living matter and, as such, matter that shows certain attributes that include responsiveness, growth, metabolism, energy transformation, and reproduction. Although a noun, as with other defined entities, the word life might be better cast as a verb to reflect its essential status as a process. Life comprises individuals, living beings, assignable to groups (taxa). Each individual is composed of one or more minimal living units, called cells (cell), and is capable of transformation of carbon-based and other compounds (metabolism), growth, and participation in reproductive acts. Life-forms present on Earth today have evolved from ancient common ancestors through the generation of hereditary variation and natural selection. Life can be traced to fossils (fossil) over 3 billion years old and is thus only slightly younger than Earth, which gravitationally accreted into a planet about 4.5 billion years ago. But this is life as a whole. More than 99.9 percent of species that have ever lived are extinct. The several branches of science that reveal the common historical, functional, and chemical basis of the evolution of all life include electron microscopy (electron microscope), genetics, paleobiology (including paleontology), and molecular biology.

The phenomenon of life can be approached in several ways: life as it is known and studied on planet Earth; life imaginable in principle; and life, by hypothesis, that might exist elsewhere in the universe (see extraterrestrial life). As far as is known, life exists only on Earth. Most life-forms reside in a thin sphere that extends about 23 km (14 miles) from 3 km (2 miles) beneath the bottom of the ocean to the top of the troposphere (lower atmosphere); the relative thickness is comparable to a coat of paint on a rubber ball. An estimated 10–30 million distinguishable species currently inhabit this sphere of life, or biosphere.

Much is known about life from points of view reflected in the various biological, or “life,” sciences. These include anatomy (the study of form at the visible level), ultrastructure (the study of form at the microscopic level), physiology (the study of function), molecular biology and biochemistry (the study of form and function at chemical levels), ecology (the study of the relations of organisms with their environments), taxonomy (the naming, identifying, and classifying of organisms), ethology (the study of animal behaviour), and sociobiology (the study of social behaviour). Specific sciences that participate in the study of life focus more narrowly on certain taxa or levels of observation—e.g., botany (the study of plants (plant)), lichenology (the study of lichens (lichen), leafy or crusty individuals composed of permanent associations between algae or photosynthetic (photosynthesis) bacteria and fungi (fungus)), herpetology (the study of amphibians (amphibian) and reptiles (reptile)), microbiology (the study of bacteria, yeast, and other unicellular fungi, archaea, protists (protozoan), viruses (virus)), zoology (the study of marine and land animals (animal)), and cytology (the study of cells (cell)). Although the scientists, technicians, and others who participate in studies of life easily distinguish living matter from inert or dead matter, none can give a completely inclusive, concise definition of life itself. Part of the problem is that the core properties of life—growth, change, reproduction, active resistance to external perturbation, and evolution—involve transformation or the capacity for transformation. Living processes are thus antithetical to a desire for tidy classification or final definition. To take one example, the number of chemical elements (chemical element) involved with life has increased with time; an exhaustive list of the material constituents of life would therefore be premature. Nonetheless, most scientists implicitly use one or more of the metabolic, physiological, biochemical, genetic, thermodynamic, and autopoietic definitions given below.

Physiological (physiology) definitions of life are popular. Life is defined as any system capable of performing functions such as eating, metabolizing, excreting, breathing, moving, growing, reproducing, and responding to external stimuli. But many such properties are either present in machines that nobody is willing to call alive or absent from organisms, such as the dormant hard-covered seed of a tree, that everybody is willing to call alive. An automobile, for example, can be said to eat, metabolize, excrete, breathe, move, and be responsive to external stimuli. A visitor from another planet, judging from the enormous number of automobiles on Earth and the way in which cities and landscapes have been designed for the special benefit of motorcars, might well believe that automobiles are not only alive but are the dominant life-form on the planet. (See physiology.)

All organisms on Earth, from the tiniest cell to the loftiest trees (tree), display extraordinary powers. They effortlessly perform complex transformations of organic molecules, exhibit elaborate behaviour patterns, and indefinitely construct from raw materials in the environment more or less identical copies of themselves. How could systems of such staggering complexity and such stunning beauty ever arise? A main part of the answer, for which today there is excellent scientific evidence, was carefully chronicled by the English naturalist Charles Darwin (Darwin, Charles) in the years before the publication in 1859 of his epoch-making work On the Origin of Species. A modern rephrasing of his theory of natural selection goes something like this: Hereditary information is carried by large molecules (molecule) known as genes (gene), composed of nucleic acids (nucleic acid). Different genes are responsible for the expression of different characteristics of the organism. During the reproduction of the organism, the genes also replicate and thereby pass on the instructions for various characteristics to the next generation. Occasionally, there are imperfections, called mutations (mutation), in gene replication. A mutation alters the instructions for one or more particular characteristics. The mutation also breeds true, in the sense that its capability for determining a given characteristic of the organism remains unimpaired for generations until the mutated gene is itself mutated. Some mutations, when expressed, will produce characteristics favourable for the organism; organisms with such favourable genes will reproduce preferentially over those without such genes. Most mutations, however, turn out to be deleterious and often lead to some impairment or to death of the organism. (To illustrate, it is unlikely that one can improve the functioning of a finely crafted watch by dropping it from a tall building. The watch may perform better, but this is highly improbable.) In this way, organisms slowly evolve toward greater complexity. This evolution occurs, however, only at enormous cost: modern humans, complex and reasonably well-adapted, exist only because of billions of deaths of organisms slightly less adapted and somewhat less complex. In short, Darwin's (Darwinism) theory of natural selection states that complex organisms evolved through time because of replication, mutation, and replication of mutations. A genetic (adaptation) definition of life therefore would be a system capable of evolution by natural selection. (See Darwinism.)

