Thehistory of biology traces the study of theliving world fromancient tomodern times. Although the concept ofbiology as a single coherent field arose in the 19th century, thebiological sciences emerged fromtraditions of medicine andnatural history reaching back toAyurveda,ancient Egyptian medicine and the works ofAristotle,Theophrastus andGalen in the ancientGreco-Roman world. This ancient work was further developed in theMiddle Ages byMuslim physicians and scholars such asAvicenna. During the EuropeanRenaissance and early modern period, biological thought was revolutionized inEurope by a renewed interest inempiricism and the discovery of many novel organisms. Prominent in this movement wereVesalius andHarvey, who used experimentation and careful observation inphysiology, andnaturalists such asLinnaeus andBuffon who began toclassify the diversity of life and thefossil record, as well as the development and behavior of organisms.Antonie van Leeuwenhoek revealed by means ofmicroscopy the previously unknown world of microorganisms, laying the groundwork forcell theory. The growing importance ofnatural theology, partly a response to the rise ofmechanical philosophy, encouraged the growth of natural history (although it entrenched theargument from design).
Over the 18th and 19th centuries, biological sciences such asbotany andzoology became increasingly professionalscientific disciplines.Lavoisier and other physical scientists began to connect the animate and inanimate worlds through physics and chemistry. Explorer-naturalists such asAlexander von Humboldt investigated the interaction between organisms and their environment, and the ways this relationship depends on geography—laying the foundations forbiogeography,ecology andethology. Naturalists began to rejectessentialism and consider the importance ofextinction and themutability of species.Cell theory provided a new perspective on the fundamental basis of life. These developments, as well as the results fromembryology andpaleontology, were synthesized inCharles Darwin's theory ofevolution bynatural selection. The end of the 19th century saw the fall ofspontaneous generation and the rise of thegerm theory of disease, though the mechanism ofinheritance remained a mystery.
In the early 20th century, the rediscovery ofMendel's work in botany byCarl Correns led to the rapid development ofgenetics applied to fruit flies byThomas Hunt Morgan and his students, and by the 1930s the combination ofpopulation genetics and natural selection in the "neo-Darwinian synthesis". New disciplines developed rapidly, especially afterWatson andCrick proposed the structure ofDNA. Following the establishment of theCentral Dogma and the cracking of thegenetic code, biology was largely split betweenorganismal biology—the fields that deal with whole organisms and groups of organisms—and the fields related tocellular andmolecular biology. By the late 20th century, new fields likegenomics andproteomics were reversing this trend, with organismal biologists using molecular techniques, and molecular and cell biologists investigating the interplay between genes and the environment, as well as the genetics of natural populations of organisms.
Theearliest humans must have had and passed on knowledge aboutplants andanimals to increase their chances of survival. This may have included knowledge of human and animal anatomy and aspects of animal behavior (such as migration patterns). However, the first major turning point in biological knowledge came with theNeolithic Revolution about 10,000 years ago. Humans first domesticated plants for farming, thenlivestock animals to accompany the resultingsedentary societies.[1]
Between around 3000 and 1200BCE, theAncient Egyptians andMesopotamians made contributions toastronomy,mathematics, andmedicine,[2][3] which later entered and shaped Greeknatural philosophy ofclassical antiquity, a period that profoundly influenced the development of what came to be known as biology.[1]
Over a dozenmedical papyri have been preserved, most notably theEdwin Smith Papyrus (the oldest extant surgical handbook) and theEbers Papyrus (a handbook of preparing and using materia medica for various diseases), both from around 1600 BCE.[2]
Ancient Egypt is also known for developingembalming, which was used formummification, in order to preserve human remains and forestalldecomposition.[1]
The Mesopotamians seem to have had little interest in the natural world as such, preferring to study how the gods had ordered the universe.Animal physiology was studied fordivination, including especially the anatomy of theliver, seen as an important organ inharuspicy.Animal behavior too was studied for divinatory purposes. Most information about the training and domestication of animals was probably transmitted orally, but one text dealing with the training of horses has survived.[4]
The ancient Mesopotamians had no distinction between "rational science" andmagic.[5][6][7] When a person became ill, doctors prescribed both magical formulas to be recited and medicinal treatments.[5][6][7] The earliest medical prescriptions appear inSumerian during theThird Dynasty of Ur (c. 2112 – c. 2004 BCE).[8] The most extensive Babylonian medical text, however, is theDiagnostic Handbook written by theummânū, or chief scholar,Esagil-kin-apli ofBorsippa,[9] during the reign of the Babylonian kingAdad-apla-iddina (1069 – 1046 BCE).[10] InEast Semitic cultures, the main medicinal authority was an exorcist-healer known as anāšipu.[5][6][7] The profession was passed down from father to son and was held in high regard.[5] Of less frequent recourse was theasu, a healer who treated physical symptoms using remedies composed of herbs, animal products, and minerals, as well as potions, enemas, and ointments orpoultices. These physicians, who could be either male or female, also dressed wounds, set limbs, and performed simple surgeries. The ancient Mesopotamians also practicedprophylaxis and took measures to prevent the spread of disease.[4]
Observations and theories regarding nature and human health, separate fromWestern traditions, had emerged independently in other civilizations such as those inChina and theIndian subcontinent.[1] In ancient China, earlier conceptions can be found dispersed across several different disciplines, including the work ofherbologists, physicians, alchemists, andphilosophers. TheTaoist tradition ofChinese alchemy, for example, emphasized health (with the ultimate goal being theelixir of life). The system ofclassical Chinese medicine usually revolved around the theory ofyin and yang, and thefive phases.[1] Taoist philosophers, such asZhuangzi in the 4th century BCE, also expressed ideas related toevolution, such as denying the fixity of biological species and speculating that species had developed differing attributes in response to differing environments.[11]
One of the oldest organised systems of medicine is known from ancient India in the form ofAyurveda, which originated around 1500 BCE fromAtharvaveda (one of the four most ancient books of Indian knowledge, wisdom and culture).
