Rates Of Aging
Everything that exists in time ages, but rates of aging within living and nonliving realms vary greatly. Geologists and paleontologists who study the earth's history use terms of reference in the hundreds of millions of years. Evidence for the earth's aging is sought in the erosion of mountain ranges or the effects of plate tectonics on the making of continents, and sedimented fossils of extinct plant and animal species mark out a precise record of the earth's aging. These, and related phenomena, beyond the human experience to discern, are subject to the restless vicissitudes of what scientists call the earth's deep time. Physicists, meanwhile, indicate the decay of subatomic particles in units of time so brief as to be unintelligible to the ordinary human mind. Living organisms also vary greatly, but within time spans ranging from minutes to millennia, rather than microseconds to millions or billions of years. From the days or weeks of unicellular organisms to the months of rodents or decades of primates, animal aging appears tied in part to size and complexity. Many plants, however, do not appear to have a "natural" lifespan: There are trees living up to three thousand years, such as the giant California sequoias (the General Sherman Tree in California's Sequoia National Park is estimated to be between three and four thousand years old). Among vertebrates, certain families of species seem to live longer than others: parrots among birds, tortoises among reptiles, and elephants and primates among mammals. What is biologically important is the tempo at which such creatures live their lives rather than the actual length of time they live. In turn, a species' or a creature's tempo is determined by laws of size, environmental niches, reproductive cycles, and metabolic rates. As Stephen Jay Gould notes, a rat may live at a faster rate than an elephant, but this does not mean that it lives any less than an elephant (Gould, 1977).
Among primates, Homo sapiens fully evince the paradox of aging posed by the higher primates. On the one hand, humans live the longest and take the longest time to mature. On the other hand, humans are the most "youthful" primate, because their lengthy neotenic, postnatal development ensures an extended retention of youthful mammalian features (such as a large brain relative to body size and a playful curiosity). Neoteny also means that humans, born relatively helpless and unformed, develop traits and characteristics outside the womb that most primates develop soon after birth, and with which most other mammals are born. Thus nature's experimentation with increased primate intelligence, carried to a high point in humans, has produced a course of life where more areas of behavior are shaped by societal and family learning than by instinct. The special product of this unique evolutionary experiment is human intelligence and the creation of culture and history as key forces in the species' development.
It is a cosmic irony, therefore, that this intelligence allows us to be aware of our own aging. As the only animal conscious of its own mortality, we have invented many different ways to deal with this knowledge across cultures and over the course of history. In Europe, Medieval and Renaissance thinkers saw aging and dying as part of the universal order, represented by the elements of the earth, the cycle of the seasons, and the movement of the planets. The modern biological and social sciences have developed theories of aging based on cellular, neurological, genetic, physiological, psychological, social, and demographic factors. Whereas cytogerontologists, such as Leonard Hayflick, locate the secrets of aging in cellular biology, those in the humanities, such as philosopher Ronald H. Manheimer, seek it in human wisdom and social relationships. Social and psychological gerontologists connect research on individual health, longevity, and cognitive abilities to wider issues of social inequality, gender, race, housing, and lifestyle. Broader still are demographic and global studies that profile the aging characteristics of whole populations. Thus, human aging, from the cell to the population, is a multifarious process that requires study using a multidisciplinary approach.