thermodynamics distinguishes between isolated, closed, and open systems. An isolated system is separated from the rest of the environment and exchanges neither light nor heat nor matter with its surroundings. A closed system exchanges energy but not matter. An open system is one in which both material and energetic exchanges occur. The second law of thermodynamics (thermodynamics) states that, in a closed system, no processes will tend to occur that increase the net organization (or decrease the net entropy) of the system. Thus, the universe taken as a whole is steadily moving toward a state of complete randomness, lacking any order, pattern, or beauty. This fate was popularized in the 19th century as the “heat death (heat)” of the universe.

Some scientists argue on grounds of quite general open-system thermodynamics that the organization of a system increases as energy flows through it. Moreover, energy flow leads to the development of cycles. An example of a biological cycle on Earth is the carbon cycle. carbon from atmospheric carbon dioxide is incorporated by photosynthetic or chemosynthetic organisms and converted into carbohydrates (carbohydrate) through the process of autotrophy. These carbohydrates are ultimately oxidized by heterotrophic organisms to extract useful energy locked in their chemical bonds (chemical bonding). In the oxidation of carbohydrates, carbon dioxide is returned to the atmosphere, thus completing the cycle. It has been shown that similar cycles develop spontaneously and in the absence of life by the flow of energy through chemical systems. Biological cycles may represent natural thermodynamic cycles reinforced by a genetic apparatus. It seems doubtful that open-system thermodynamic processes in the absence of replication lead to the sorts of complexity that characterize biological systems; replication, however, may be interpreted as an especially efficient thermodynamic means of gradient breakdown—a kind of special, slow-burning “fire.” In any case, it is clear that much of the complexity of life on Earth has arisen through replication, with thermodynamically favoured pathways being used by energy-transforming organisms.

A newer definition of life revolves around the idea of autopoiesis. This idea was put forth by Chilean biologists Humberto Maturana and Francisco Varela and emphasizes the peculiar closure of living systems, which are alive and maintain themselves metabolically whether they succeed in reproduction or not. Unlike machines, whose governing functions are embedded by human designers, organisms are self-governing. The autopoietic definition of life resembles the physiological definition but emphasizes life's maintenance of its own identity, its informational closure, its cybernetic self-relatedness, and its ability to make more of itself. Autopoiesis refers to self-producing, self-maintaining, self-repairing, and self-relational aspects of living systems. Living beings maintain their form by the continuous interchange and flow of chemical components. Cellular autopoietic systems are bounded by a dynamic material made by the system itself. In life on Earth the limiting material is lipid membrane studded with transport proteins (protein) fabricated by the incessantly active cell. A source of usable energy flows to all living or autopoietic systems—either light in the visible or near-visible spectrum or specific organic carbon or other chemicals such as hydrogen, hydrogen sulfide, or ammonia. Energy sources that have never been adequate to maintain autopoiesis on Earth include heat, sound waves, and electromagnetic radiation outside the visible or near-visible spectrum.

The smallest autopoietic system on Earth is a living bacterial cell (bacteria). (Viruses, plasmids (plasmid), and other replicating molecules cannot, even in principle, behave as an autopoietic system; no matter how much food, liquid water, and serviceable energy they are provided, they still require cells for their continuity and duplication.) Some cells, such as Carsonella ruddii, have fewer than 200 genes and proteins, but they, like organelles and viruses, are not autopoietic, since they must be inside an autopoietic system (living cell) to metabolize and survive. No self-bounded autopoietic system smaller than a cell with at least 450 proteins and the genes that code for these proteins has ever been described. Larger than bacteria are other autopoietic systems of intermediate size such as protists (protist), fungal spores, mules and other individual mammals (mammal), and plants such as oak trees or poppies (poppy). Autopoietic entities at even larger levels include ecosystems (ecosystem) such as coral reefs (coral reef), prairies (prairie), or ponds. The maximal or largest single autopoietic system known is referred to as “Gaia (climate),” named by English atmospheric scientist James E. Lovelock for Gaea, the ancient Greek personification of Earth. Gaia is basically a closed thermodynamic system because there is little interchange of matter with the extraterrestrial environment. There is evidence that the global, Gaian system of life shows organism-like properties, such as regulation of atmospheric chemistry, global mean temperature, and oceanic salinity (biosphere) over multimillion-year time spans. Such regulation may be understood as part of life's organization as a complex and cyclical open thermodynamic system.

