Phases of Stellar Evolution

The initial phase of stellar evolution is contraction of the protostar from the interstellar gas, which consists of mostly hydrogen, some helium, and traces of heavier elements. In this stage, which typically lasts millions of years, half the gravitational potential energy released by the collapsing protostar is radiated away and half goes into increasing the temperature of the forming star. Eventually the temperature becomes high enough for thermonuclear reactions to begin; if the mass of the protostar is too small to raise the temperature to the ignition point for the thermonuclear reaction, the result is a brown dwarf, or "failed star." In these thermonuclear reactions, loosely called "hydrogen burning," four hydrogen nuclei are fused to form a helium nucleus (see nucleosynthesis). This point in time is conventionally called age zero.

Many protostar contractions have been observed in isolated gas clouds; that is, where one cloud contracted to form one star. However, in 1995, the first example of a star-forming region was found in the Eagle Nebula, some 7,000 light-years from the earth. In this region, stars are being formed at the tips of long, fingerlike columns stretching from a huge cloud of interstellar gas and dust; the columns are being eroded by radiation (a process called photoevaporation) from stars in the vicinity, leaving scattered knots of matter that contract into stars.

Once formed, a star settles into a long "middle age" during which it shines steadily as it converts its hydrogen supply into helium. For stars of a given chemical composition, the mass alone determines the luminosity, surface temperature, and size of the star. The luminosity increases very sharply with an increase in the mass; doubling the mass (which is proportional to the energy supply) increases the luminosity (which is proportional to the rate of using energy) more than 10 times. Hence the more massive and luminous a star is, the faster it depletes its hydrogen and the faster it evolves.

Because the middle age of a star is the longest period in stellar evolution, one would expect most of the observed stars to be at this stage and to show a strong correlation of luminosity with color (color is a measure of stellar temperature). This prediction is confirmed by plotting stars on a Hertzsprung-Russell diagram, in which the majority of stars fall along a diagonal line called the main sequence. The main sequence is most heavily populated at the low luminosity end; these are the stars that evolve most slowly and so remain longest on the main sequence.

As a star's hydrogen is converted into helium, its chemical composition becomes inhomogeneous: helium-rich in the core, where the nuclear reactions occur, and more nearly pure hydrogen in the surrounding envelope. The hydrogen near the center of the core is consumed first. As this is depleted, the site of the nuclear reactions moves out from the center of the core and fusion occurs in successive concentric shells. Finally fusion occurs only in a thin, outer shell of the core, the only place where both the hydrogen content and the temperature are high enough to sustain the reactions.

As the helium content of the star's core builds up, the core contracts and releases gravitational energy, which heats up the core and actually increases the rates of the nuclear reactions. Thus the rate of hydrogen consumption rises as the hydrogen is used up. To accommodate the higher luminosity resulting from the increased reaction rates, the envelope must expand to allow an increased flow of energy to the surface of the star. As the outer regions of the star expand, they cool.

The star now consists of a dense, helium rich core surrounded by a huge, tenuous envelope of relatively cool gas; the star has become a red giant. Eventually, the contracting stellar core will reach temperatures in excess of 100 million degrees Kelvin. At this point, helium burning sets in. With the ignition of that process, the expansion of the envelope is halted and then reversed; the star retreats from the red giant phase, shrinking in size and luminosity, and reapproaches the main sequence. The exact course of evolution is uncertain, but as the star recrosses the main sequence, it will probably become unstable. The star may eject some of its mass or become an exploding nova or supernova star; at the very least, it will become a pulsating variable star, possibly a Cepheid variable.

In the later stages of evolution, further contraction and elevation of temperature open up new thermonuclear reactions. It is believed that the heavier elements in the universe, up to iron, were synthesized in the interiors of stars by a variety of intricate nuclear reactions, many involving neutron absorption. Elements heavier than iron are made in supernova explosions. As a result of the nuclear reactions, the chemical composition of the late-stage star becomes highly inhomogeneous; its structure is fractionated into a number of concentric shells consisting of different elements around an iron core.

The final outcome of stellar evolution depends critically on the remaining mass of the old star. The vast majority of stars do not develop iron cores. If the mass is not greater than the Chandrasekhar mass limit (1.5 times the sun's mass), the star will become a white dwarf, glowing feebly for billions of years by radiating away its remaining heat energy until it becomes a black dwarf, a totally dead star. If the star is too massive to become a stable white dwarf, contraction will continue until the temperature reaches about 5 billion degrees Kelvin. At this temperature the iron nuclei in the core begin to absorb electrons; this creates neutron-rich isotopes and simultaneously deprives the core of its pressure. With further collapse and increase in density, the core becomes a special kind of rigid solid. At still higher density, the solid "evaporates" as the nuclei break up into free neutrons. The resulting neutron fluid forms the core of a new astrophysical body, called a neutron star, of which pulsars are examples. If the stellar mass is too great to be stable even as a neutron star, complete gravitational collapse will ensue and a black hole will form.

