Life Span, the length of time that embraces all the events of an organism’s life, from conception to death. Nearly all multicellular organisms pass through an embryonic phase, starting with the first division of the fertilized egg; a juvenile phase, in which the organism grows to sexual maturity; and an adult phase, in which the organism reaches a peak of sexual productivity and begins a decline of physiological activities that ends in death. Different species have characteristic lifespans that vary greatly in total length and in the duration of individual phases. For example, many mayfly species are adults for only one day of their one-year lifespan, whereas humans spend nearly 70 percent of their total lifespan as adults.
II AVERAGE LIFE SPAN
Maximum lifespan is the greatest age that a member of a species has been known to reach, whereas average lifespan is the average age at which that organism is expected to die. Average lifespan, which is the more useful concept, reflects in part the relative hospitality of the environment, among other considerations.
The maximum lifespan for humans (authenticated at 122 years) has probably changed very little in the last several centuries. The average lifespan, however, has increased greatly. For people living in industrialized countries, the average lifespan has risen from 35 or 40 years of age at the end of the 18th century to about twice that age today (about 78 years in the United Kingdom).
Humans have the longest average lifespan of any mammal. The average lifespans of other animals are as follows: elephants, 70 years; dogs, 18 years; cats, 14 years; horses, 20 years; whales, 50 years; carp, 30 years; guppies, 5 years; eagles, owls, and parrots, 60 years; parakeets, 12 years; and box tortoises, 100 years. The longest-lived animal is the giant tortoise, which is believed to attain a maximum age of about 200 years. Some plants, such as trees, however, live much longer than any animal; redwoods, for example, may live for more than 3,000 years. See also Creosote Bush; Sequoia.
III THEORIES ON LIFE-SPAN LIMITS
Although biologists largely agree that an organism’s lifespan is determined and limited by natural selection, no single theory prevails about the mechanisms by which a species rids itself of one generation to make room for the next. According to one theory, called the error theory, ageing is caused by the accumulation of small flaws in genetic information passed on as the body cells reproduce. The rate at which such flaws accumulate varies greatly. For example, ageing in humans takes many years, whereas in lampreys an “error catastrophe”, or rapid tissue decline, occurs, only hours after spawning.
According to another theory, ageing is programmed into the cells of an organism. Cells taken from humans and other mammals and grown in the laboratory have been found to go into a senescent phase and die before reaching 50 transfers of daughter cells to a new culture medium. According to this concept, the rate at which individual cells age defines not only the average lifespan of a species but also that of the species’ sexes (females commonly outlive males).
In the immune theory of ageing (see Immune System), the body of an organism slowly loses its ability to defend itself from harmful organisms that invade from without and cellular anomalies that subvert from within. In humans, the thymus gland, which plays a main role in the immune system by generating antibodies, diminishes to a fraction of its original bulk and function by a person’s 50th year. Proponents of the immune theory believe that the entire immune system loses its ability to distinguish friend from foe within the body and attacks the body’s own tissues, producing what are called autoimmune diseases.
Unsurprisingly, since it was discovered that reducing calorie intake can extend the lifespan of several species, much work has been conducted on the reasons behind this. Many drugs and therapies have been studied to see if they can mimic this effect without actually starving an organism, but as of yet, there is no direct way to increase the length of human lives.