Reconnaissance missions to the planets of the inner solar system have revealed stark and barren landscapes. From the heavily cratered and atmosphereless surfaces of both Mercury and the moon to the hot sulfurous fogs of Venus and the dusty, windswept surface of Mars, no sign of life is apparent anywhere. The biosphere, by definition the place where all Earth's life dwells, is a delight with its green, wet contrast. Austrian geologist Eduard Suess (Suess, Eduard) invented the term biosphere to match the other envelopes of the planet: the atmosphere of gas; the hydrosphere of oceans (ocean), lakes (lake), rivers (river), springs (spring) and other waters; and the lithosphere, or the solid rock surface of the outer portion of Earth. Yet it was the great Russian crystallographer and mineralogist Vladimir I. Vernadsky (Vernadsky, Vladimir Ivanovich) who brought the term into common parlance with his book of the same name. In The Biosphere (1926) Vernadsky outlines his view of life as a major geological force. Living matter, Vernadsky contends, erodes, levels, transports, and chemically transforms surface rocks, minerals, and other features of Earth. If the biosphere is the place where life is found, the biota (or the biomass as a whole) is the sum of all living forms: Flora, Fauna, and microbiota.

Thus far, we have seen that the biosphere, by structure, composition, and physical makeup, is completely enclosed by the domain of life, which has so adapted itself to biospheric conditions that there is no place 【on Earth】 in which it is unable to manifest itself in one way or another.

The specific carrier of the genetic information in all organisms is the nucleic acid known as DNA, short for deoxyribonucleic acid. DNA is a double helix, two molecular coils wrapped around each other and chemically bound one to another by bonds connecting adjacent bases (base). Each long ladderlike DNA helix has a backbone that consists of a sequence of alternating sugars (sugar) and phosphates (phosphate). Attached to each sugar is a “base” consisting of the nitrogen-containing compound adenine, guanine, ctyosine, or thymine. Each sugar-phosphate-base “rung” is called a nucleotide. A very significant one-to-one pairing between bases occurs that ensures the connection of adjacent helices. Once the sequence of bases along one helix (half the ladder) has been specified, the sequence along the other half is also specified. The specificity of base pairing plays a key role in the replication of the DNA molecule. Each helix makes an identical copy of the other from molecular building blocks in the cell. These nucleic acid replication events are mediated by enzymes called DNA polymerases. With the aid of enzymes, DNA can be produced in the laboratory.

The genetic code was first broken in the 1960s. Three consecutive nucleotides (base-sugar-phosphate rungs) are the code for one amino acid of a protein molecule. By controlling the synthesis of enzymes, DNA controls the functioning of the cell. Of the four different bases taken three at a time, there are 4³, or 64, possible combinations. The meaning of each of these combinations, or codons, is known. Most of them represent one of the 20 particular amino acids found in protein. A few of them represent punctuation marks—for example, instructions to start or stop protein synthesis (metabolism). Some of the code is called degenerate. This term refers to the fact that more than one nucleotide triplet may specify a given amino acid. This nucleic acid–protein interaction underlies living processes in all organisms on Earth today. Not only are these processes the same in all cells of all organisms, but even the particular “dictionary” that is used for the transcription of DNA information into protein information is essentially the same. Moreover, this code has various chemical advantages over other conceivable codes. The complexity, ubiquity, and advantages argue that the present interactions among proteins and nucleic acids are themselves the product of a long evolutionary history. They must interact as a single reproductive, autopoietic system that has not failed since its origin. The complexity reflects time during which natural selection could accrue variations; the ubiquity reflects a reproductive diaspora from a common genetic source; and the advantages, such as the limited number of codons, may reflect an elegance born of use. DNA's “staircase” structure allows for easy increases in length. At the time of the origin of life, this complex replication and transcription apparatus could not have been in operation. A fundamental problem in the origin of life is the question of the origin and early evolution of the genetic code.