Validating the Theory of Stellar Evolution

Because the computed lifetimes of stars range from millions to billions of years, one cannot follow an individual star through its life history observationally, or even observe significant changes in the whole span of human history, except from the violent events of nova and supernova explosions. However, new stars are continually being formed and hence stars of all ages exist at the present epoch; examples of the various stages of stellar evolution can be found in different stars. The age of a star is not a directly observable characteristic but must be inferred from the very evolutionary theory one is trying to validate. Confidence in this circular reasoning results from its self-consistency and its ability to draw together into a unified picture a wide variety of observational data on individual stars, clusters of stars, and galaxies.

See cosmology; star clusters.

The Evolutionary Tree

Humans are mammals of the Primate order. The earliest primates evolved about 65 million years ago in the geological period known as the Paleocene epoch. They were small-brained, arboreal fruit eaters, similar to modern tree shrews. Primates of the Eocene epoch (55 to 38 million years ago) were similar and ancestral to contemporary tarsiers, lemurs, and tree shrews, and are classified as lower primates or prosimians. During the late Eocene, the higher primates, or anthropoids, developed from prosimian ancestors and, aided by continental drift, diverged into New World (or platyrrhine) and Old World (or catarrhine) monkeys. The branching of Old World monkeys and hominoids apparently occurred in the late Oligocene (38 to 25 million years ago) or early Miocene (25 to 8 million years ago), a time period poorly represented in the fossil record. The lesser apes (gibbons and siamangs) and other hominoid lines diverged about 20 million years ago, while the Asian great apes (the orangutan being the only surviving form) diverged from the African hominoids about 15 to 10 million years ago. Genetic evidence suggests that the ancestral lines of gorillas diverged about 8 million years ago and that chimpanzees and hominids diverged about 5 million years ago.

Hominid Evolution

The earliest known hominids are members of the genus Australopithecus, the earliest of which date to more than 4 million years ago. Unlike other primates, but like all hominids, australopithecines were bipedal. Their crania, however, were small and apelike, with an average cranial capacity of about 450 cc in the gracile species and 600 cc in the robust forms. Australopithecines that have been considered ancestral in the lineage leading to the human genus Homo include A. afarensis (an important skeleton of which is popularly known as Lucy) and A. africanus. The exact position of these and other early species on the hominid family tree continues to be disputed.

The first member of the genus Homo, a small gracile species known as H. habilis, was present in east Africa at least 2 million years ago. H. habilis was the first hominid to exhibit the marked expansion of the brain (with an average cranial capacity of about 750 cc) that would become a hallmark of subsequent hominid evolutionary history. By about 1.6 million years ago, H. habilis had evolved into a larger, more robust, and larger-brained species known as Homo erectus. Cranial capacities ranged from about 900 cc in early specimens to 1050 cc in later ones. H. erectus persisted for well over a million years and migrated off the African continent into Asia, Indonesia, and Europe.

Between 500,000 and 250,000 years ago, H. erectus evolved into H. sapiens. Transitional forms between H. erectus and H. sapiens are referred to as archaic H. sapiens. With the exception of H. sapiens neandertalensis (see Neanderthal man), no additional subspecies are recognized. Indeed, some scientists consider Neanderthal a separate species. Archaic H. sapiens changed gradually, becoming somewhat larger, more gracile and larger-brained through time. Cranial capacity, for example, increased from about 1150 cc in early transitional forms to the current world average of just over 1350 cc. By 150,000 years ago in Africa and Asia and 28,000 years ago in Europe (see Cro-Magnon man), the transition to H. sapiens was complete, and fully modern humans became the single surviving hominid species.

The Evolution of Culture

Among hominids, a parallel evolutionary process involving increased intelligence and cultural complexity is apparent in the material record. Evidence of greater behavioral flexibility and adaptability presumably reflects the decreased influence of genetically encoded behaviors and the increased importance of learning and social interaction in transmitting and maintaining behavioral adaptations (see culture). Because the organization of neural circuitry is more significant than overall cranial capacity in establishing mental capabilities, direct inferences from the fossil record are likely to be misleading. Contemporary humans, for example, exhibit considerable variability in cranial capacity (1150 cc to 1600 cc), none of which is related to intelligence.

