well In physical cosmology, the Big Bang is the scientific theory of how the universe emerged from a tremendously dense and hot state about 13.7 billion years ago. The theory is based on the observed expanding of space (in accord with the Friedmann-Lemaître model of general relativity) as indicated by the Hubble redshift of distant galaxies taken together with the cosmological principle.
Extrapolated into the past, these observations show that the universe has expanded from a state in which all the matter and energy in the universe was at an immense temperature and density. Physicists do not widely agree on what happened before this, although general relativity predicts a gravitational singularity (for reporting on some of the more notable speculation on this issue, see cosmogony).
Based on measurements of the expansion of the universe using Type 1a supernovae, measurements of the lumpiness of the cosmic microwave background, and measurements of the correlation function of galaxies, the universe has a calculated age of 13.7 ± 0.2 billion years. The agreement of these three independent measurements is considered strong evidence for the so-called ΛCDM model that describes the detailed nature of the contents of the universe.
The early universe was filled homogeneously and isotropically with an incredibly high energy density and concomitantly huge temperatures and pressures. It expanded and cooled, going through phase transitions analogous to the condensation of steam or freezing of water as it cools, but related to elementary particles.
Approximately 10−35 seconds after the Planck epoch a phase transition caused the universe to experience exponential growth during a period called cosmic inflation. After inflation stopped, the material components of the universe were in the form of a quark-gluon plasma (also including all other particles—and perhaps experimentally produced recently as a quark-gluon liquid [3]) in which the constituent particles were all moving relativistically. As the universe continued growing in size, the temperature dropped. At a certain temperature, by an as-yet-unknown transition called baryogenesis, the quarks and gluons combined into baryons such as protons and neutrons, somehow producing the observed asymmetry between matter and antimatter. Still lower temperatures led to further symmetry breaking phase transitions that put the forces of physics and elementary particles into their present form. Later, some protons and neutrons combined to form the universe's deuterium and helium nuclei in a process called Big Bang nucleosynthesis. As the universe cooled, matter gradually stopped moving relativistically and its rest mass energy density came to gravitationally dominate that of radiation. After about 300,000 years the electrons and nuclei combined into atoms (mostly hydrogen); hence the radiation decoupled from matter and continued through space largely unimpeded. This relic radiation is the cosmic microwave background.
Over time, the slightly denser regions of the nearly uniformly distributed matter gravitationally attracted nearby matter and thus grew even denser, forming gas clouds, stars, galaxies, and the other astronomical structures observable today. The details of this process depend on the amount and type of matter in the universe. The three possible types are known as cold dark matter, hot dark matter, and baryonic matter. The best measurements available (from WMAP) show that the dominant form of matter in the universe is cold dark matter. The other two types of matter make up less than 20% of the matter in the universe.
The universe today appears to be dominated by a mysterious form of energy known as dark energy. Approximately 70% of the total energy density of today's universe is in this form. This dark energy causes the expansion of the universe to deviate from a linear velocity-distance relationship, observed as a faster than expected expansion at very large distances. Dark energy in its simplest formulation takes the form of a cosmological constant term in Einstein's field equations of general relativity, but its composition is unknown and, more generally, the details of its equation of state and relationship with the standard model of particle physics continue to be investigated both observationally and theoretically.
All these observations are encapsulated in the ΛCDM model of cosmology, which is a mathematical model of the Big Bang with six free parameters. Mysteries appear as one looks closer to the beginning, when particle energies were higher than can yet be studied by experiment. There is no compelling physical model for the first 10−33 seconds of the universe, before the phase transition that grand unification theory predicts. At the "first instant", Einstein's theory of gravitation predicts a gravitational singularity where densities become infinite. To resolve this paradox, a theory of quantum gravitation is needed. Understanding this period of the history of the universe is one of the greatest unsolved problems in physics.
See also: Timeline of the Big Bang
The term Big Bang is used both in a narrow sense to refer to a point in time when the observed expansion of the universe (Hubble's law) began — calculated to be 13.7 billion (1.37 × 1010) years ago (±2%) — and in a more general sense to refer to the prevailing cosmological paradigm explaining the origin and expansion of the universe, as well as the composition of primordial matter through nucleosynthesis as predicted by the Alpher-Bethe-Gamow theory.[1]
From this model, George Gamow in 1948 was able to predict, at least qualitatively, the existence of cosmic microwave background radiation (CMB).[2] The CMB was discovered in the 1960s and further validated the Big Bang theory over its chief rival, the steady state theory.