Many other commonalities exist among organisms on Earth. Only one class of molecules (molecule) stores energy for biological processes until the cell has use for it; these molecules are all nucleotide phosphates (phosphate). The most common example is adenosine triphosphate (ATP). For the very different function of energy storage, a molecule identical to one of the building blocks of the nucleic acids (both DNA and RNA) is employed. Metabolically ubiquitous molecules—flavin adenine dinucleotide (FAD) and coenzyme A—include subunits similar to the nucleotide phosphates. nitrogen-rich ring compounds, called porphyrins (porphyrin), represent another category of molecules; they are smaller than proteins and nucleic acids and common in cells. Porphyrins are the chemical bases of the heme in hemoglobin, which carries oxygen molecules through the bloodstream of animals and the nodules of leguminous plants. chlorophyll, the fundamental molecule mediating light absorption during photosynthesis in plants and bacteria, is also a porphyrin. In all organisms on Earth, many biological molecules have the same “handedness” (these molecules can have both “left-” and “right-handed” forms that are mirror images of each other; see below The earliest living systems (life)). Of the billions of possible organic compounds, fewer than 1,500 are employed by contemporary life on Earth, and these are constructed from fewer than 50 simple molecular building blocks.

Besides chemistry, cellular life has certain supramolecular structures in common. Organisms as diverse as single-celled paramecia (Paramecium) and multicellular pandas (panda, giant) (in their sperm tails), for example, possess little whiplike appendages called cilia (or flagella (flagellum), a term that is also used for completely unrelated bacterial structures; the correct generic term is undulipodia (cilium)). These “moving cell hairs” are used to propel the cells through liquid. The cross-sectional structure of undulipodia shows nine pairs of peripheral tubes and one pair of internal tubes made of proteins called microtubules (microtubule). These tubules are made of the same protein as that in the mitotic spindle, the structure to which chromosomes are attached in cell division. There is no immediately obvious selective advantage of the 9:1 ratio. Rather, these commonalities indicate that a few functional patterns based on common chemistry are used over and over again by the living cell. The underlying relations, particularly where no obvious selective advantage exists, show all organisms on Earth are related and descended from a very few common cellular ancestors—or perhaps one.

Green plants are typical photoautotrophs. Plants absorb sunlight to generate ATP (adenosine triphosphate) and to disassociate water into oxygen and hydrogen. To break down the water molecule, H₂O, into hydrogen and oxygen requires much energy. The hydrogen from water is then combined in the “dark reactions” with carbon dioxide, CO₂. The result is the production of such energy-rich organic molecules as sugars, amino acids, and nucleotides. The oxygen becomes the gas O₂, which is released as waste back into the atmosphere. Animals, which are strictly heterotrophs, cannot live on carbon dioxide, sunlight, and water with a few salts (salt) like plants do. They must breathe in the atmospheric oxygen. Animals combine oxygen chemically with hydrogen atoms that they remove from their food—that is, from organic materials such as sugar, protein, and amino acids. Animals release water as a waste product from the oxygen respiration. Animals, like all heterotrophs, use organic materials as their sole source of carbon. This conversion of carbon provides an example of an aspect of an ecological cycle in which a required element flows through different types of organisms as it changes its oxidation state from CO₂ to (CH₂O)_n and back to CO₂.

All ATP biological electron-transfer reactions lead to the net production of ATP molecules (molecule). Two of the three phosphates (PO₄) of this molecule are held by energy-rich bonds sufficiently stable to survive for long periods of time in the cell but not so strong that the cell cannot tap these bonds for energy when needed. ATP and similar molecules (such as guanosine triphosphate 【GTP】) have a five-carbon sugar and three phosphates (phosphate). As far as is known, such molecules are the general and unique energy currency of living systems on Earth.

All life is composed of cells of one of two types: prokaryotes (those that lack a nucleus) or eukaryotes (those with a nucleus). Even in one-celled organisms this distinction is very clear.

Prior to the recognition of microbial life, the living world was too easily divided into animals that moved in pursuit of food and plants that produced food from sunlight. The futility of this simplistic classification scheme has been underscored by entire fields of science. Many bacteria both swim (like animals) and photosynthesize (like plants), yet they are best considered neither. Many algae (e.g., euglenids, dinomastigotes, chlorophytes) also swim and photosynthesize simultaneously. Molecular biological measurements of the DNA that codes for components of the ribosomes (ribosome) (organelles that are universally distributed) consistently find fungi to be extremely different from plants. Indeed, fungi genetically resemble animals more than plants.

ploidy, the concept of the number of complete sets of genes (gene) organized into chromosomes, is inapplicable to prokaryotes. Ploidy in protists, depending on species, varies so greatly and regularly that it is obvious that sexual cycles evolved in this diverse group of eukaryotes. Fungal cell nuclei are haploid (one set of chromosomes) or dikaryotic (two distinct nuclei from two different parents, each with one set of chromosomes sharing the same cell). Plant cell nuclei have two sets of chromosomes (diploid, in the sporophyte generation) or one (haploid, in the gametophyte generation). Animal cell nuclei, except in the gametes (gamete) (sperm and egg), tend to have two sets of chromosomes (they are diploid).

