Aging/Senescence

Senescence follows growth, development, and reproductive adulthood in organisms that differentiate their somatic and germline cells. It includes all biological processes: loss of cellular integrity, crosslinking of macromolecules, and damage due to reactive oxygen species (ROS) and sugars, that increase the probability of death within the next unit of time. Aging includes the phenotypic signs of senescence: increased susceptibility to infectious and chronic diseases, loss of resistance to external and internal stressors, and inabilities to maintain and repair somatic systems. Senescence leads to cellular deteriorations that underlie age-related changes in physiology, physique, and phenotype - aging. Although often used interchangeably these terms are not the same. The evolutionary origins of senescence are still debated. Two prevailing views are a genetically programmed versus a stochastic process, secondary to the terminal differentiation of somatic as opposed to germline cells. Terminally differentiated cells exchange their ability to survive indefinitely to complete their tasks as myocytes, skin cells, or brain cells within multicellular organisms. Longevity is the amount of time that an organism lives in sidereal time, measured as hours, days, or years. Living systems generate detrimental molecules during energy production (ROS) and somatic responses to stress (cortisol, catecholamines). Long-lived species appear to have evolved multiple housekeeping mechanisms that reduce the ill effects of ROS and stress-related hormones, while expressing multiople repair systems for damaged DNA, allowing them to survive longer. Long-lived species tend to be larger (whales, great apes), able to escape predation (birds, bats), or show increased reproductive probability with increasing age (sharks, tortoises). Humans show extended life spans compared to other primates, even chimpanzees, likely due in large part to unique patterns of biocultural evolution. Many models for the uniqueness of human life history and longevity have been proposed. However, the same pocesses of senescence are seen in humans as in other primates.