The Big Bang theory developed from observations and theoretical considerations. Observationally, it was determined that most spiral nebulae were receding from Earth, but those who made the observation weren't aware of the cosmological implications, nor that the supposed nebulae were actually galaxies outside our own Milky Way.[3] In 1927, Georges Lemaître, a Roman Catholic priest, independently derived the Friedmann-Lemaître-Robertson-Walker equations from Albert Einstein's equations of general relativity and proposed, on the basis of the recession of spiral nebulae, that the universe began with the "explosion" of a "primeval atom"—what was later called the Big Bang.[4]
In 1929, Edwin Hubble provided an observational basis for Lemaître's theory. He discovered that, seen from Earth, light from other galaxies is red-shifted in direct proportion to their distance from the Earth. This fact is now known as Hubble's law.[5] Given the cosmological principle whereby the universe, when viewed on sufficiently large distance scales, has no preferred directions or preferred places, Hubble's law suggested that the universe was expanding, contradicting the infinite and unchanging static universe scenario developed by Einstein.
Artist depiction of the WMAP satellite gathering data to help scientists understand the Big Bang.This idea allowed for two opposing possibilities. One was Lemaître's Big Bang theory, advocated and developed by George Gamow. The other possibility was Fred Hoyle's steady state model in which new matter would be created as the galaxies moved away from each other. In this model, the universe is roughly the same at any point in time.[6] It was actually Hoyle who coined the name of Lemaître's theory, referring to it sarcastically as "this big bang idea" during a program broadcast on March 28, 1949 by the BBC Third Programme. Hoyle repeated the term in further broadcasts in early 1950, as part of a series of five lectures entitled The Nature of Things. The text of each lecture was published in The Listener a week after the broadcast, the first time that the term "big bang" appeared in print. [2]
For a number of years the support for these theories was evenly divided. However, the observational evidence began to support the idea that the universe evolved from a hot dense state. Since the discovery of the cosmic microwave background radiation in 1965 it has been regarded as the best theory of the origin and evolution of the cosmos. Virtually all theoretical work in cosmology now involves extensions and refinements to the basic Big Bang theory. Much of the current work in cosmology includes understanding how galaxies form in the context of the Big Bang, understanding what happened at the Big Bang, and reconciling observations with the basic theory.
Huge advances in Big Bang cosmology were made in the late 1990s and the early 21st century as a result of major advances in telescope technology in combination with large amounts of satellite data such as that from COBE, the Hubble Space Telescope and WMAP. Such data have allowed cosmologists to calculate many of the parameters of the Big Bang to a new level of precision and led to the unexpected discovery that the expansion of the universe appears to be accelerating. (See dark energy.)
In biology, evolution is the change in the heritable traits of a population over successive generations, as determined by shifts in the allele frequencies of genes. Through the course of time, this process results in the origin of new species from existing ones (speciation). All contemporary organisms are related to each other through common descent, the products of cumulative evolutionary changes over billions of years. Evolution is the source of the vast diversity of extant and extinct life on Earth.[1][2]
The basic mechanisms that produce evolutionary change are natural selection (which includes ecological, sexual, and kin selection) and genetic drift; these two mechanisms act on the genetic variation created by mutation, genetic recombination and gene flow. Natural selection is the process by which individual organisms with favorable traits are more likely to survive and reproduce. If those traits are heritable, they pass them to their offspring, with the result that beneficial heritable traits become more common in the next generation.[3][4][5] Given enough time, this passive process can result in varied adaptations to changing environmental conditions.[6]
The modern understanding of evolution is based on the theory of natural selection, which was first set out in a joint 1858 paper by Charles Darwin and Alfred Russel Wallace and popularized in Darwin's 1859 book The Origin of Species. In the 1930s, Darwinian natural selection was combined with the theory of Mendelian heredity to form the modern evolutionary synthesis, also known as "Neo-Darwinism". The modern synthesis describes evolution as a change in the allele frequency within a population from one generation to the next.[6]
The theory of evolution has become the central organizing principle of modern biology, relating directly to topics such as the origin of antibiotic resistance in bacteria, eusociality in insects, and the staggering biodiversity of the living world. The modern evolutionary synthesis is broadly received as scientific consensus and has replaced earlier explanations for the origin of species, including Lamarckism, and is currently the most powerful theory explaining biology.
