Eventually, this process results in death, often as the result of disease and often because of internal failure. We know neither the causes nor in most cases, the mechanisms of aging. We do not know whether it is intrinsic to development or an aberration of it. We do not know whether the fundamental process of aging occurs on the level of cells, on the level of the whole organism or somewhere in between. There are hints suggesting these possibilities but none are convincing.
Scientists know much about death rate and the related life span of many organisms.
They vary a great deal even among related species. Furthermore, in humans and most other complex organisms, the death rate increases rapidly with age. In humans over the age of thirty years, the death rate doubles every seven years. Therefore each species has a fairly well defined life span. Most organisms that die of disease live to an age that is not very different from that of their species.