LIFE SPAN

the period of time between the birth and death of an organism. It is a commonplace that all organisms die. Some die after only a brief existence, like that of the mayfly, whose adult life burns out in a day, and others like that of the gnarled bristlecone pines, which have lived thousands of years. The limits of the life span of each species appear to be determined ultimately by heredity. Locked within the code of the genetic material are instructions that specify the age beyond which a species cannot live given even the most favourable conditions. And many environmental factors act to diminish that upper age limit. the period of time between the birth and death of an organism, ranging from one day for the mayfly to thousands of years for the bristlecone pine. A maximum life span probably exists for each species. The life span of each organism depends on different environmental pressures (availability of food, shelter, climate, predators), variation in physical condition (disease, accidents), and heredity (offspring from long-lived parents have a longer life expectancy than those from short-lived parents). Among single-celled organisms, which reproduce by cell division, the concept of an individual life span loses some validity, as in a sense the individual continues to exist indefinitely. Thus arbitrary definitions of individual lives, such as the period between reproductive divisions, may be used to estimate life span, but such estimates are not comparable to those for sexually reproducing organisms. In most species the maximum life span can be estimated from the longest observed survival of individual members of the species. The maximum human life span is reported to be between 115 and 150 years. In many animals, the maximum life span has been calculated from survival in captivity, where safety from predators allows many individuals to live to an advanced old age seldom reached in the wild. Typical vertebrate life spans range from about one year for some small rodents to 177 years for the giant tortoise; the longest-lived mammal appears to be the human. Invertebrate animals generally have shorter life spans, some surviving for only a few days, but some snails, crayfish, and beetles have been observed to live for up to 30 years. Although the maximum life span of humans is over 100 years, the average life span today is only about 70 years, compared with 30 years in the 1700s. More humans are thus likely to survive to the hypothetical maximum life span in the present generation. Nevertheless, there is no evidence that maximum life span has increased, and persons surviving the risks of infancy and childhood in historic times had life spans roughly similar to present-day averages. The life span of plants is more difficult to determine than that of animals, but in general, plant species seem to have longer maximum survivals than animals. Bristlecone pine trees in California and Nevada have been confirmed to have lived for 4,900 years, and several other trees, including oaks, redwoods, and junipers, may live for more than 1,000 years. Some flowering plants, such as English ivy, also may have life spans in the hundreds of years, and the mycelia of fungi may survive 400 years, putting forth transient reproductive bodies such as mushroom caps whenever conditions are appropriate. Except for trees, which have annual growth rings that make possible an accurate assessment of age, it is almost impossible to state the age of a given plant unless there are clear records of its individual existence from planting onward. Some mosses have been estimated to have lived for as long as 2,800 years, but the proven survival of these species is considerably less. Several herbaceous plant species survive for only a single growing season, producing flowers and seeds and then dying off; for such plants, called annuals, average and maximum life spans are more or less equivalent. Other herbaceous plants live for two growing seasons, storing food in the first and putting forth flowers and seeds in the second; again, average and maximum life spans are the same. Other plants, called perennials, survive for several years before reproducing. Estimates of plant life spans are also made more difficult because the life span of plants is sometimes determined from the formation of seeds or spores rather than from their germination. Many seeds retain viability for years, producing normal plants when they are finally planted; in such cases, the period between germination and death of the plant may be considerably less than the total life span from the first formation of the seed. Additional reading Studies of longevity include Leonid A. Gavrilov and Natalia S. Gavrilova, The Biology of Life Span: A Quantitative Approach, rev. and updated ed. (1991; originally published in Russian, 2nd ed., rev. and updated, 1991); and Joep M.A. Munnichs et al. (eds.), Life-Span and Change in Gerontological Perspective (1985), a report of diverse research on behavioral development across the life span. United Nations Demographic Yearbook includes mortality statistics for all countries, showing the influence of economic, social, and climatic factors on mortality. The origin of life Hypotheses of origins Perhaps the most fundamental and at the same time the least understood biological problem is the origin of life. It is central to many scientific and philosophical problems and to any consideration of extraterrestrial life. Most of the hypotheses of the origin of life will fall into one of four categories: The origin of life is a result of a supernatural event; that is, one permanently beyond the descriptive powers of physics and chemistry. Lifeparticularly simple formsspontaneously and