Jim Gold, age 15, of Marquette, Mich., for her question:
HOW LONG IS THE AVERAGE LIFE SPAN?
Lifespan is the length of time that embraces all the events of an organism's life, from conception to death. 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 . For example, the average life of finches in the wild is probably only about one year, but a caged finch has been known to live as long as 29 years.
The maximum life span for humans, authenticated at 113 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 to 40 years of age at the end of the 18th century to about twice that age today.
The 1980 United States census revealed that the number of Americans over 65 years old has doubled since 1950 and jumped from 20 million to 25.5 million since 1970, largely because of advances in medical science and nutrition. This increase of 28 percent far exceeded the overall population increase of 11 percent over the same 10 year period.
In 1980, women outnumbered men 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 span of American women now exceeds that of American men by more than seven years. For women, the 1980 average age was 77.7 years and for men it was 70.1.
Humans have the longest average life span of any mammal. The average life spans 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, five years; eagles, owls and parrots, 60 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.
Every higher organism has a species characteristic average life span that is defined and limited by the best interest 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 gentic makeup, are better suited to meet the challenges of a changing environment than an immortal population of gentically unalterable individules.