Life Span

I

Introduction

Life Span, 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 (see Death and Dying). Different species have characteristic life spans that vary greatly in total length and in duration of individual phases. For example, many mayfly species are adults for only one day of their one-year life span, whereas humans spend nearly 70 percent of their total life span as adults.

II

Average Life Span

Maximum life span is the greatest age that a member of a species has been known to reach, whereas average life span is the average age at which that organism is expected to die. Average life span, which is the more useful concept, reflects in part the relative hospitality of the environment, among other considerations.

The maximum life span for humans (authenticated at 122 years) has probably changed very little in the last several centuries. The average life span, however, has increased greatly. For persons living in industrialized countries, the average life span has risen from 35 or 40 years of age at the end of the 18th century to about twice that age today, about 77.7 years in the United States in 2005. The 2000 U.S. census revealed that the number of Americans over 65 years old has more than doubled since 1950 and increased from 31.1 million to 34.91 million from 1990 to 2000, largely because of continuing advances in medical science and nutrition. In 2000, women outnumbered men about three to two in the over-65 age bracket, and in the over-85 age bracket they outnumbered men by more than two to one. The average life expectancy of American women now exceeds that of American men by about five years (79 for females born in 2000, and 74 for males born in that year).

Humans have the longest average life span of any mammal. The average life spans of other animals vary. Elephants, for example, typically live about 70 years, while mice only live about 3 years. One of the longest-lived animals is the giant tortoise, which usually lives between 100 and 150 years, but has a maximum age of about 200 years. Evidence suggests some bowhead whales may live more than 200 years. The slow-growing ocean quahog, a type of mollusk, can live more than 400 years. Some plants, such as trees, live much longer than any animal; redwoods, for example, may live for more than 3,000 years. Some specimens of the creosote bush, a plant found in the southern United States and South and Central America, date back almost 11,700 years.

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III

Evolutionary Considerations

Every higher organism has a species-characteristic average life span that is defined and limited by the best interests of species survival. For an adult plant or animal to continue to live indefinitely after it has procreated would deplete nutrients or food supplies and jeopardize the survival of the organism's population. More important, to have such immortal adults would slow down or bring to a halt the process of natural selection, which is one of the major mechanisms by which a species adapts to environmental changes. Generations of mortal individuals, each produced sexually and endowed with a unique genetic makeup, are better suited to meet the challenges of a changing environment than an immortal population of genetically unalterable individuals.

These selective mechanisms apparently work so efficiently that only some bacteria and protozoans have not adopted them and can continue to survive by reproducing in an exclusively asexual manner. Such unicellular organisms do not have life spans in the sense that individuals are conceived and die, each with unique sets of genetic information. Rather, they may be said to go through rejuvenation by producing identical daughter cells.

IV

Theories on Life-Span Limits

Although biologists largely agree that an organism's life span is determined and limited by the laws of 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, aging 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, aging in humans takes many years, whereas in lampreys an “error catastrophe,” or rapid tissue decline, occurs only hours after spawning.

According to another theory, aging is programmed into the cells of an organism and the rate at which individual cells age defines the average life span of a species. 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 cell divisions (Mitosis). In 1998 scientists discovered that cells injected with the enzyme telomerase divide up to 90 times before going into a senescent phase and dying. Learning more about telomerase, and its ability to extend cell life span, may lead to a better understanding of just how aging is programmed into cells.

In the immune theory of aging (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.

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