The ancient IndianAyurveda tradition independently developed the concept of three humours, resembling that of thefour humours ofancient Greek medicine, though the Ayurvedic system included further complications, such as the body being composed offive elements and seven basictissues. Ayurvedic writers also classified living things into four categories based on the method of birth (from the womb, eggs, heat & moisture, and seeds) and explained the conception of afetus in detail. They also made considerable advances in the field ofsurgery, often without the use of humandissection or animalvivisection.[1] One of the earliest Ayurvedic treatises was theSushruta Samhita, attributed to Sushruta in the 6th century BCE. It was also an earlymateria medica, describing 700 medicinal plants, 64 preparations from mineral sources, and 57 preparations based on animal sources.[12]
Thepre-Socratic philosophers asked many questions about life but produced little systematic knowledge of specifically biological interest—though the attempts of theatomists to explain life in purely physical terms would recur periodically through the history of biology. However, the medical theories ofHippocrates and his followers, especiallyhumorism, had a lasting impact.[1]
The philosopherAristotle was the most influential scholar of the living world fromclassical antiquity.[13] Though his early work in natural philosophy was speculative,Aristotle's later biological writings were more empirical, focusing on biological causation and the diversity of life. He made countless observations of nature, especially the habits andattributes ofplants andanimals in the world around him, which he devoted considerable attention tocategorizing. In all, Aristotle classified 540 animal species, and dissected at least 50. He believed that intellectual purposes,formal causes, guided all natural processes.[14]
Aristotle's successor at theLyceum,Theophrastus, wrote a series of books on botany, theHistory of Plants, which survived as the most important contribution of antiquity to botany, even into theMiddle Ages. Many of Theophrastus' names survive into modern times, such askarpós for fruit, andperikárpion for seed vessel.Dioscorides wrote a pioneering andencyclopedicpharmacopoeia,De materia medica, incorporating descriptions of some 600 plants and their uses inmedicine.Pliny the Elder, in hisNatural History, assembled a similarly encyclopaedic account of things in nature, including accounts of many plants and animals.[15] Aristotle, and nearly all Western scholars after him until the 18th century, believed that creatures were arranged in a graded scale of perfection rising from plants on up to humans: thescala naturae orGreat Chain of Being.[16]
A few scholars in theHellenistic period under thePtolemies—particularlyHerophilus of Chalcedon andErasistratus of Chios—amended Aristotle's physiological work, even performing dissections and vivisections.[17]Claudius Galen became the most important authority on medicine and anatomy. Though a few ancientatomists such asLucretius challenged theteleological Aristotelian viewpoint that all aspects of life are the result of design or purpose, teleology (and after the rise ofChristianity,natural theology) would remain central to biological thought essentially until the 18th and 19th centuries.Ernst W. Mayr argued that "Nothing of any real consequence happened in biology after Lucretius and Galen until the Renaissance."[18] The ideas of the Greek traditions of natural history and medicine survived, but they were generally taken unquestioningly inmedieval Europe.[19]
The decline of theRoman Empire led to the disappearance or destruction of much knowledge, though physicians still incorporated many aspects of the Greek tradition into training and practice. InByzantium and theIslamic world, many of the Greek works were translated intoArabic and many of the works of Aristotle were preserved.[20]
During theHigh Middle Ages, a few European scholars such asHildegard of Bingen,Albertus Magnus andFrederick II wrote on natural history. Therise of European universities, though important for the development of physics and philosophy, had little impact on biological scholarship.[21]
TheEuropean Renaissance brought expanded interest in both empirical natural history and physiology. In 1543,Andreas Vesalius inaugurated the modern era of Western medicine with his seminalhuman anatomy treatiseDe humani corporis fabrica, which was based on dissection of corpses. Vesalius was the first in a series of anatomists who gradually replacedscholasticism withempiricism in physiology and medicine, relying on first-hand experience rather than authority and abstract reasoning. Viaherbalism, medicine was also indirectly the source of renewed empiricism in the study of plants.Otto Brunfels,Hieronymus Bock andLeonhart Fuchs wrote extensively on wild plants, the beginning of a nature-based approach to the full range of plant life.[22]Bestiaries—a genre that combines both the natural and figurative knowledge of animals—also became more sophisticated, especially with the work ofWilliam Turner,Pierre Belon,Guillaume Rondelet,Conrad Gessner, andUlisse Aldrovandi.[23]