Because of its potential implications for the origins of humankind, evolutionary theory has been at the center of many social and religious controversies since its inception
The idea of biological evolution has existed since ancient times, notably among Greek philosophers such as Anaximander and Epicurus and Indian philosophers such as Patañjali. Scientific theories of evolution were proposed in the 18th and 19th centuries, by scientists such as Jean-Baptiste Lamarck and Charles Darwin.
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Classical Darwinian theory
The transmutation of species was accepted by many scientists before 1859, but Charles Darwin's On The Origin of Species by Means of Natural Selection provided the first convincing exposition[7] of a mechanism by which evolutionary change could occur: natural selection. Darwin worked in private for many years, developing comprehensive justification for his theory, then brought forward publication of his work on evolution after receiving a letter from Alfred Russel Wallace in which Wallace revealed his own independent discovery of natural selection. Accordingly, Wallace is sometimes given shared credit for originating the theory.[8]
The publication of Darwin's book sparked a great deal of scientific and social debate. Darwin's work relied on many different fields of scientific inquiry for its evidence, and as a consequence debates over the theory took place in many different arenas. The book also was very popular among the literate public, and was soon translated into many languages.
Darwin was able to observe variation, and infer natural selection and thereby adaptation. However, the basis of heritability wasn't known, so Darwin couldn't explain how variation might arise, or be altered over generations. Darwin's proposal of a hereditary mechanism (pangenesis) lacked scientific support and was not incorporated into the modern synthesis[9], being replaced by genetics.
Although the occurrence of evolution of some sort came to be widely accepted by scientists, Darwin's specific ideas about evolution—that it occurred gradually, through natural selection—were actively attacked and contested. From the end of the 19th century through the early 20th century, forms of neo-Lamarckism, "progressive" evolution (orthogenesis), and an evolution which worked by "jumps" (saltationism, as opposed to gradualism) became popular, although a form of neo-Darwinism, led by August Weismann, also enjoyed some minor success. The biometric school of evolutionary theory, resulting from the work of Darwin's cousin, Francis Galton, emerged as well, using statistical approaches to biology which emphasized gradualism and some aspects of natural selection.[10]
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Modern synthesis
Main article: Modern evolutionary synthesis
See also: Mendelian inheritance
Gregor Mendel's work on the inheritance of traits in pea plants laid the foundation for genetics.Darwin's lack of a hereditary mechanism is often seen today as a major stumbling block in the historical acceptance of his theory, but in his time it was not a pressing issue as questions of the development of an organism were seen as more important than questions of the transmission of hereditary traits; Darwin and other biologists of his day thought that the answers to heredity would be found in embryology rather than in breeding experiments. Work on plant hybridity by a contemporary of Darwin's, an obscure Augustinian monk in Bohemia named Gregor Mendel, revealed that certain traits in peas occurred in discrete forms (that is, they were either one distinct trait or another, such as "round" or "wrinkled") and were inherited in a well-defined and predictable manner. Mendel's Law of Segregation and Law of Independent Assortment would eventually become key theories in the development of genetics, but in Darwin's time their significance was not seen (even by Mendel himself).[10]
When Mendel's work was "rediscovered" in 1901, it was initially interpreted as supporting an anti-Darwinian "jumping" form of evolution. The convinced Mendelians, such as William Bateson and Charles Benedict Davenport, and biometricians, such as Walter Frank Raphael Weldon and Karl Pearson, became embroiled in a bitter debate, with Mendelians charging that the biometricians did not understand biology, and biometricians arguing that most biological traits exhibited continuous variation rather than the "jumps" expected by the early Mendelian theory (we now know that the Mendelians were investigating Mendelian traits, those traits where existing variation is controlled by one gene and therefore is discrete, and the biometricians were investigating complex traits, where those traits were controlled by multiple genes, and the variation is therefore continuous). However, the simple version of the theory of early Mendelians soon gave way to the classical genetics of Thomas Hunt Morgan and his school, which thoroughly grounded and articulated the applications of Mendelian laws to biology. Eventually, it was shown that a rigorous statistical approach to Mendelism was reconcilable with the data of the biometricians by the work of statistician and population geneticist R.A. Fisher in the 1930s. Following this, the work of population geneticists —notably Sewall Wright and J. B. S. Haldane — and zoologists in the 1930s and 1940s synthesized Darwinian evolution with genetics, creating the modern evolutionary synthesis.[10] Genes were then still theoretical entities, and many paleontologists and embryologists were inclined to dismiss them as being of no, or minor, importance. [11]
Debates over various aspects of how evolution occurs have continued. One prominent debate was over the theory of punctuated equilibrium, proposed in 1972 by paleontologists Niles Eldredge and Stephen Jay Gould to explain the paucity of gradual transitions between species in the fossil record, as well as the absence of change or stasis that is observed over significant intervals of time.
