Miller, on the other hand, said he believes that no clear distinction can be made between disease and other frailties of aging. “Anything a doctor can charge for we call disease, but wrinkled skin, white hair or not feeling good in the morning, these we don’t call disease,” he said.
He said he thinks that the idea of intrinsic aging is not well defined and that contrary to the theories of the evolutionary biologists, there may be simple ways to intervene in the aging process.
In the view of evolutionary biologists, the life span of each species is adapted to the nature of its environment. Mice live at most a year in the wild because owls, cats and freezing to death are such frequent hazards. Mice with genes that allow longer life can rarely be favored by natural selection. Rather, the mice that leave the most progeny are those that devote resources to breeding at as early an age as possible.
According to this theory, if mice had wings and could escape their usual predators, natural selection ought to favor longer life. And indeed the maximum life span of bats is 3.5 times greater than flightless mammals of the same size, according to research by Gerald Wilkinson of the University of Maryland.
In this view, cells are so robust that they do not limit life span. Instead the problem, especially for longer-lived species, is to keep them under control lest they cause cancer. Cells have not blocked the evolution of extremely long life spans, like that of the bristlecone pine, which lives 5,000 years, or certain deep sea corals, whose age has been found to exceed 4,000 years.
Some species seem to be imperishable. A tiny freshwater animal known as a hydra can regenerate itself from almost any part of its body, apparently because it makes no distinction between its germ cells and its ordinary body cells. In people the germ cells, the egg and sperm, do not age; babies are born equally young, whatever the age of their parents. The genesis of aging was the division of labor in the first multicellular animals between the germ cells and the body cells.
That division put the role of maintaining the species on the germ cells and left the body cells free to become specialized, like neurons or skin cells. But in doing so the body cells made themselves disposable. The reason we die, in the view of Thomas Kirkwood, an expert on the theory of aging, is that constant effort is required to keep the body cells going. “This, in the long run, is unwarranted — in terms of natural selection, there are more important things to do,” he writes.
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