Artists such asAlbrecht Dürer andLeonardo da Vinci, often working with naturalists, were also interested in the bodies of animals and humans, studying physiology in detail and contributing to the growth of anatomical knowledge.[24] The traditions ofalchemy andnatural magic, especially in the work ofParacelsus, also laid claim to knowledge of the living world. Alchemists subjected organic matter to chemical analysis and experimented liberally with both biological and mineralpharmacology.[25] This was part of a larger transition in world views (the rise of themechanical philosophy) that continued into the 17th century, as the traditional metaphor ofnature as organism was replaced by thenature as machine metaphor.[26]
Systematizing, naming and classifying dominated natural history throughout much of the 17th and 18th centuries.Carl Linnaeus published a basictaxonomy for the natural world in 1735 (variations of which have been in use ever since), and in the 1750s introducedscientific names for all his species.[27] While Linnaeus conceived of species as unchanging parts of a designed hierarchy, the other great naturalist of the 18th century,Georges-Louis Leclerc, Comte de Buffon, treated species as artificial categories and living forms as malleable—even suggesting the possibility ofcommon descent. Though he was opposed to evolution, Buffon is a key figure in thehistory of evolutionary thought; his work would influence the evolutionary theories of bothLamarck andDarwin.[28]
The discovery and description of new species and thecollection of specimens became a passion of scientific gentlemen and a lucrative enterprise for entrepreneurs; many naturalists traveled the globe in search of scientific knowledge and adventure.[29]
Extending the work of Vesalius into experiments on still living bodies (of both humans and animals),William Harvey and other natural philosophers investigated the roles of blood, veins and arteries. Harvey'sDe motu cordis in 1628 was the beginning of the end for Galenic theory, and alongsideSantorio Santorio's studies of metabolism, it served as an influential model of quantitative approaches to physiology.[30]
In the early 17th century, the micro-world of biology was just beginning to open up. A few lensmakers and natural philosophers had been creating crudemicroscopes since the late 16th century, andRobert Hooke published the seminalMicrographia based on observations with his own compound microscope in 1665. But it was not untilAntonie van Leeuwenhoek's dramatic improvements in lensmaking beginning in the 1670s—ultimately producing up to 200-fold magnification with a single lens—that scholars discoveredspermatozoa,bacteria,infusoria and the sheer strangeness and diversity of microscopic life. Similar investigations byJan Swammerdam led to a new interest inentomology and built the basic techniques of microscopic dissection andstaining.[31]
As the microscopic world was expanding, the macroscopic world was shrinking. Botanists such asJohn Ray worked to incorporate the flood of newly discovered organisms shipped from across the globe into a coherent taxonomy, and a coherent theology (natural theology).[32] Debate over another flood, theNoachian, catalyzed the development ofpaleontology; in 1669Nicholas Steno published an essay on how the remains of living organisms could be trapped in layers of sediment and mineralized to producefossils. Although Steno's ideas about fossilization were well known and much debated among natural philosophers, an organic origin for all fossils would not be accepted by all naturalists until the end of the 18th century due to philosophical and theological debate about issues such as the age of the earth andextinction.[33]
Up through the 19th century, the scope of biology was largely divided between medicine, which investigated questions of form and function (i.e., physiology), and natural history, which was concerned with the diversity of life and interactions among different forms of life and between life and non-life. By 1900, much of these domains overlapped, while natural history (and its counterpartnatural philosophy) had largely given way to more specialized scientific disciplines—cytology,bacteriology,morphology,embryology,geography, andgeology.
FromGreekβίος (bíos) 'life', (fromProto-Indo-European root *gwei-, to live) andλογία (logia) 'study of'. The compound appears in the title of Volume 3 ofMichael Christoph Hanow'sPhilosophiae naturalis sive physicae dogmaticae: Geologia, biologia, phytologia generalis et dendrologia, published in 1766. The termbiology in its modern sense appears to have been introduced independently byThomas Beddoes (in 1799),[34]Karl Friedrich Burdach (in 1800),Gottfried Reinhold Treviranus (Biologie oder Philosophie der lebenden Natur, 1802) andJean-Baptiste Lamarck (Hydrogéologie, 1802).[35][36][37]
Beforebiology, there were several terms used for the study of animals and plants.Natural history referred to the descriptive aspects of biology, though it also includedmineralogy and other non-biological fields; from the Middle Ages through the Renaissance, the unifying framework of natural history was thescala naturae orGreat Chain of Being.Natural philosophy andnatural theology encompassed the conceptual and metaphysical basis of plant and animal life, dealing with problems of why organisms exist and behave the way they do, though these subjects also included what is nowgeology,physics,chemistry, andastronomy. Physiology and (botanical) pharmacology were the province of medicine.Botany,Zoology, and (in the case of fossils)Geology replacednatural history andnatural philosophy in the 18th and 19th centuries beforebiology was widely adopted.[38][39] To this day, "botany" and "zoology" are widely used, although they have been joined by other sub-disciplines of biology.
