life

Ability to grow, reproduce, and respond to such stimuli as light, heat, and sound. Life on Earth may have begun about 4 billion years ago when a chemical reaction produced the first organic substance. Over time, life has evolved from primitive single-celled organisms to complex multicellular ones. There are now some 10 million different species of plants and animals living on the Earth. The earliest fossil evidence of life is threadlike chains of cells discovered in 1980 in deposits in northwestern Australia; these ‘stromatolites’ have been dated as being 3.5 billion years old.

Biology is the study of living organisms - their evolution, structure, functioning, classification, and distribution - while biochemistry is the study of the chemistry of living organisms. Biochemistry is especially concerned with the function of the chemical components of organisms such as proteins, carbohydrates, lipids, and nucleic acids.

Life probably originated in the primitive oceans. The original atmosphere, 4 billion years ago, consisted of carbon dioxide, nitrogen, and water. Laboratory experiments have shown that more complex organic molecules, such as amino acids and nucleotides, can be produced from these ingredients by passing electric sparks through a mixture. The climate of the early atmosphere was probably very violent, with lightning a common feature, and these conditions could have resulted in the oceans becoming rich in organic molecules, producing the so-called ‘primeval soup’. These molecules may then have organized themselves into clusters capable of reproducing and eventually developing into simple cells. Soon after life developed, photosynthesis would have become the primary source of energy for life. By this process, life would have substantially affected the chemistry of the atmosphere and, in turn, that of its own environment. Once the atmosphere had changed to its present composition, life could only be created by the replication of living organisms (a process called biogenesis).

Definition of life Although biologists have a vast knowledge of living things, they find difficulty in defining life and locating the dividing line between living and nonliving things. For example, a virus is a lifeless particle until it becomes active inside a living cell. Almost all living organisms share certain basic characteristics, which include reproduction, growth, metabolism, movement, responsiveness, and adaptation, but not every organism displays all these features, and even inorganic substances may exhibit some of them. Living things are not only greatly dependent on their physical surroundings, they are also interdependent on other life forms. At the same time, each organism is adapted to its own particular environment which must provide the right conditions for it to survive. For example, all living things require water and a range of other chemical substances, and life as we know it can exist only within a limited range of temperatures. Conditions capable of supporting life occur on and near the surface of the Earth in a thin region called the biosphere.

Evolution of life Chemical evolution is one means of explaining the origin of life on Earth through the formation of complex organic compounds capable of reproducing themselves - such as the DNA molecule. Other explanations that have been put forward include a supernatural origin of life, and spontaneous generation - the belief that living organisms arise spontaneously from nonorganic matter. This belief was finally dispelled by the French chemist Louis Pasteur in the mid-19th century when he demonstrated that even the most minute bacteria do not arise spontaneously, but develop from other bacteria. After Pasteur's experiments, most biologists accepted the idea that all life comes from existing life. Another explanation put forward is the arrival on Earth of spores, or ‘life seeds’, from elsewhere in the universe in the form of complex organic molecules present in meteors or comets. However, this argument does not really offer a viable explanation of the origins of life, since these primitive life forms must themselves have been created somehow and somewhere.

Chemical evolution The concept of the chemical evolution of life by the formation of complex organic compounds from simpler chemical elements or compounds was developed simultaneously, but independently, in the 1920s by the English biologist J B S Haldane and the Russian biochemist Alexandr Ivanovich Oparin. In 1924 Oparin published his theory on the chemical origin of life, and this was substantiated by experiments carried out in 1953 by two American chemists, Stanley Miller and Harold Urey. By passing electric discharges through a mixture of methane, ammonia, water, and hydrogen - the basic elements of the primeval atmosphere - they were able to produce an organic mixture of sugars, organic bases, fatty acids, and amino acids - the important chemical components of living organisms. Biochemistry has shown that all living organisms are composed of similar organic compounds, in particular proteins and nucleic acids. Thus, these could have been synthesized in the conditions existing 4 billion years ago on Earth and, by a process of chemical evolution, produced DNA nucleic acid, a substance capable of replicating itself. Amino acids have been detected in rocks that are more than 3 billion years old.

Biogenesis Such a complex organic mixture would have provided the essential nutrients and amino acids for the continued development of life, and the first living cells may have been evolved as anaerobic organisms able to function in an atmosphere without oxygen. Once a living organism had been formed from nonliving organic compounds, more living things would have arisen by biogenesis. Hence, the diversity of living organisms currently existing on Earth may have had a common ancestry in the original primitive soup of organic materials.

Lifespans The various forms of life on Earth have an enormous range of lifespans. Although bacteria are the first known species of life on Earth, their individual lifespans are very brief - a matter of just a few hours or days. Flowers and insects, too, have very short lifespans; small mammals live for a year or two, while larger mammals such as cats and dogs live 15-20 years. The longest living organisms are trees, some of which can exist for hundreds, or even thousands, of years.

Extraterrestrial life Human excursions into space have shown that conditions on Earth at the time life began are not unique to this planet. Similar conditions prevail elsewhere in the Solar System, and traces of carbon compounds have been found in the interstellar regions. Recent space probes have located water in the polar regions of the Moon (1998), and also on Titan, the largest of Saturn's moons. Since the presence of water is fundamental to life as we know it, the discovery of life on other planets becomes more of a possibility.

Life

US weekly magazine of photojournalism, which recorded US and world events pictorially from 1936-72, 1978- . It was founded by Henry Luce, owner of Time Inc., who bought the title of an older magazine. It ceased publication in 1972, although a few ‘Special Report’ issues occasionally appeared after that date. In 1978 the magazine was revived, issued monthly, focusing more on personalities than on current news. From May 2000, the magazine ceased to be published monthly, but only in special editions. Its publication of books continued.