. It is objective, as it relates to a clear event of the organismās life, and harmonizes with the common intuition concerning the progressive nature of the ageing process that causes the elderly to die at a faster rate than the youth. Recognition of the GompertzāMakeham law in other human populations and other species, including invertebrates, was important for further support for its biological value (Gavrilov & Gavrilova, 2006; Olshansky, 2010).
However, it should be pointed out that the data were collected in populations in specific environments. In fact, humans do not live in the wild as they have regular access to food and are medicated when ill; the other studied animals were confined to protected laboratory environments, therefore avoiding exposure to the hazards of wild-living, where death by accident, famine or predation is common.
The improvement of human living standards observed throughout the first half of the 20th century changed the Gompertz curves because of the decline in mortality rates in infancy. In more recent decades, however, continued sanitary improvements have modified the mortality rates of the remaining members of the cohort. Consequently, while the curves established in the 19th century fit well to mortality trends of the adult human population until the age of around 80 years old, they fail to do so after that because the logarithmic increase in the death rate tends to level off and even decelerate (Vaupel et al., 1998; Rau et al., 2008). As a result, postponement of mortality has continued and the number of centenarians has increased in all developed countries. Another consequence, more subtle and assuming that improvements in public health as a whole will continue, is the upward shift in the limits of human longevity (Vaupel, 2010). Surprising as it may be, estimates have shown that there has been an increase in life expectancy of 3 months each year since the middle of the 19th century (Oeppen & Vaupel, 2002) that is likely to continue, raising the possibility that, in the not so distant future, a significant number of humans will live beyond 100 years.
Therefore, the conviction that Gompertz curves reflect an intrinsic biological principle of ageing, as was thought (Sas...