Widespread travel by naturalists in the early-to-mid-19th century resulted in a wealth of new information about the diversity and distribution of living organisms. Of particular importance was the work ofAlexander von Humboldt, which analyzed the relationship between organisms and their environment (i.e., the domain ofnatural history) using the quantitative approaches ofnatural philosophy (i.e.,physics andchemistry). Humboldt's work laid the foundations ofbiogeography and inspired several generations of scientists.[40]
The emerging discipline of geology also brought natural history and natural philosophy closer together; the establishment of thestratigraphic column linked the spatial distribution of organisms to their temporal distribution, a key precursor to concepts of evolution.Georges Cuvier and others made great strides incomparative anatomy andpaleontology in the late 1790s and early 19th century. In a series of lectures and papers that made detailed comparisons between living mammals andfossil remains Cuvier was able to establish that the fossils were remains of species that had becomeextinct—rather than being remains of species still alive elsewhere in the world, as had been widely believed.[41] Fossils discovered and described byGideon Mantell,William Buckland,Mary Anning, andRichard Owen among others helped establish that there had been an 'age of reptiles' that had preceded even the prehistoric mammals. These discoveries captured the public imagination and focused attention on the history of life on earth.[42] Most of these geologists held tocatastrophism, butCharles Lyell's influentialPrinciples of Geology (1830) popularisedHutton'suniformitarianism, a theory that explained the geological past and present on equal terms.[43]
The most significant evolutionary theory before Darwin's was that ofJean-Baptiste Lamarck; based on theinheritance of acquired characteristics (an inheritance mechanism that was widely accepted until the 20th century), it described a chain of development stretching from the lowliest microbe to humans.[44] The British naturalistCharles Darwin, combining the biogeographical approach of Humboldt, the uniformitarian geology of Lyell,Thomas Malthus's writings on population growth, and his own morphological expertise, created a more successful evolutionary theory based onnatural selection; similar evidence ledAlfred Russel Wallace to independently reach the same conclusions.[45]
The 1859 publication of Darwin's theory inOn the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life is often considered the central event in the history of modern biology. Darwin's established credibility as a naturalist, the sober tone of the work, and most of all the sheer strength and volume of evidence presented, allowedOrigin to succeed where previous evolutionary works such as the anonymousVestiges of Creation had failed. Most scientists were convinced of evolution andcommon descent by the end of the 19th century. However, natural selection would not be accepted as the primary mechanism of evolution until well into the 20th century, as most contemporary theories of heredity seemed incompatible with the inheritance of random variation.[46]
Wallace, following on earlier work byde Candolle,Humboldt and Darwin, made major contributions tozoogeography. Because of his interest in the transmutation hypothesis, he paid particular attention to the geographical distribution of closely allied species during his field work first inSouth America and then in theMalay Archipelago. While in the archipelago he identified theWallace line, which runs through theSpice Islands dividing the fauna of the archipelago between an Asian zone and aNew Guinea/Australian zone. His key question, as to why the fauna of islands with such similar climates should be so different, could only be answered by considering their origin. In 1876 he wroteThe Geographical Distribution of Animals, which was the standard reference work for over half a century, and a sequel,Island Life, in 1880 that focused on island biogeography. He extended the six-zone system developed byPhilip Sclater for describing the geographical distribution of birds to animals of all kinds. His method of tabulating data on animal groups in geographic zones highlighted the discontinuities; and his appreciation of evolution allowed him to propose rational explanations, which had not been done before.[47][48]
The scientific study ofheredity grew rapidly in the wake of Darwin'sOrigin of Species with the work ofFrancis Galton and thebiometricians. The origin ofgenetics is usually traced to the 1866 work of themonkGregor Mendel, who would later be credited with thelaws of inheritance. However, his work was not recognized as significant until 35 years afterward. In the meantime, a variety of theories of inheritance (based onpangenesis,orthogenesis, or other mechanisms) were debated and investigated vigorously.[50]Embryology andecology also became central biological fields, especially as linked to evolution and popularized in the work ofErnst Haeckel. Most of the 19th century work on heredity, however, was not in the realm of natural history, but that of experimental physiology.