Viral and plasmid nucleic acids (nucleic acid) pass from cell to cell where the DNA or RNA perform their replication and coding functions efficiently. These “small replicons”—viruses and plasmids—are essentially strands of nucleic acid with a protein coat that depend entirely on the host cell for their continual existence. Pieces of the genetic material, virus-sized, pass from one cell into another cell of the same kind. Traveling small replicons of DNA produce genetic and permanently heritable changes in their new locations. Alternatively, part of the virus nucleic acid may be permanently bound to the nuclear DNA of the cell in which it resides. Viruses may be thought of as degenerate forms that are highly specialized in order to live in specific host cells of free-living organisms. Only cells are capable of performing the metabolic tasks that viruses and plasmids require. Viruses and plasmids must use the genetic transcription apparatus of cells. Bacterial viruses, or bacteriophages (bacteriophage), may be extremely efficient in turning the luckless bacterium from a “factory” for the production of more bacteria into a factory for making more virus particles. It may take no more than 10 minutes for a bacterium infected by a single virus to produce 100 new virus particles, bursting forth from the victim bacterium by destroying it. Plasmids do not burst forth; rather, they benignly incorporate their DNA into that of their host cell.

Most familiar organisms on Earth are of course sensitive to extreme temperature in their surroundings. Mammals and birds (bird) have evolved internal regulation (thermoregulation) of their temperatures. Humans cannot tolerate body temperatures below 30 °C (86 °F) or above 40 °C (104 °F). Cold-climate organisms have special insulating layers of fat and fur. Other organisms adjust to seasonal temperature drops by developing dormant (dormancy) propagules such as spores, eggs, or tuns, which are hardy desiccation- and radiation-resistant forms produced by microscopic animals called tardigrades, also known as “water bears.” Dormancy is often accompanied by dehydration.

Life has been detected in the stratosphere and in the ocean's major depths. Mud-loving photosynthetic bacteria live in pools at Yellowstone National Park at temperatures above 90 °C (194 °F), whereas some unidentified dark-dwelling marine bacteria at marine hot vents have been recorded to grow at a superhot temperature of 113 °C (235 °F). (Because the water is under pressure, it is not above the boiling point.) Sulfate-reducing bacteria taken from the ocean grow and reproduce at 104 °C (219 °F) under high hydrostatic pressure. Many organisms employ organic or inorganic antifreezes to lower the freezing point of their internal liquids. Many kinds can live at several tens of degrees below 0 °C (32 °F). Some insects use dimethyl (dimethyl sulfoxide) sulfoxide as an antifreeze. Other organisms live in briny pools in which dissolved salts lower the freezing point. San Juan Pond in Antarctica, for example, contains about one molecule of calcium chloride for every two water molecules. Not until –45 °C (–49 °F) does the pond freeze. A type of cryophilic (cold-loving) bacteria that lives there continues to metabolize down to at least –23 °C (–9 °F). Biological activity does not cease at the freezing point of water. In certain sea urchins, some microtubule proteins form the tubules of mitosis best at –2 °C (28 °F), and some enzymes (enzyme) are actually more active in ice than in water. Many bacteria are routinely frozen at –80 °C (–112 °F). They are thawed with no decrease in activity. Freezing temperatures alone cause no damage. Rather, frozen water removes tissue fluidity and leaves dangerous salt concentrations in its wake. The combination of expansion and contraction attendant to freezing and thawing harms membranes. Some arthropods (arthropod) can be severely dehydrated and then revived simply by the addition of water. Once dehydrated, these animals can be brought to any temperature from close to absolute zero (–273 °C, or –460 °F) to above the boiling point of water (100 °C, or 212 °F) without apparent damage. When encysted in response to dehydration, these arthropods at first glance are indistinguishable from a weathered grain of sand.

The sizes of organisms on Earth vary greatly and are not always easy to estimate. On the large end, great stands of poplar trees entirely connected by common roots are really a single organism. A variety of influences place an upper limit to the size of organisms. One is the strength of biological materials. Sequoia redwood trees, some of which exceed 90 metres (300 feet), are apparently near the upper limit of height for an organism. The Italian astronomer Galileo calculated in 1638 that a tree taller than roughly 90 metres would buckle under its own weight when displaced slightly from the vertical (for example, by a breeze). Because of the buoyancy of water, large animals such as whales (whale) are not presented with such stability problems. Other size-related difficulties arise. The volume of tissues to be nourished increases as the cube of the characteristic length of the organism, but the surface of the gut, which absorbs the ingested food, increases only as the square of the length for a fixed morphology. As an organism's length is increased, a point of diminishing returns is ultimately reached where nutrition is irreversibly impeded in an animal.