Tool use was once thought to be the hallmark of members of the genus Homo, beginning with H. habilis, but is now known to be common among chimpanzees. The earliest stone tools of the lower Paleolithic, known as Oldowan tools and dating to about 2 to 2.5 million years ago, were once thought to have been manufactured by H. habilis. Recent finds suggest that Oldowan tools may also have been made by robust australopithecines. The simultaneous emergence of H. erectus and the more complex Achuelian tool tradition may indicate shifting adaptations as much as increased intelligence.

While it is clear that H. erectus was much more versatile than any of its predecessors, adapting its technologies and behaviors to diverse environmental conditions, the extent and limitations of its intellectual endowment remain a subject of heated debate. This is also the case for both archaic H. sapiens and Neanderthals, the latter associated with the more sophisticated technologies of the middle Paleolithic. However impressive the achievements of H. erectus and early H. sapiens, most material remains predating 40,000 years ago reflect utilitarian concerns. Nonetheless, there is now scattered African archaeological evidence from before that time (in one case as early as 90,000 years ago) of the production by H. sapiens of beads and other decorative work, perhaps indicating a gradual development of the aesthetic concerns and other symbolic thinking characteristic of later human societies. Whether the emergence of modern H. sapiens corresponds to the explosion of technological innovations and artistic activities associated with Cro-Magnon culture or was a more prolonged process of development is a subject of archaeological debate.

History of Evolutionary Theory

Evolutionary concepts appeared in some early Greek writings, e.g., in the works of Thales, Empedocles, Anaximander, and Aristotle. Under the restraining influence of the Church, no evolutionary theories developed during some 15 centuries of the Christian era to challenge the belief in special creation and the literal interpretation of the first part of Genesis; however, much data was accumulated that was to be utilized by later theorists. With the growth of scientific observation and experimentation, there began to appear from about the middle of the 16th cent. glimpses of the theory of evolution that emerged in the mid 19th cent. The invention of the microscope, making possible the study of reproductive cells and the growth of the science of embryology, was a factor in overthrowing hampering theories founded in false ideas of the reproductive process; studies in classification (taxonomy or systematics) and anatomy, based on dissection, were also influential.

Linnaeus, in his later years, showed an inclination toward belief in the mutability of species as a result of his observations of the many variations among species. Buffon, on the basis of his work in comparative anatomy, suggested the influence of use and disuse in molding the organs of vertebrate animals. Lamarck was the first to present a clearly stated evolutionary theory, but because it included the inheritance of acquired characteristics as the operative force of evolution, his whole theory was ridiculed and discredited for many years.

Although special creation of each species was the prevalent belief even among scientists in the first half of the 19th cent., the evidence in favor of evolution had by that time been uncovered. It remained for someone to assemble and interpret the evidence and to formulate a scientifically credible theory. This was accomplished simultaneously by A. R. Wallace and Charles Robert Darwin, who set forth the concepts that came to be known as Darwinism. In 1859 appeared the first edition of Darwin's Origin of Species. The influence of this evolutionary theory upon scientific thought and experimentation cannot be overestimated. In the years following the promulgation of Darwin's theory of evolution, many accepted and many denied its validity.

The theory found an opposing force in some religious creeds that declared it incompatible with their basic tenets. For a time evolution, sometimes falsely interpreted as meaning human descent from monkeys rather than descent from an ancient and extinct ancestor, became a target for attack by both church and educational authorities. Feeling ran high even as late as the time of the Scopes trial. Nevertheless, the theory of evolution became firmly entrenched as a scientific principle, and in most creeds it has been reconciled with religious teachings. Some Christian fundamentalists, however, do not accept the theory and have striven to have biblical creationism taught in the schools as an alternative theory. (For the evolution of human beings, see human evolution.)

Evolutionary theory has undergone modification in the light of later scientific developments. As more and more information has accumulated, the facts from a number of fields of investigation have provided corroboration and mutual support. Evidence that evolution has occurred still rests substantially on the same grounds that Darwin emphasized; comparative anatomy, embryology, geographical distribution, and paleontology. But additional recent evidence has come from biochemistry and molecular biology, which reveals fundamental similarities and relations in metabolism and hereditary mechanisms among disparate types of organisms. In general, both at the visible level and at the biochemical, one can detect the kinds of gradations of relatedness among organisms expected from evolution.