Over the course of the 19th century, the scope of physiology expanded greatly, from a primarily medically oriented field to a wide-ranging investigation of the physical and chemical processes of life—including plants, animals, and even microorganisms in addition to man.Living things as machines became a dominant metaphor in biological (and social) thinking.[51]
Advances inmicroscopy also had a profound impact on biological thinking. In the early 19th century, a number of biologists pointed to the central importance of thecell. In 1838 and 1839,Schleiden andSchwann began promoting the ideas that (1) the basic unit of organisms is the cell and (2) that individual cells have all the characteristics oflife, though they opposed the idea that (3) all cells come from the division of other cells. Thanks to the work ofRobert Remak andRudolf Virchow, however, by the 1860s most biologists accepted all three tenets of what came to be known ascell theory.[56]
Cell theory led biologists to re-envision individual organisms as interdependent assemblages of individual cells. Scientists in the rising field ofcytology, armed with increasingly powerful microscopes and newstaining methods, soon found that even single cells were far more complex than the homogeneous fluid-filled chambers described by earlier microscopists.Robert Brown had described thenucleus in 1831, and by the end of the 19th century cytologists identified many of the key cell components:chromosomes,centrosomesmitochondria,chloroplasts, and other structures made visible through staining. Between 1874 and 1884Walther Flemming described the discrete stages of mitosis, showing that they were notartifacts of staining but occurred in living cells, and moreover, that chromosomes doubled in number just before the cell divided and a daughter cell was produced. Much of the research on cell reproduction came together inAugust Weismann's theory of heredity: he identified the nucleus (in particular chromosomes) as the hereditary material, proposed the distinction betweensomatic cells andgerm cells (arguing that chromosome number must be halved for germ cells, a precursor to the concept ofmeiosis), and adoptedHugo de Vries's theory ofpangenes. Weismannism was extremely influential, especially in the new field of experimentalembryology.[57]
By the mid-1850s themiasma theory of disease was largely superseded by thegerm theory of disease, creating extensive interest in microorganisms and their interactions with other forms of life. By the 1880s,bacteriology was becoming a coherent discipline, especially through the work ofRobert Koch, who introduced methods for growing pure cultures onagar gels containing specific nutrients inPetri dishes. The long-held idea that living organisms could easily originate from nonliving matter (spontaneous generation) was attacked in a series of experiments carried out byLouis Pasteur, while debates overvitalism vs.mechanism (a perennial issue since the time of Aristotle and the Greek atomists) continued apace.[58]
In chemistry, one central issue was the distinction between organic and inorganic substances, especially in the context of organic transformations such asfermentation andputrefaction. Since Aristotle these had been considered essentially biological (vital) processes. However,Friedrich Wöhler,Justus Liebig and other pioneers of the rising field oforganic chemistry—building on the work of Lavoisier—showed that the organic world could often be analyzed by physical and chemical methods. In 1828 Wöhler showed that the organic substanceurea could be created by chemical means that do not involve life, providing a powerful challenge tovitalism. Cell extracts ("ferments") that could effect chemical transformations were discovered, beginning withdiastase in 1833. By the end of the 19th century the concept ofenzymes was well established, though equations ofchemical kinetics would not be applied to enzymatic reactions until the early 20th century.[59]
Physiologists such asClaude Bernard explored (through vivisection and other experimental methods) the chemical and physical functions of living bodies to an unprecedented degree, laying the groundwork forendocrinology (a field that developed quickly after the discovery of the firsthormone,secretin, in 1902),biomechanics, and the study ofnutrition anddigestion. The importance and diversity of experimental physiology methods, within both medicine and biology, grew dramatically over the second half of the 19th century. The control and manipulation of life processes became a central concern, and experiment was placed at the center of biological education.[60]
At the beginning of the 20th century, biological research was largely a professional endeavour. Most work was still done in thenatural history mode, which emphasized morphological and phylogenetic analysis over experiment-based causal explanations. However, anti-vitalist experimental physiologists and embryologists, especially in Europe, were increasingly influential. The tremendous success of experimental approaches to development, heredity, and metabolism in the 1900s and 1910s demonstrated the power of experimentation in biology. In the following decades, experimental work replaced natural history as the dominant mode of research.[61]
In the early 20th century, naturalists were faced with increasing pressure to add rigor and preferably experimentation to their methods, as the newly prominent laboratory-based biological disciplines had done.Ecology had emerged as a combination of biogeography with thebiogeochemical cycle concept pioneered by chemists; field biologists developed quantitative methods such as thequadrat and adapted laboratory instruments and cameras for the field to further set their work apart from traditional natural history. Zoologists and botanists did what they could to mitigate the unpredictability of the living world, performing laboratory experiments and studying semi-controlled natural environments such as gardens; new institutions like theCarnegie Station for Experimental Evolution and theMarine Biological Laboratory provided more controlled environments for studying organisms through their entire life cycles.[62]
