It’s a challenge to imagine artifacts surviving from the Persian conquest of Jerusalem in 614, let alone 10-millimeter long freshwater creatures. Even so, American biologists are describing just such a creature, Hydra magnipapillata, as “effectively immortal” due to its extraordinary ability to continuously reproduce its cells.

In a creature whose entire body measures less than a centimeter and consists of just a mouth, foot and tentacles, this may seem an insignificant feat, but it has given the creature the ability to live for as much as 1,000 years or more. Its secret is nothing resembling an elixir of life or fountain of youth; rather, it is a simple immunity to the aging process we are accustomed to envisioning.

While the animals do age, the process is a nonlinear cycle. When cells become old, new ones are generated to take their place; the process continues for up to hundreds of years. Scientists estimate that in 1,400 years, five percent of a Hydra population could still be living given continued laboratory conditions.

Hydra magnipapillata are not much to look at, unless you happen to have a microscope at hand. Even so, the creatures resemble nothing so much as thin, pale, translucent straws, with equally translucent (but somewhat thinner) pale tentacles protruding from one end. Hydra appear rather unremarkable, aside from the obviously unusual quality of near-immortality, at least in laboratory settings.

Dr. Owen Jones, an evolutionary biologist of the University of Southern Denmark, notes that our natural inclination is to regard human aging as a ubiquitous process and equally applicable to every living thing. As it turns out, there are many variations in the phenomenon of aging across different species. Lobsters, for instance, never age. “Many people,” Dr. Jones observes, “tend to think that aging is inevitable and occurs in all organisms on earth as it does for humans — that every species becomes weaker with age and more likely to die. But that is not the case.” Fertility patterns also vary significantly among species.

While Hydra magnipapillata live in a continued state of low mortality (that is, it is no more likely to die after 100 years than after ten), some species experience decreasing mortality throughout their aging processes. The desert tortoise is such a creature, experiencing the highest risk of mortality early in life, but living for up to 80 years after surviving childhood. Among plant species, this aging pattern is yet more common.

Curiously (from the human perspective at least), some species of both plants and animals also experience increased fertility in their later years. One such species is the agave plant, native to Mexico and South American countries. With these other patterns of fertility and mortality in mind, Dr. Jones proposes giving “aging” a redefinition. It’s nonsensical, given what we know, to define the process of aging in terms of how long something can live. Rather, he suggests, we should consider aging as “based on the shape of mortality trajectories.” If our comparison is interspecies, I suppose we could say that aging is relative after all.