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Molecular genetics
The most significant recent developments in evolutionary biology have been the improved understanding of and advances in genetics.[12] In the 1940s, following up on Griffith's experiment, Avery, MacLeod and McCarty definitively identified DNA (deoxyribonucleic acid) as the "transforming principle" responsible for transmitting genetic information. In 1953, Francis Crick and James D. Watson published their famous paper on the structure of DNA, based on the research of Rosalind Franklin and Maurice Wilkins. These developments ignited the era of molecular biology and transformed the understanding of evolution into a molecular process (see molecular evolution): the mutation of segments of DNA. George C. Williams' 1966 Adaptation and natural selection: A Critique of some Current Evolutionary Thought marked a departure from the idea of group selection towards the modern notion of the gene as the unit of selection. In the mid-1970s, Motoo Kimura formulated the neutral theory of molecular evolution, firmly establishing the importance of genetic drift as a mechanism of evolution.
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Academic disciplines
Scholars in a number of academic disciplines continue to document examples of the theory of evolution, contributing to a deeper understanding of its underlying mechanisms. Every subdiscipline within biology both informs and is informed by knowledge of the details of evolution, such as in ecological genetics, human evolution, molecular evolution, and phylogenetics. Areas of mathematics (such as bioinformatics), physics, chemistry and other fields all make important foundational contributions to the theory of evolution. Even disciplines as far removed as geology and sociology play a part, since the process of biological evolution has coincided in time and space with the development of both the Earth and human civilization.
Evolutionary biology is a subdiscipline of biology concerned with the origin and descent of species, as well as their changes over time. It was originally an interdisciplinary field including scientists from many traditional taxonomically-oriented disciplines. For example, it generally includes scientists who may have a specialist training in particular organisms, such as mammalogy, ornithology, or herpetology, but who use those organisms to answer general questions in evolution. Evolutionary biology as an academic discipline in its own right emerged as a result of the modern evolutionary synthesis in the 1930s and 1940s. It was not until the 1970s and 1980s, however, that a significant number of universities had departments that specifically included the term evolutionary biology in their titles.
Evolutionary developmental biology (informally, evo-devo) is a field of biology that compares the developmental processes of different animals in an attempt to determine the ancestral relationship between organisms and how developmental processes evolved. The discovery of genes regulating development in model organisms allowed for comparisons to be made with genes and genetic networks of related organisms.
Physical anthropology emerged in the late 19th century as the study of human osteology, and the fossilized skeletal remains of other hominids. At that time, anthropologists debated whether their evidence supported Darwin's claims, because skeletal remains revealed temporal and spatial variation among hominids, but Darwin had not offered an explanation of the specific mechanisms that produce variation. With the recognition of Mendelian genetics and the rise of the modern synthesis, however, evolution became both the fundamental conceptual framework for, and the object of study of, physical anthropologists. In addition to studying skeletal remains, they began to study genetic variation among human populations (population genetics); thus, some physical anthropologists began calling themselves biological anthropologists.