readily arises from nonliving matter in short periods of time, today as in the past. Life is coeternal with matter and has no beginning; life arrived on the Earth at the time of the origin of the earth or shortly thereafter. Life arose on the early Earth by a series of progressive chemical reactions. Such reactions may have been likely or may have required one or more highly improbable chemical events. Hypothesis 1, the traditional contention of theology and some philosophy, is in its most general form not inconsistent with contemporary scientific knowledge, although this knowledge is inconsistent with a literal interpretation of the biblical accounts given in chapters 1 and 2 of Genesis and in other religious writings. Hypothesis 2 (not of course inconsistent with 1) was the prevailing opinion for centuries. A typical 17th-century view follows: doubt whether, in cheese and timber, worms are generated, or, if beetles and wasps, in cowdung, or if butterflies, locusts, shellfish, snails, eels, and such life be procreated of putrefied matter, which is to receive the form of that creature to which it is by formative power disposed To question this is to question reason, sense, and experience. If he doubts this, let him go to Egypt, and there he will find the fields swarming with mice begot of the mud of the Nylus , to the great calamity of the inhabitants. It was only in the Renaissance, with its burgeoning interest in anatomy, that such transformations were realized to be impossible. A British physiologist, William Harvey, during the mid-17th century, in the course of his studies on the reproduction and development of the king's deer, made the basic discovery that every animal comes from an egg. An Italian biologist, Francesco Redi, in the latter part of the 17th century, established that the maggots in meat came from flies' eggs, deposited on the meat. And an Italian priest, Lazzaro Spallanzani, in the 18th century, showed that spermatozoa were necessary for the reproduction of mammals. But the idea of spontaneous generation died hard. Even though it was proved that the larger animals always came from eggs, there was still hope for the smaller ones, the microorganisms. It seemed obvious that, because of their ubiquity, these microscopic creatures must be generated continually from inorganic matter. Meat could be kept from going maggoty by covering it with a flyproof net, but grape juice could not be kept from fermenting by putting over it any netting whatever. This was the subject of a great controversy between the famous French bacteriologists Louis Pasteur and F.A. Pouchet in the 1850s, in which Pasteur triumphantly showed that even the minutest creatures came from germs floating in the air, but that they could be guarded against by suitable filtration. Actually, Pouchet was arguing that life must somehow arise from nonliving matter; if not, how had life come about in the first place? Toward the end of the 19th century Hypothesis 3 gained currency, particularly with the suggestion by a Swedish chemist, S.A. Arrhenius, that life on Earth arose from panspermia, microorganisms or spores wafted through space by radiation pressure from planet to planet or solar system to solar system. Such an idea of course avoids rather than solves the problem of the origin of life. In addition, it is extremely unlikely that any microorganism could be transported by radiation pressure to the Earth over interstellar distances without being killed by the combined effects of cold, vacuum, and radiation. Pasteur's work discouraged many scientists from discussing the origin of life at all. Moreover they were anxious not to offend religious feeling by probing too deeply into the subject. Although Darwin would not commit himself on the origin of life, others subscribed to Hypothesis 4 more resolutely, notably the famous British biologist T.H. Huxley in his Protoplasm, the Physical Basis of Life (1869), and the British physicist John Tyndall in his Belfast Address of 1874. Although Huxley and Tyndall asserted that life could be generated from inorganic chemicals, they had extremely vague ideas about how this might be accomplished. The very phrase organic molecule implies that there exists a special class of chemicals uniquely of biological origin, despite the fact that organic molecules have been routinely produced from inorganic chemicals since 1828. In the following discussion the word organic carries no imputation of biological origin. In fact the problem largely reduces to finding an abiological source of appropriate organic molecules. The primitive atmosphere Darwin's attitude was: It is mere rubbish thinking at present of the origin of life; one might as well think of the origin of matter. The two problems are, in fact, curiously connected, and modern scientists are thinking about the origin of matter. There is convincing evidence that thermonuclear reactions and subsequent explosions in the interiors of stars generate all the chemical elements more massive than hydrogen and helium and then distribute them into the interstellar medium from which subsequent generations of stars and planets form. Because of the commonality of these thermonuclear processes, and because some thermonuclear reactions are more probable than others, there exists a cosmic distribution of the major elements, so far as is known, throughout the universe. Table 1 compares, for some atoms of interest, the relative numerical abundances in the universe as a whole, on the Earth, and in living organisms. There is of course some variation in composition from star to star, from place to place on the Earth, and from organism to organism, but such comparisons are nevertheless very instructive. The