The chief weakness of Darwinian evolution lay in gaps in its explanations of the mechanism of evolution and of the origin of species. The Darwinian concept of natural selection is that inheritable variations among the individuals of given types of organisms continually arise in nature and that some variations prove advantageous under prevailing conditions in that they enable the organism to leave relatively more surviving offspring. But how these variations initially arise or are transmitted to offspring, and hence to subsequent generations, was not understood by Darwin. The science of genetics, originating at the beginning of the 20th cent. with the recognition of the importance of the earlier work of Mendel, provided a satisfactory explanation for the origin and transmission of variation. In 1901, de Vries presented his theory that mutation, or suddenly appearing and well-defined inheritable variation (as opposed to the slight, cumulative changes stressed by Darwin), is a force in the origin and evolution of species. Mutation in genes is now accepted by most biologists as a fundamental concept in evolutionary theory. The gene is the carrier of heredity and determines the attributes of the individual; thus changes in the genes can be transmitted to the offspring and produce new or altered attributes in the new individual.

Still prevalent misunderstandings of evolution are the beliefs that an animal or plant changes in order to better adapt to its environment—for example, that it develops an eye for the purpose of seeing—and that actual physical competition among individuals is required. Since mutation is a random process, changes can be either useful, unfavorable, or neutral to the individual's or species' survival. However, a new characteristic that is not detrimental may sometimes better enable the organism to survive or leave offspring in its environment, especially if that environment is changing, or to penetrate a new environment—such as the development of a lunglike structure that enables an aquatic animal to survive on land (see lungfish), where there may be more food and fewer predators.

Evolution of geographically separated groups in such a way that they show physical resemblances. A notable example is the similarity between the marsupial mammals of Australia and placental mammals elsewhere, which have arrived at remarkably similar forms through the separate courses of their evolution.

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Evolution of modern human beings from extinct nonhuman and humanlike forms. Genetic evidence points to an evolutionary divergence between the lineages of humans and the great apes on the African continent 8–5 million years ago (mya). The earliest fossils considered to be remains of hominins (members of the human lineage) date to at least 4 mya in Africa; they are classified as genus Australopithecus. The next major evolutionary stage, Homo habilis, inhabited sub-Saharan Africa about 2–1.5 mya. Homo habilis appears to have been supplanted by a taller and more humanlike species, Homo erectus, which lived from circa 1,700,000 to 200,000 years ago, gradually migrating into Asia and parts of Europe. Between circa 600,000 and 200,000 years ago, Homo heidelbergensis, sometimes called archaic Homo sapiens, lived in Africa, Europe, and perhaps parts of Asia. Having features resembling those of both H. erectus and modern humans, H. heidelbergensis may have been an ancestor of modern humans and also of the Neanderthals (H. neanderthalensis), who inhabited Europe and western Asia from circa 200,000 to 28,000 years ago. Fully modern humans (H. sapiens) seem to have emerged in Africa only circa 150,000 years ago, perhaps having descended directly from H. erectus or from an intermediate species such as H. heidelbergensis.

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Biological theory that animals and plants have their origin in other preexisting types and that the distinguishable differences are due to modifications in successive generations. It is one of the keystones of modern biological theory. In 1858 Charles Darwin and Alfred Russel Wallace jointly published a paper on evolution. The next year Darwin presented his major treatise On the Origin of Species by Means of Natural Selection, which revolutionized all later biological study. The heart of Darwinian evolution is the mechanism of natural selection. Surviving individuals, which vary (see variation) in some way that enables them to live longer and reproduce, pass on their advantage to succeeding generations. In 1937 Theodosius Dobzhansky applied Mendelian genetics (see Gregor Mendel) to Darwinian theory, contributing to a new understanding of evolution as the cumulative action of natural selection on small genetic variations in whole populations. Part of the proof of evolution is in the fossil record, which shows a succession of gradually changing forms leading up to those known today. Structural similarities and similarities in embryonic development among living forms also point to common ancestry. Molecular biology (especially the study of genes and proteins) provides the most detailed evidence of evolutionary change. Though the theory of evolution is accepted by nearly the entire scientific community, it has sparked much controversy from Darwin's time to the present; many of the objections have come from religious leaders and thinkers (see creationism) who believe that elements of the theory conflict with literal interpretations of the Bible. Seealso Hugo de Vries, Ernst Haeckel, human evolution, Ernst Mayr, parallel evolution, phylogeny, sociocultural evolution, speciation.

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In the theory of evolution, the rise of a system that cannot be predicted or explained from antecedent conditions. The British philosopher of science G.H. Lewes (1817–78) distinguished between resultants and emergents—phenomena that are predictable from their constituent parts (e.g., a physical mixture of sand and talcum powder) and those that are not (e.g., a chemical compound such as salt, which looks nothing like sodium or chlorine). The evolutionary account of life is a continuous history marked by stages at which fundamentally new forms have appeared. Each new mode of life, though grounded in the conditions of the previous stage, is intelligible only in terms of its own ordering principle. These are thus cases of emergence. In the philosophy of mind, the primary candidates for the status of emergent properties are mental states and events.

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