Time between birth and death. It ranges from a mayfly's day to certain trees' thousands of years. Its limit appears to depend on heredity, but such factors as (in humans) disease, natural disasters, war, diet, and habits such as smoking reduce it. Maximum life span is theoretical; more meaningful is average life span, which life-insurance companies and actuaries analyze and tabulate. Long-lived progenitors tend to beget long-lived descendants. A very-low-calorie diet appears to prolong life. Reduced infant mortality and improved sanitation and nutrition account for much of the increase since circa 1800—from about 35 to over 70 years in most industrialized countries. The oldest well-documented age reached by a human is 122 years.
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Reflections on longevity have usually gone beyond acknowledging the basic shortness of human life and have included thinking about methods to extend life. Longevity has been a topic not only for the scientific community but also for writers of travel, science fiction and utopian novels. There are many difficulties in authenticating the longest human lifespan ever, due to inaccurate birth statistics; though fiction, legend, and mythology have proposed or claimed vastly longer lifespans in the past or future and longevity myths frequently allege them to exist in the present.
Longevity according to the psalms of the Bible was estimated on average to be "threescore and ten", that is 70 years, and "by reason of strength be extended to fourscore", that is 80 years. In addition, Solon, the famous lawgiver of Ancient Greece, in his dialogue with Croesus stated 70 as the allotted length of life for man. The longest living person as recorded in the Old Testament was Methuselah, who was said to have lived nearly a millennium.
There are many organizations dedicated to exploring the causes behind aging, ways to prevent aging, and ways to reverse aging. Despite the fact that it is human nature not to wish to surrender to old age and death, a few organizations are against antiaging because they believe it sacrifices the best interests of the new generation and/or that it is unnatural and/or unethical. Others are dedicated to it, seeing it as a form of transhumanism and the pursuit of immortality. Even among those who do not wish for eternal life, longevity may be desired to experience more of life or to provide a greater contribution to humanity.
A remarkable statement mentioned by Diogenes Laertius (c. 250) is the earliest (or at least one of the earliest) references about plausible centenarian longevity given by a scientist, the astronomer Hipparchus of Nicea (c.185—c.120 B.C.), who, according to the doxographer, assured that the philosopher Democritus of Abdera (c.470/460—c.370/360 B.C.) lived 109 years. All other accounts given by the ancients about the age of Democritus appear to, without giving any specific age, agree on the fact that the philosopher lived over 100 years. This is a possibility that turns out to be likely, given the fact that many ancient Greek philosophers are thought to have lived over the age of 90 (e.g., Xenophanes of Colophon, c.570/565—c.475/470 B.C., Pyrrho of Ellis, c.360—c.270 B.C., Eratosthenes of Cirene c.285—c.190 B.C., etc.) and because of the difference that the case of Democritus evidences from the case of, for example, Epimenides of Crete (VII, VI centuries B.C.), who is said to have lived 154, 157 or 290 years, as has been said about countless elders even during the last centuries as well as in the present time. These cases are not verifiable by modern means.
Population longevities can be seen as increasing due to increases in life expectancies around the world:
The current validated longevity records can be found in the list of supercentenarians. Notable individuals include:
One claim of Christian scholars is that the life span of humans has changed; that originally man was to have everlasting life, but due to man's sin, God progressively shortened man's life in the "four falls of mankind" — first to less than 1000 years, then to under 500, 200, and eventually 120 years. After those long living people died around the time of the Biblical Flood, God decided that humans would not be permitted to live more than 120 years However, since later biblical figures (and more recent people) such as Sarah lived for longer than that, 120 years should be considered the "usual" upper limit to man's lifespan. Some individuals can live slightly longer than that.
It has been hypothesized that there is a trade-off between cancerous tumor suppression and tissue repair capacity, and that by lengthening telomeres we might slow aging and in exchange increase vulnerability to cancer (Weinstein and Ciszek, 2002). Experimentation with telomeres on worms has yielded increased worm life spans by about 20% (Joeng et al., 2004). Even if further study shows that telomeres specifically are not tied to aging, the concept that some sort of DNA damage can cause genetically accelerated aging cannot be abandoned, thus providing a rational explanation for longevity and a subsequent reduction of longevity post-flood.
Many cultures like the Sumerians and Indus Valley also document groups of people who have lived for hundreds of years.
A more commonly accepted explanation is that such stories are longevity myths; age exaggeration tends to be greater in "mythical" periods in many cultures; the early emperors of Japan or China often ruled for more than a century, according to tradition. With the advent of modern accountable record-keeping, age claims fell to realistic levels. Even later in the Bible King David died at 70 years; other kings in their 30s, 40s, and 50s.
Recent increases in the rates of lifestyle diseases, such as obesity, diabetes, hypertension, and heart disease, may however drastically slow or reverse this trend toward increasing life expectancy in the developed world.
Oeppen and Vaupel (see Science:1029, 2002) have observed that since 1840 record life expectancy has risen linearly for men and women, albeit more slowly for men. For women the increase has been almost three months per year. In light of steady increase, without any sign of limitation, the suggestion that life expectancy will top out must be treated with caution. Oeppen and Vaupel observe that experts who assert that "life expectancy is approaching a ceiling ... have repeatedly been proven wrong." It is thought that life expectancy for women has increased more dramatically due to the considerable advances in medicine related to childbirth.
Some argue that molecular nanotechnology will greatly extend human lifespans. If the rate of increase of lifespan can be raised with these technologies to a level of not three months per year, but twelve months per year, we will have achieved effective immortality. This is the goal of radical life extension.