Theecological succession concept, pioneered in the 1900s and 1910s byHenry Chandler Cowles andFrederic Clements, was important in early plant ecology.[63]Alfred Lotka'spredator-prey equations,G. Evelyn Hutchinson's studies of the biogeography and biogeochemical structure of lakes and rivers (limnology) andCharles Elton's studies of animalfood chains were pioneers among the succession of quantitative methods that colonized the developing ecological specialties. Ecology became an independent discipline in the 1940s and 1950s afterEugene P. Odum synthesized many of the concepts ofecosystem ecology, placing relationships between groups of organisms (especially material and energy relationships) at the center of the field.[64]
In the 1960s, as evolutionary theorists explored the possibility of multipleunits of selection, ecologists turned to evolutionary approaches. Inpopulation ecology, debate overgroup selection was brief but vigorous; by 1970, most biologists agreed that natural selection was rarely effective above the level of individual organisms. The evolution of ecosystems, however, became a lasting research focus. Ecology expanded rapidly with the rise of the environmental movement; theInternational Biological Program attempted to apply the methods ofbig science (which had been so successful in the physical sciences) to ecosystem ecology and pressing environmental issues, while smaller-scale independent efforts such asisland biogeography and theHubbard Brook Experimental Forest helped redefine the scope of an increasingly diverse discipline.[65]
1900 marked the so-calledrediscovery of Mendel byCarl Correns, who arrived atMendel's laws (which were not actually present in Mendel's work).[66] Soon after, cytologists (cell biologists) proposed thatchromosomes were the hereditary material. This was taken up byCarl Correns and others between 1910 and 1915 as the "Mendelian-chromosome theory" of heredity.Thomas Hunt Morgan and the "Drosophilists" in his fly lab applied this to a new model organism.[67] They hypothesizedcrossing over to explain linkage and constructedgenetic maps of the fruit flyDrosophila melanogaster, which became a widely usedmodel organism.[68]
Hugo de Vries tried to link the new genetics with evolution; building on his work with heredity andhybridization, he proposed a theory ofmutationism, which was widely accepted in the early 20th century.Lamarckism, or the theory of inheritance of acquired characteristics also had many adherents.Darwinism was seen as incompatible with the continuously variable traits studied bybiometricians, which seemed only partially heritable. In the 1920s and 1930s—following the acceptance of the Mendelian-chromosome theory— the emergence of the discipline ofpopulation genetics, with the work ofR.A. Fisher,J.B.S. Haldane andSewall Wright, unified the idea of evolution bynatural selection withMendelian genetics, producing themodern synthesis. Theinheritance of acquired characters was rejected, while mutationism gave way as genetic theories matured.[69]
In the second half of the century the ideas of population genetics began to be applied in the new discipline of the genetics of behavior,sociobiology, and, especially in humans,evolutionary psychology. In the 1960sW.D. Hamilton and others developedgame theory approaches to explainaltruism from an evolutionary perspective throughkin selection. The possible origin of higher organisms throughendosymbiosis, and contrasting approaches to molecular evolution in thegene-centered view (which held selection as the predominant cause of evolution) and theneutral theory (which madegenetic drift a key factor) spawned perennial debates over the proper balance ofadaptationism and contingency in evolutionary theory.[70]
In the 1970sStephen Jay Gould andNiles Eldredge proposed the theory ofpunctuated equilibrium which holds that stasis is the most prominent feature of the fossil record, and that most evolutionary changes occur rapidly over relatively short periods of time.[71] In 1980Luis Alvarez andWalter Alvarez proposed the hypothesis that animpact event was responsible for theCretaceous–Paleogene extinction event.[72] Also in the early 1980s, statistical analysis of the fossil record of marine organisms published byJack Sepkoski andDavid M. Raup led to a better appreciation of the importance ofmass extinction events to the history of life on earth.[73]
By the end of the 19th century all of the major pathways ofdrug metabolism had been discovered, along with the outlines of protein and fatty acid metabolism and urea synthesis.[74] In the early decades of the 20th century, the minor components of foods in human nutrition, thevitamins, began to be isolated and synthesized. Improved laboratory techniques such aschromatography andelectrophoresis led to rapid advances in physiological chemistry, which—asbiochemistry—began to achieve independence from its medical origins. In the 1920s and 1930s, biochemists—led byHans Krebs andCarl andGerty Cori—began to work out many of the centralmetabolic pathways of life: thecitric acid cycle,glycogenesis andglycolysis, and the synthesis ofsteroids andporphyrins. Between the 1930s and 1950s,Fritz Lipmann and others established the role ofATP as the universal carrier of energy in the cell, andmitochondria as the powerhouse of the cell. Such traditionally biochemical work continued to be very actively pursued throughout the 20th century and into the 21st.[75]
Following the rise of classical genetics, many biologists—including a new wave of physical scientists in biology—pursued the question of the gene and its physical nature.Warren Weaver—head of the science division of theRockefeller Foundation—issued grants to promote research that applied the methods of physics and chemistry to basic biological problems, coining the termmolecular biology for this approach in 1938; many of the significant biological breakthroughs of the 1930s and 1940s were funded by the Rockefeller Foundation.[76]
Like biochemistry, the overlapping disciplines ofbacteriology andvirology (later combined asmicrobiology), situated between science and medicine, developed rapidly in the early 20th century.Félix d'Herelle's isolation ofbacteriophage during World War I initiated a long line of research focused on phage viruses and the bacteria they infect.[77]