water, which is crucial for life, is the major molecule in all organisms. Unless a massive mineral skeleton is present, the dry matter of most organisms is about one-half carbon by weight. This reflects the fact that all organic molecules (molecule) are composed of carbon bound at least to hydrogen. Metabolism uses a wide variety of other chemical elements. Amino acids are made of nitrogen and sulfur in addition to carbon, hydrogen, and oxygen. Nucleic acids (nucleic acid) are made of phosphorus in addition to hydrogen, nitrogen, oxygen, and carbon. sodium, potassium, and calcium are used to maintain electrolyte balance and to signal cells. silicon is used as a structural material in the diatom shell, the radiolarian and heliozoan spicule, and the chrysophyte exoskeleton. iron plays a fundamental role in the transport of molecular oxygen as part of the hemoglobin molecule. In some ascidians (sea squirts (sea squirt)), however, vanadium replaces iron. Ascidian blood also contains unusually large amounts of niobium, titanium, chromium, manganese, molybdenum, and tungsten. The vanadium and niobium compounds in ascidian blood may be adaptations to low oxygen levels. Some bacteria use selenium, tellurium, or even arsenic as electron acceptors. Others produce the fully saturated gas hydrides of carbon, arsenic, phosphorus, or silicon as a metabolic waste. Still others form compounds of carbon with such halogens (halogen element) as chlorine or iodine. Not only the foregoing elements but also copper, zinc, cobalt, and possibly gallium, boron, and scandium perform particular functions in the enzymatic apparatus of particular cells. These elements, both the uncommon ones and those as common as phosphorus, are much more concentrated in living matter than in the environment where the living matter resides. This concentration suggests that such rare chemicals play unique functional roles that other, more abundant elements cannot serve.

Although any given organism is severely limited in its range of behaviour (animal behaviour) patterns and sensory capabilities, life as a whole is remarkably sensitive to aspects of its local social and physical environment. A bird raised from the egg in the absence of other members of its species migrates when the season beckons, builds the proper nest, and engages in elaborate courtship rituals. Those birds that fail to perpetuate the behaviour pattern do not leave descendants. Such behavioural accuracy itself must have evolved. Rats that pass through mazes easily interbreed, as do rats that pass through with difficulty; eventually two populations with inherited characteristics called “maze-smart” and “maze-dumb” are produced. fruit fly populations attracted to light can be separated from those that avoid light. Classical genetic-crossing experiments reveal that the two populations differ largely in a small number of genes for phototropism. Similar genetic determinants of behaviour exist in humans. For example, possession of a supernumerary Y-chromosome in males is strikingly correlated with aggressive tendencies.

Humans use only a limited region of the electromagnetic spectrum, the part called visible light, which extends from 400 to 700 nanometres in wavelength. While plants, algae, photosynthetic bacteria, and most animals are sensitive to this same range of wavelengths, many are sensitive to other wavelengths as well. Many plants present flower patterns visible only in the ultraviolet range at wavelengths below 400 nanometres, where pollinating insects are sensitive. Honeybees (honeybee) use polarized light—which the unaided human eye is unable to detect—for direction finding on partly cloudy days. The “pit” of such pit vipers (viper) as the rattlesnake is an infrared (heat) receptor that serves as a direction finder. These reptiles (reptile) sense the thermal radiation emitted by mammals and birds, their warm-blooded prey. Humans are entirely insensitive to this thermal radiation.

That some animals such as dogs (dog) are sensitive to sounds (sound) that the human ear cannot detect is obvious to those who use dog whistles. Bats (bat) emit and detect sound waves at ultrahigh frequencies, in the vicinity of 100,000 cycles per second, about five times the highest frequency to which the human ear is sensitive. Bats have echolocated their prey by use of these sounds for millions of years before humans invented radar and sonar. The audio receptors of many moths (moth) that are prey to bats respond only to the frequencies emitted by the bats. When the bat sounds are heard, the moths (moth) take evasive action. Dolphins (dolphin) communicate via a very wide frequency range. They employ a “click” echolocator.

smell and taste, or some form of detection of specific chemical molecules, are universal. The ultimate in olfactory specialization may be male moths, whose feathery antennae are underlain by splayed microtubules, each of which is covered by a membrane at the distal end. They smell essentially nothing except the epoxide compound called disparlure, the chemical sex attractant discharged by the female. Only 40 molecules per second need impact on the antennae to produce a marked response. One female silkworm moth need release only 10^–8 gram (4 × 10^–10 ounce) of sex attractant (pheromone) per second in order to attract every male silkworm moth within a few kilometres.