composition of life is intermediate between the average composition of the universe and the average composition of the Earth. Ninety-nine percent both of the universe and of life is made of the six atoms, hydrogen (H), helium (He), carbon (C), nitrogen (N), oxygen (O), and neon (Ne). Can it be that life on Earth arose when the chemical composition of the Earth was much closer to the average cosmic composition, and that some subsequent events have changed the gross chemical composition of the Earth? The Jovian planets (Jupiter, Saturn, Uranus, and Neptune) are much closer to cosmic composition than is the Earth. They are largely gaseous, with atmospheres composed principally of hydrogen and helium. Methane (CH4) and ammonia (NH3) have been detected in smaller quantities, and neon and water are suspected. This circumstance very strongly suggests that the Jovian planets were formed out of material of typical cosmic composition. They have very large masses, and because they are so far from the sun their upper atmospheres are very cold. Therefore it is impossible for atoms in the upper atmospheres of the Jovian planets to escape from their gravitational fields; escape was probably very difficult even during planetary formation. The Earth and the other planets of the inner solar system, however, are much less massive and most have hotter upper atmospheres. It is possible for hydrogen and helium to escape from the Earth today, and it may well have been possible for much heavier gases to have escaped during the formation of the Earth. It is reasonable to expect that in the very early history of the Earth a much larger abundance of hydrogen prevailed, which has subsequently been lost to space. Thus the atoms carbon, nitrogen, and oxygen were present on the primitive Earth, not as CO2 (carbon dioxide), N2, and O2 as they are today but rather in the form of their fully saturated hydrides, CH4 (methane), NH3 (ammonia), and H2O. In the geological record, the presence of such reduced minerals as uraninite (UO2) and pyrite (FeS2) in sediments formed several billions of years ago implies that conditions then were considerably less oxidizing than they are today. In the 1920s J.B.S. Haldane in Britain and A.I. Oparin in the Soviet Union recognized that the abiological production of organic molecules in the present oxidizing atmosphere of the Earth is highly unlikely; but that, if the Earth once had more reducing (in this context, hydrogen-rich) conditions, the possible abiogenic production of organic molecules would have been much more likely. If large numbers of organic molecules were somehow synthesized on the primitive Earth, there would not necessarily be much trace of them today. In the present oxygen atmosphere, largely produced by green-plant photosynthesis, such molecules would tend, over geological time, to be oxidized to carbon dioxide, nitrogen, and water. In addition, as Darwin recognized, the first microorganisms would consume prebiological organic matter produced prior to the origin of life. any mechanical device that enables a person to live and usually work in an environment such as outer space or underwater in which he could not otherwise function or survive for any appreciable amount of time. Life-support systems provide all or some of the elements essential for maintaining physical well being, as for example oxygen, nutrients, water, disposal of body wastes, and control of temperature and pressure. The danger of contaminants and psychological factors must also be considered. Life-support systems are designed not only to enable survival in inhospitable environments but also to obviate the extreme difficulty people sometimes have in working under such conditions; thus life-support systems promote comfort, efficiency, and safety as well. The development of life-support systems can be traced to the work of Paul Bert, a 19th-century French physiologist, engineer, and physician. During the 1870s Bert conceptualized the basic principle of using supplementary oxygen to supply balloonists and mountain climbers who had ascended beyond the levels at which the oxygen in air is sufficient for breathing. Two of Bert's colleagues took a supply of about 150 litres of 70 percent oxygen on a balloon flight in 1875; but they failed to use it soon enough, and only one of the two survived. On this flight the oxygen was stored at ambient pressure in goldbeater's bags (made from cow's intestines) and was to be inhaled by mouth tube through a humidifier containing an aromatic liquid whose purpose was both to humidify the gas and to counteract the odour of the bags. Bert also designed a tank and regulator system with a capacity of 330 litres whereby mountain climbers could breathe oxygen near the peak on their ascent. Since Bert's pioneering efforts, various kinds of sophisticated life-support systems have been developed. They include the pressurized cabins and auxiliary environmental control mechanisms of high-altitude aircraft, spacecraft, and submarines and other submersibles. Examples of personal life-support devices are the pressure suits and extravehicular activity (EVA) backpacks (i.e., portable systems that contain cooling fluid, oxygen flow and recirculation equipment, waste containment unit, power source, and communications apparatus) worn by astronauts when working outside of their spacecraft; the self-contained underwater breathing equipment (scuba gear) used by divers; and the protective garments and breathing systems employed by firefighters. Another variety of devices that are sometimes classified as life-support systems include the anesthesia machine and the incubator unit (apparatus for housing premature or sick babies) utilized in hospitals.