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Evidence of evolution
Main article: Evidence of evolution
Evolution has left numerous records which reveal the history of different species. Fossils, together with the comparative anatomy of present-day plants and animals, constitute the morphological, or anatomical, record. By comparing the anatomies of both modern and extinct species, paleontologists can infer the lineages of those species. Important fossil evidence includes the connection of distinct classes of organisms by so-called "transitional" species, such as the Archaeopteryx, which provided early evidence for the link between dinosaurs and birds,[13] and the recently-discovered Tiktaalik, which clarifies the development from fish to animals with four limbs.[14]
The development of molecular genetics, and particularly of DNA sequencing, has allowed biologists to study the record of evolution left in the organisms' genetic structures. The degree of similarity and difference in the DNA sequences of modern species allows geneticists to reconstruct their lineages. It is from DNA sequence comparisons that figures such as the 95% similarity between humans and chimpanzees come.[15][16]
Other evidence used to demonstrate evolutionary lineages includes the geographical distribution of species. For instance, monotremes and most marsupials are found only in Australia, showing that their common ancestor with placental mammals lived before the submerging of the ancient land bridge between Australia and Asia.
Scientists correlate all of the above evidence, drawn from paleontology, anatomy, genetics, and geography, with other information about the history of Earth. For instance, paleoclimatology attests to periodic ice ages during which the world's climate was much cooler, and these are often found to match up with the spread of species which are better-equipped to deal with the cold, such as the woolly mammoth.
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Morphological evidence
Letter c in the picture indicates the undeveloped hind legs of a baleen whale, vestigial remnants of its terrestrial ancestors.Fossils are critical evidence for estimating when various lineages originated. Since fossilization of an organism is an uncommon occurrence, usually requiring hard parts (like teeth, bone or pollen), the fossil record is traditionally thought to provide only sparse and intermittent information about ancestral lineages. Fossilization of organisms without hard body parts is rare, but happens under unusual circumstances, such as rapid burial, low oxygen environments, or microbial action[17].
The fossil record provides several types of data important to the study of evolution. First, the fossil record contains the earliest known examples of life itself, as well as the earliest occurrences of individual lineages. For example, the first complex animals date from the early Cambrian period, approximately 520 million years ago. Second, the records of individual species yield information regarding the patterns and rates of evolution, showing for example if species evolve into new species (speciation) gradually and incrementally, or in relatively brief intervals of geologic time. Thirdly, the fossil record is a document of large scale patterns and events in the history of life, many of which have influenced the evolutionary history of numerous lineages. For example, mass extinctions frequently resulted in the loss of entire groups of species, such as the non-avian dinosaurs, while leaving others relatively unscathed. Recently, molecular biologists have used the time since divergence of related lineages to calibrate the rate at which mutations accumulate, and at which the genomes of different lineages evolve.
Phylogenetics, the study of the ancestry of species, has revealed that structures with similar internal organization may perform divergent functions. Vertebrate limbs are a common example of such homologous structures. The appendages on bat wings, for example, are very structurally similar to human hands, and may constitute a vestigial structure. Other examples include the presence of hip bones in whales and snakes. Such structures may exist with little or no function in a more current organism, yet have a clear function in an ancestral species of the same. Examples of vestigial structures in humans include wisdom teeth, the coccyx and the vermiform appendix.
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Molecular evidence
Comparison of the DNA sequences allows organisms to be grouped by sequence similarity, and the resulting phylogenetic trees are typically congruent with traditional taxonomy, and are often used to strengthen or correct taxonomic classifications. Sequence comparison is considered a measure robust enough to be used to correct erroneous assumptions in the phylogenetic tree in instances where other evidence is scarce. For example, neutral human DNA sequences are approximately 1.2% divergent (based on substitutions) from those of their nearest genetic relative, the chimpanzee, 1.6% from gorillas, and 6.6% from baboons.[18] Genetic sequence evidence thus allows inference and quantification of genetic relatedness between humans and other apes.[19][20] The sequence of the 16S rRNA gene, a vital gene encoding a part of the ribosome, was used to find the broad phylogenetic relationships between all extant life. The analysis, originally done by Carl Woese, resulted in the three-domain system, arguing for two major splits in the early evolution of life. The first split led to modern Bacteria and the subsequent split led to modern Archaea and Eukaryote.
The proteomic evidence also supports the universal ancestry of life. Vital proteins, such as the ribosome, DNA polymerase, and RNA polymerase are found in the most primitive bacteria to the most complex mammals. The core part of the protein is conserved across all lineages of life, serving similar functions. Higher organisms have evolved additional protein subunits, largely affecting the regulation and protein-protein interaction of the core. Other overarching similarities between all lineages of extant organisms, such as DNA, RNA, amino acids, and the lipid bilayer, give support to the theory of common descent. The chirality of DNA, RNA, and amino acids is conserved across all known life. As there is no functional advantage to right or left handed molecular chirality, the simplest hypothesis is that the choice was made randomly in the early beginnings of life and passed on to all extant life through common descent.