The evidence is overwhelming that all life on Earth has evolved from common ancestors in an unbroken chain since its origin. Darwin's principle of evolution is summarized by the following facts. All life tends to increase: more organisms are conceived, born, hatched, germinated from seed, sprouted from spores, or produced by cell division (or other means) than can possibly survive. Each organism so produced varies, however little, in some measurable way from its relatives. In any given environment at any given time, those variants best suited to that environment will tend to leave more offspring than the others. Offspring resemble their ancestors. Variant organisms will leave offspring like themselves. Therefore, organisms will diverge from their ancestors with time. The term natural selection is shorthand for saying that all organisms do not survive to leave offspring with the same probability. Those alive today have been selected relative to similar ones that never survived or procreated. All organisms on Earth today are equally evolved since all share the same ancient original ancestors who faced myriad threats to their survival. All have persisted since the beginning of the Archean Eon (3.5 billion years ago), products of the great evolutionary process with its identical molecular biological bases. Because the environment of Earth is so varied, the particular details of any organism's evolutionary history differ from those of another species in spite of chemical similarities.

Everywhere the environment of Earth is heterogeneous. Mountains, oceans, and deserts suffer extremes of temperature, humidity, and water availability. All ecosystems contain diverse microenvironments: oxygen-depleted oceanic oozes, sulfide- or ammonia-rich soils, mineral outcrops with a high radioactivity content, or boiling organic-rich springs, for example. Besides these physical factors, the environment of any organism involves the other organisms in its surroundings. For each environmental condition, there is a corresponding ecological niche. The variety of ecological niches populated on Earth is quite remarkable. Even wet cracks in granite are replete with “rock eating” bacteria. Ecological niches in the history of life have been filled independently several times. For example, quite analogous to the ordinary placental mammalian wolf was the marsupial wolf, the thylacine (extinct since 1936) that lived in Australia; the two predatory mammals have striking similarities in physical appearance and behaviour. The same streamlined shape for high-speed marine motion evolved independently at least four times: in Stenopterygius and other Mesozoic reptiles; in tuna, which are fish; and in dolphins and seals (seal), which are mammals. Convergent evolution in hydrodynamic form arises from the fact that only a narrow range of solutions to the problem of high-speed marine motion by large animals exists. The eye (eye, human), a light receptor that makes an image, has evolved independently more than two dozen times not only in animals on Earth but in protists such as the dinomastigote Erythropsodinium. Apparently eyelike structures best solve the problem of visual recording. Where physics or chemistry establishes one most efficient solution to a given ecological problem, evolution in distinct lineages will often tend toward similar, nearly identical solutions. This phenomenon is known as convergent evolution (evolution).

Past time on Earth, as inferred from the rock record, is divided into four immense periods of time called eons. These are the Hadean (4.6 to 4 billion years ago), the Archean (Archean Eon) (4 to 2.5 billion years ago), the Proterozoic (Proterozoic Eon) (2.5 billion to 542 million years ago), and the Phanerozoic (Phanerozoic Eon) (542 million years ago to the present). For the Hadean Eon, the only record comes from meteorites (meteorite) and lunar rocks. No rocks of Hadean age survive on Earth. In the figure-->, eons are divided into eras, periods, and epochs. Such entries in the geologic time scale are often called “geologic time intervals.”

Among the oldest known fossils (fossil) are those found in the Fig Tree (Fig Tree microfossils) Chert from the Transvaal, dated over three billion years ago. These organisms have been identified as bacteria, including oxygenic photosynthetic bacteria (cyanobacteria)—i.e., prokaryotes rather than eukaryotes. Even prokaryotes, however, are exceedingly complicated organisms that grow and reproduce efficiently. Structures of communities of microorganisms, layered rocks called stromatolites (stromatolite), are found from more than three billion years ago. Since Earth is about 4.6 billion years old, these finds suggest that the origin of life must have occurred within a few hundred million years of that time.

The Proterozoic Eon, once thought to be devoid of fossil evidence for life, is now known to be populated by overwhelming numbers of various kinds of bacteria and protist fossils—including acritarchs (spherical, robust unidentified fossils) and the entire range of Ediacaran fauna (Ediacara fauna). The Ediacarans—large, enigmatic, and in some cases animal-like extinct life-forms—are probably related to extant protists. Almost 100 species are known from some 30 locations worldwide, primarily sandstone formations. Most Ediacarans, presumed to have languished in sandy seaside locales, probably depended on their internal microbial symbionts (photo- or chemoautotrophs) for nourishment. No evidence that they were animals exists. In addition to the Ediacarans, acritarchs, and other abundant microfossils, clear evidence for pre-Phanerozoic, or Precambrian, life includes the massive banded-iron formations (banded-iron formation) (BIFs). Most BIFs date from 2.5 to 1.8 billion years ago. They are taken as indirect evidence for oxygen-producing, metal-depositing microscopic Proterozoic life. Investigations that use the electron microprobe (an instrument for visualizing structure and chemical composition simultaneously) and other micropaleontological techniques unfamiliar to classical geology have been employed to put together a much more complete picture of pre-Phanerozoic life.

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 irretrievably beyond the descriptive powers of physics, chemistry, and other science.

● Life, particularly simple forms, spontaneously (spontaneous generation) 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 Earth at the time of Earth's origin or shortly thereafter.

