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New demographic data on humans, animals and plants for the first time unveil such an extraordinary diversity of aging processes that no existing evolutionary theory can account for.
Despite aging being one the hottest topic in the media recently, scientists have no coherent explanation for it. New demographic data on humans, animals and plants for the first time unveil such an extraordinary diversity of aging processes that no existing evolutionary theory can account for. Both life spans and mortalities vary from species to species. The fact that the probability of dying rises with age applies to humans, but is not principally true. This is shown by a catalogue of 46 species with their respective mortality and fertility rates, which has now been published in the science journal Nature
. It is the result of a long-term data collection project led by scientists at the Max Planck Institute for Demographic Research (MPIDR) in Rostock, Germany, and at the Max-Planck Odense Center on the Biodemography of Aging (MaxO) in Odense, Denmark.
Not only are previous explanations unable to deal with life spans ranging from a few days (fruit fly), to decades (humans), to centuries (hydra), but they are also unable to account for variations in the death rate. Common theories assert that the probability of dying rises with age, as for humans. However, the researchers cataloged species such as the white mangrove and the desert tortoise whose probability of dying actually decreases with age. In addition, fertility periods of some species also challenge common theories.
Previous attempts to explain aging claim that creatures only invest in self-preservation until they have reproduced successfully and raised their offspring. Following this line of reasoning, when the end of the fertility period approaches, the body should start to decay -- which is known as senescence, or aging.
For humans this is only partly true. According to the Nature study, mortality of modern Japanese women rises constantly after childhood. But contradictorily, humans still live for a long time after fertility has ceased. Today, many people stay healthy until they are grandparents and their probability of dying is correspondingly small. Only at advanced ages is mortality growing rapidly. For example, in Japanese women 100 years old, mortality reaches more than 20 times their lifetime average.
This makes humans a real oddity. No other species in the researcher's catalogue has a mortality curve which rises that sharply. Even among other mammals, death rates reach no more than five times the lifetime average. Why evolution developed such big differences is a mystery to Biologists.