Molecular evidence also offers a mechanism for large evolutionary leaps and macroevolution. Horizontal gene transfer, the process in which an organism transfers genetic material (i.e. DNA) to another cell that is not its offspring, allows for large sudden evolutionary leaps in a species by incorporating beneficial genes evolved in another species. The Endosymbiotic theory explains the origin of mitochondria and plastids (e.g. chloroplasts), which are organelles of eukaryotic cells, as the incorporation of an ancient prokaryotic cell into ancient eukaryotic cell. Rather than evolving eukaryotic organelles slowly, this theory offers a mechanism for a sudden evolutionary leap by incorporating the genetic material and biochemical composition of a separate species. This evolutionary mechanism has been observed. Heneta, a protist, is an extant organism that is undergoing endosymbiotic evolution[21][22].
Further evidence for reconstructing ancestral lineages comes from junk DNA such as pseudogenes, i.e., 'dead' genes, which steadily accumulate mutations. [23]
Since metabolic processes do not leave fossils, research into the evolution of the basic cellular processes is done largely by comparison of existing organisms. Many lineages diverged when new metabolic processes appeared, and it is theoretically possible to determine when certain metabolic processes appeared by comparing the traits of the descendants of a common ancestor or by detecting their physical manifestations. As an example, the appearance of oxygen in the earth's atmosphere is linked to the evolution of photosynthesis.
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Evidence from studies of complex iteration
"It has taken more than five decades, but the electronic computer is now powerful enough to simulate evolution" assisting bioinformatics in its attempt to solve biological problems.[24] Computer science allows the iteration of self changing complex systems to be studied, allowing a mathematically exact understanding of the nature of the processes behind evolution and providing evidence for the hidden causes of known evolutionary events. The evolution of specific cellular mechanisms like spliceosomes that can turn the cell's genome into a vast workshop of billions of interchangeable parts can be studied for the first time in an exact way.
Christoph Adami et al., for example, make this point in Evolution of biological complexity:
To make a case for or against a trend in the evolution of complexity in biological evolution, complexity needs to be both rigorously defined and measurable. A recent information-theoretic (but intuitively evident) definition identifies genomic complexity with the amount of information a sequence stores about its environment. We investigate the evolution of genomic complexity in populations of digital organisms and monitor in detail the evolutionary transitions that increase complexity. We show that, because natural selection forces genomes to behave as a natural "Maxwell Demon," within a fixed environment, genomic complexity is forced to increase. [25]
David J. Earl and Michael W. Deem also make this point in Evolvability is a selectable trait:
Not only has life evolved, but life has evolved to evolve. That is, correlations within protein structure have evolved, and mechanisms to manipulate these correlations have evolved in tandem. The rates at which the various events within the hierarchy of evolutionary moves occur are not random or arbitrary but are selected by Darwinian evolution. Sensibly, rapid or extreme environmental change leads to selection for greater evolvability. This selection is not forbidden by causality and is strongest on the largest-scale moves within the mutational hierarchy. Many observations within evolutionary biology, heretofore considered evolutionary happenstance or accidents, are explained by selection for evolvability. For example, the vertebrate immune system shows that the variable environment of antigens has provided selective pressure for the use of adaptable codons and low-fidelity polymerases during somatic hypermutation. A similar driving force for biased codon usage as a result of productively high mutation rates is observed in the hemagglutinin protein of influenza A. [26]
"Computer simulations of the evolution of linear sequences have demonstrated the importance of recombination of blocks of sequence rather than point mutagenesis alone. Repeated cycles of point mutagenesis, recombination, and selection should allow in vitro molecular evolution of complex sequences, such as proteins." [27] Evolutionary molecular engineering, also called "directed evolution" or "in vitro molecular evolution", involves the iterated cycle of mutation, multiplication with recombination, and selection of the fittest of individual molecules (proteins, DNA and RNA). The process of natural evolution can be reconstructed, showing possible paths from catalytic cycles based on proteins to ones based on RNA to ones based on DNA
which one do ya beileve and agree and understand?