● Life arose on the early Earth by a series of progressive chemical reactions (chemical reaction). Such reactions may have been likely or may have required one or more highly improbable chemical events.

【May one】 doubt whether, in cheese and timber, worms are generated, or, if beetles and wasps, in cow's dung, or if butterflies, locusts, shellfish, snails, eels, and suchlike be procreated of putrefied matter, which is apt to receive the form of that creature to which it is by the formative power disposed. To question this is to question reason, sense, and experience. If he doubts of this, let him go to Egypt, and there he will find the fields swarming with mice begot of the mud of the Nylus 【Nile】, to the great calamity of the inhabitants.

(Alexander Ross, Arcana Microcosmi, 1652.)

Relative abundances of the elementsDarwin's (Darwin, Charles) 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. Indeed, modern astrophysicists do think about the origin of matter. The evidence is convincing that thermonuclear reactions (thermonuclear reaction), either in stellar interiors or in supernova explosions, generate all the chemical elements (chemical element) of the periodic table more massive than hydrogen and helium. Supernova explosions and stellar winds then distribute the elements into the interstellar medium, from which subsequent generations of stars (star) and planets form. These thermonuclear processes are frequent and well-documented. Some thermonuclear reactions are more probable than others. These facts lead to the idea that a certain cosmic (Cosmos) distribution of the major elements occurs throughout the universe. Some atoms of biological interest, their relative numerical abundances in the universe as a whole, on Earth, and in living organisms are listed in the table (Relative abundances of the elements). Even though elemental composition varies from star to star, from place to place on Earth, and from organism to organism, these comparisons are instructive: the composition of life is intermediate between the average composition of the universe and the average composition of Earth. Ninety-nine percent of the mass both of the universe and of life is made of six atoms: hydrogen (H), helium (He), carbon (C), nitrogen (N), oxygen (O), and neon (Ne). Might not life on Earth have arisen when Earth's chemical composition was closer to the average cosmic composition and before subsequent events changed Earth's gross chemical composition?

The Jovian planets ( Jupiter, Saturn, Uranus, and Neptune) are much closer to cosmic composition than is Earth. They are largely gaseous, with atmospheres composed principally of hydrogen and helium. methane, ammonia, neon, and water have been detected in smaller quantities. This circumstance very strongly suggests that the massive Jovian planets formed from material of typical cosmic composition. Because they are so far from the Sun, their upper atmospheres (atmosphere) are very cold. Atoms in the upper atmospheres of the massive, cold Jovian planets cannot now escape from their gravitational fields, and escape was probably difficult even during planetary formation.

Daniel B. Botkin (ed.), Forces of Change: A New View of Nature (2000), surveys interdisciplinary science concerning the biosphere. Loren Eiseley, The Immense Journey (1957), is regarded as a classic work of nature writing that combines science, poetry, personal memoir, and philosophical speculation. James Lovelock, Gaia: A New Look at Life on Earth (1979), argues that life is a planetary-level thermodynamic phenomenon; i.e., Earth's surface shows bodylike attributes of regulation of temperature, atmospheric chemistry, and other global environmental variables. Lynn Margulis and Dorion Sagan, What Is Life? (1995), explores the title question from a viewpoint combining biology and philosophy. Lynn Margulis and K.V. Schwartz, Five Kingdoms, 3rd ed. (1998), is a compendium of a popular, more-than-genetic classification system that divides all life on Earth into five kingdoms: bacteria, protoctists, fungi, plants, and animals. R. Morrison, The Spirit in the Gene: Humanity's Proud Illusion and the Laws of Nature (1999), argues that humanity's gene- and brain-based inclination to believe in its superiority is pushing humans to the edge of extinction. Eric D. Schneider and Dorion Sagan, Into the Cool: Energy Flow, Thermodynamics, and Life (2005), explores life as one member of a class of naturally complex structures cycling matter in regions of energy flow. James E. Strick, Sparks of Life: Darwinism and the Victorian Debates over Spontaneous Generation (2000), reviews the controversies of the late 19th century between evolutionists who supported the idea of “life from nonlife” and their responses to Louis Pasteur's religious view that only the Deity can make life. Sidney Liebes, Elisabet Sahtouris, and Brian Swimme, A Walk Through Time: From Stardust to Us: The Evolution of Life on Earth (1998), dramatizes the events from life's original appearance almost four billion years ago to the relatively extremely recent appearance of human beings. V.I. Vernadsky, The Biosphere (1998; originally published in Russian, 1926), popularized the term biosphere before the space-age photographs of Earth from space. Vernadsky sees life as a planetary phenomenon and examines it as a mineralogist might a strange new mineral. E.O. Wilson, Biophilia (1984), discusses the importance of cultivating a natural love of life, or “biophilia,” for the good of humanity and the biosphere.Carl Sagan Lynn Margulis Dorion Sagan