Science has an ugly word for it: senescence. It means getting old. Your bones start to creak, your brain begins to wander, and each day gets a little harder until, finally, you die.
That's nature's way, biologists have told us. But is it really?
Scientists at 13 institutions around the world decided to see whether that grim fate faces every organism, and they have made a startling discovery.
Aging is very different for some species in both the plant and animal worlds, so no one size fits all. Some actually get healthier as they get older, dying when they are reaching the apex -- not the pits -- of their lives.
"Many people, including scientists, tend to think that ageing 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," evolutionary biologist Owen Jones of the University of Southern Denmark said in releasing a report published in a leading science journal, Nature.
Jones, lead author of the study, and 13 colleagues, identified 46 organisms that don't follow the rules that were presumed to apply to all living tissues on this planet. So what does that mean?
It means maybe we don't know nearly as much about the aging process as we had thought.
The species include 11 mammals, 12 other vertebrates, 10 invertebrates, 12 plants and one algae.
One tiny animal, the freshwater Hydra magnipapillata, found in streams and lakes all over the Earth, is particularly interesting. The researchers think it could potentially live 1400 years and still be healthy. It probably doesn't live nearly that long in the real world, because it is a predator in a sometimes unfriendly environment, but some of them could, the researchers think, in the ideal conditions of a lab.
The scientists admit they were surprised at the diversity in aging that they found among the species they studied, although some followed the traditional pattern of aging: declining health leading up to death. But they didn't follow all the rules of evolution. Some, for example, remained reproductive until the end.
"Although it has been predicted that evolution should inevitably lead to increasing mortality and declining fertility with age after maturity, there is great variation among these species," the study concludes. "The diversity challenges theoreticians to develop broader perspectives on the evolution of ageing and empiricists to study the demography of more species."
The researchers are not challenging evolution itself. They are challenging our understanding of it.
Maybe studying these and other species that live and die by different rules will lead to better ways to cope with aging, which for humans and most other mammals, is a very complex, unforgiving and poorly understood process.
One star among the 46 species is the desert tortoise, whose rate of mortality actually declines as it ages. The decline, the study notes, "persists up to the terminal age."
The terminal age, or an animal's potential lifespan, is the species' oldest age at which 5 percent are still alive. For humans, Japanese women are the long livers, with 5 percent still breathing at the age of 102.
For other species, the champ is the Hydra, with a potential lifespan of 1,400 years. The shortest lifespan among the 46 belongs to the nematode worm -- 25 days.
Like humans, several species, including killer whales and Bali mynah birds, live long after they have passed their reproductive stage.