Animals almost without exception (see Hydra) die in due course from senescence. Intervening phenomena which commonly bring death earlier include malnutrition, predation, disease, accidents resulting in terminal physical injury, or, in extreme circumstances, grave ecosystem disruption. Intentional human activity causing death includes suicide, homicide, and war. Roughly 150,000 people die each day across the globe. Death in the natural world can also occur as an indirect result of human activity: an increasing cause of species depletion in recent times has been destruction of ecological systems as a consequence of the widening spread of industrial technology.

The chief concern of medical science has been to postpone and avert death. Death in this context is now seen as less an event than a process: conditions once considered indicative of death are now reversible. Where in the process a dividing line is drawn between life and death depends on factors beyond the presence or absence of vital signs. In general, clinical death is neither necessary nor sufficient for a determination of legal death. A patient with working heart and lungs determined to be brain dead can be pronounced legally dead without clinical death occurring. Precise medical definition of death, in other words, becomes more problematic, paradoxically, as scientific knowledge and technology advance. Death remains a central mystery of life itself.

Natural selection

Death is an important part of the process of natural selection. Organisms that are less adapted to their current environment than others are more likely to die having produced fewer offspring, reducing their contribution to the gene pool of succeeding generations. Their genes are thus eventually bred out of a population, leading to processes such as speciation and extinction. It should be noted however that reproduction plays an equally important role in determining survival. For example, an organism that dies young but leaves many offspring will have a much greater Darwinian fitness than a long-lived organism which leaves only one.

Extinction

Extinction is the cessation of existence of a species or group of taxa, reducing biodiversity. The moment of extinction is generally considered to be the death of the last individual of that species (although the capacity to breed and recover may have been lost before this point). Because a species' potential range may be very large, determining this moment is difficult, and is usually done retrospectively. This difficulty leads to phenomena such as Lazarus taxa, where a species presumed extinct abruptly "reappears" (typically in the fossil record) after a period of apparent absence. New species arise through the process of speciation, an aspect of evolution. New varieties of organisms arise and thrive when they are able to find and exploit an ecological niche — and species become extinct when they are no longer able to survive in changing conditions or against superior competition.