Senescence is the process of aging, both in whole organisms and individual cells within these organisms. Researchers have focused heavily on human senescence, looking at how humans age, and why, in the hopes of perhaps someday treating the problems associated with senescence, which range from eventual breakdown of the human body to mental problems. Senescence is also a topic of interest more generally in the field of biology, as it occurs in all living organisms on Earth.
The process of senescence is incredibly complex, and accompanied with a myriad of chemical and physical reactions. As organisms age, they slowly break down, experiencing tissue death and more general malfunction, whether they are plants, animals, fungi, or one-celled organisms. Without the processes of senescence, organisms would be immortal, only subject to death if they were severely injured or killed, and the Earth would be choked with living organisms as a result.
From a biological standpoint, senescence promotes evolution, and keeps Earth from being too crowded. Many organisms are designed to reproduce at a very young age, sinking their resources into the creation and nurturing of young, and as a result, their bodies eventually give out. Researchers have suggested that this is a calculated decision on the part of nature, which is well aware that chances of survival go down with each year of life, due to accidents, injury, attacks by predators, and so forth. By making early reproduction a priority so that species can survive, individuals, in turn, are subject to senescence.
People can study cellular senescence, which focuses on the aging of specific cells, as well as organismal senescence, which looks at the aging of an entire organism as a whole. Cellular senescence can be very interesting, as it can be used to see why some cells age differently than others, and how aging impacts the ability of a cell to function. One general rule of cellular senescence seems to be that cells cannot reproduce themselves again after 50 divisions, and this in turn explains why many organisms tend to experience a variety of problems all at once with age, as all of their cells break down.
Organismal senescence is also a very interesting field of study, especially for researchers who have access to extremely old individuals. For example, tortoises and parrots are famous to living to great age in their natural environments, and study of these animals could explain why they age so slowly, and live so long. Individual long-lived representatives of a species can also provide interesting fruit for study among researchers who want to learn more about the process of aging and eventual death.