The development of standard, genetically uniform organisms that could produce repeatable experimental results was essential for the development ofmolecular genetics. After early work withDrosophila andmaize, the adoption of simplermodel systems like the bread moldNeurospora crassa made it possible to connect genetics to biochemistry, most importantly withBeadle andTatum'sone gene–one enzyme hypothesis in 1941. Genetics experiments on even simpler systems liketobacco mosaic virus andbacteriophage, aided by the new technologies ofelectron microscopy andultracentrifugation, forced scientists to re-evaluate the literal meaning oflife; virus heredity and reproducingnucleoprotein cell structures outside the nucleus ("plasmagenes") complicated the accepted Mendelian-chromosome theory.[78]
Oswald Avery showed in 1943 thatDNA was likely the genetic material of the chromosome, not its protein; the issue was settled decisively with the 1952Hershey–Chase experiment—one of many contributions from the so-calledphage group centered around physicist-turned-biologistMax Delbrück. In 1953James Watson andFrancis Crick, building on the work ofMaurice Wilkins andRosalind Franklin, suggested that the structure of DNA was a double helix. In their famous paper "Molecular structure of Nucleic Acids", Watson and Crick noted coyly, "It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material."[80] After the 1958Meselson–Stahl experiment confirmed thesemiconservative replication of DNA, it was clear to most biologists that nucleic acid sequence must somehow determineamino acid sequence in proteins; physicistGeorge Gamow proposed that a fixedgenetic code connected proteins and DNA. Between 1953 and 1961, there were few known biological sequences—either DNA or protein—but an abundance of proposed code systems, a situation made even more complicated by expanding knowledge of the intermediate role ofRNA. In 1961, it was demonstrated that when agene encodes aprotein, three sequential bases of a gene’sDNA specify each successive amino acid of the protein.[81] Thus thegenetic code is a triplet code, where each triplet (called a codon) specifies a particular amino acid. Furthermore, it was shown that the codons do not overlap with each other in the DNA sequence encoding a protein, and that each sequence is read from a fixed starting point. To actually decipher the code, it took an extensive series of experiments in biochemistry and bacterial genetics, between 1961 and 1966—most importantly the work ofNirenberg andKhorana.[82]During 1962–1964, numerous conditional lethal mutants of a bacterial virus were isolated.[83] These mutants were used in several different labs to advance fundamental understanding of the functions and interactions of the proteins employed in the machinery ofDNA replication,DNA repair,DNA recombination, and in the assembly of molecular structures.
In addition to the Division of Biology atCaltech, theLaboratory of Molecular Biology (and its precursors) atCambridge, and a handful of other institutions, thePasteur Institute became a major center for molecular biology research in the late 1950s.[84] Scientists at Cambridge, led byMax Perutz andJohn Kendrew, focused on the rapidly developing field ofstructural biology, combiningX-ray crystallography withMolecular modelling and the new computational possibilities ofdigital computing (benefiting both directly and indirectly from themilitary funding of science). A number of biochemists led byFrederick Sanger later joined the Cambridge lab, bringing together the study ofmacromolecular structure and function.[85] At the Pasteur Institute,François Jacob andJacques Monod followed the 1959PaJaMo experiment with a series of publications regarding thelacoperon that established the concept ofgene regulation and identified what came to be known asmessenger RNA.[86] By the mid-1960s, the intellectual core of molecular biology—a model for the molecular basis of metabolism and reproduction— was largely complete.[87]
The late 1950s to the early 1970s was a period of intense research and institutional expansion for molecular biology, which had only recently become a somewhat coherent discipline. In what organismic biologistE. O. Wilson called "The Molecular Wars", the methods and practitioners of molecular biology spread rapidly, often coming to dominate departments and even entire disciplines.[88] Molecularization was particularly important ingenetics,immunology,embryology, andneurobiology, while the idea that life is controlled by a "genetic program"—a metaphor Jacob and Monod introduced from the emerging fields ofcybernetics andcomputer science—became an influential perspective throughout biology.[89] Immunology in particular became linked with molecular biology, with innovation flowing both ways: theclonal selection theory developed byNiels Jerne andFrank Macfarlane Burnet in the mid-1950s helped shed light on the general mechanisms of protein synthesis.[90]
Resistance to the growing influence of molecular biology was especially evident inevolutionary biology.Protein sequencing had great potential for the quantitative study of evolution (through themolecular clock hypothesis), but leading evolutionary biologists questioned the relevance of molecular biology for answering the big questions of evolutionary causation. Departments and disciplines fractured as organismic biologists asserted their importance and independence:Theodosius Dobzhansky made the famous statement that "nothing in biology makes sense except in the light of evolution" as a response to the molecular challenge. The issue became even more critical after 1968;Motoo Kimura'sneutral theory of molecular evolution suggested thatnatural selection was not the ubiquitous cause of evolution, at least at the molecular level, and that molecular evolution might be a fundamentally different process frommorphological evolution. (Resolving this "molecular/morphological paradox" has been a central focus of molecular evolution research since the 1960s.)[91]
Biotechnology in the general sense has been an important part of biology since the late 19th century. With the industrialization ofbrewing andagriculture, chemists and biologists became aware of the great potential of human-controlled biological processes. In particular,fermentation proved a great boon to chemical industries. By the early 1970s, a wide range of biotechnologies were being developed, from drugs likepenicillin andsteroids to foods likeChlorella and single-cell protein togasohol—as well as a wide range ofhybrid high-yield crops and agricultural technologies, the basis for theGreen Revolution.[92]
Biotechnology in the modern sense ofgenetic engineering began in the 1970s, with the invention ofrecombinant DNA techniques.[93]Restriction enzymes were discovered and characterized in the late 1960s, following on the heels of the isolation, then duplication, then synthesis of viralgenes. Beginning with the lab ofPaul Berg in 1972 (aided byEcoRI fromHerbert Boyer's lab, building on work withligase byArthur Kornberg's lab), molecular biologists put these pieces together to produce the firsttransgenic organisms. Soon after, others began usingplasmidvectors and adding genes forantibiotic resistance, greatly increasing the reach of the recombinant techniques.[94]
Wary of the potential dangers (particularly the possibility of a prolific bacteria with a viral cancer-causing gene), the scientific community as well as a wide range of scientific outsiders reacted to these developments with both enthusiasm and fearful restraint. Prominent molecular biologists led by Berg suggested a temporary moratorium on recombinant DNA research until the dangers could be assessed and policies could be created. This moratorium was largely respected, until the participants in the 1975Asilomar Conference on Recombinant DNA created policy recommendations and concluded that the technology could be used safely.[95]
Following Asilomar, new genetic engineering techniques and applications developed rapidly.DNA sequencing methods improved greatly (pioneered byFrederick Sanger andWalter Gilbert), as didoligonucleotide synthesis andtransfection techniques.[96] Researchers learned to control the expression oftransgenes, and were soon racing—in both academic and industrial contexts—to create organisms capable of expressing human genes for the production of human hormones. However, this was a more daunting task than molecular biologists had expected; developments between 1977 and 1980 showed that, due to the phenomena of split genes andsplicing, higher organisms had a much more complex system ofgene expression than the bacteria models of earlier studies.[97] The first such race, for synthesizing humaninsulin, was won byGenentech. This marked the beginning of the biotech boom (and with it, the era ofgene patents), with an unprecedented level of overlap between biology, industry, and law.[98]
By the 1980s, protein sequencing had already transformed methods ofscientific classification of organisms (especiallycladistics) but biologists soon began to use RNA and DNA sequences ascharacters; this expanded the significance ofmolecular evolution within evolutionary biology, as the results ofmolecular systematics could be compared with traditional evolutionary trees based onmorphology. Following the pioneering ideas ofLynn Margulis onendosymbiotic theory, which holds that some of theorganelles ofeukaryotic cells originated from free livingprokaryotic organisms throughsymbiotic relationships, even the overall division of the tree of life was revised. Into the 1990s, the five domains (Plants, Animals, Fungi, Protists, and Monerans) became three (theArchaea, theBacteria, and theEukarya) based onCarl Woese's pioneeringmolecular systematics work with16S rRNA sequencing.[99]
The development and popularization of thepolymerase chain reaction (PCR) in mid-1980s (byKary Mullis and others atCetus Corp.) marked another watershed in the history of modern biotechnology, greatly increasing the ease and speed of genetic analysis.[100] Coupled with the use ofexpressed sequence tags, PCR led to the discovery of many more genes than could be found through traditional biochemical or genetic methods and opened the possibility of sequencing entire genomes.[101]
The unity of much of themorphogenesis of organisms from fertilized egg to adult began to be unraveled after the discovery of thehomeobox genes, first in fruit flies, then in other insects and animals, including humans. These developments led to advances in the field ofevolutionary developmental biology towards understanding how the variousbody plans of the animal phyla have evolved and how they are related to one another.[102]
TheHuman Genome Project—the largest, most costly single biological study ever undertaken—began in 1988 under the leadership ofJames D. Watson, after preliminary work with genetically simpler model organisms such asE. coli,S. cerevisiae andC. elegans.Shotgun sequencing and gene discovery methods pioneered byCraig Venter—and fueled by the financial promise of gene patents withCelera Genomics— led to a public–private sequencing competition that ended in compromise with the first draft of the human DNA sequence announced in 2000.[103]
At the beginning of the 21st century, biological sciences converged with previously differentiated new and classic disciplines likephysics into research fields likebiophysics. Advances were made inanalytical chemistry and physics instrumentation including improved sensors,optics, tracers, instrumentation, signal processing, networks,robots, satellites, and compute power for data collection, storage, analysis, modeling, visualization, and simulations. These technological advances allowed theoretical and experimental research including internet publication of molecularbiochemistry,biological systems, and ecosystems science. This enabled worldwide access to better measurements, theoretical models, complex simulations, theory predictive model experimentation, analysis, worldwide internet observationaldata reporting, open peer-review, collaboration, and internet publication. New fields of biological sciences research emerged includingbioinformatics,neuroscience,theoretical biology,computational genomics,astrobiology andsynthetic biology.
"As far as biology as a whole is concerned, it was not until the late eighteenth and early nineteenth century that the universities became centers of biological research."
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