Evolution of birds
The evolution of birds began in the Jurassic Period, with the earliest birds derived from a clade of theropoda dinosaurs named Paraves. Birds are categorized as Aves. For more than a century, the small theropod dinosaur Archaeopteryx lithographica from the Late Jurassic period was considered to have been the earliest bird. Modern phylogenies place birds in the dinosaur clade Theropoda. According to the current consensus, Aves and a sister group, the order Crocodilia, together are the sole living members of an unranked "reptile" clade, the Archosauria. Four distinct lineages of bird survived the Cretaceous–Paleogene extinction event 66 million years ago, giving rise to ostriches and relatives and relatives, ground-living fowl, "modern birds". Phylogenetically, Aves is defined as all descendants of the most recent common ancestor of a specific modern bird species, either Archaeopteryx, or some prehistoric species closer to Neornithes. If the latter classification is used the larger group is termed Avialae.
The relationship between dinosaurs and modern birds is still under debate. There is significant evidence that birds emerged within theropod dinosaurs that birds are members of Maniraptora, a group of theropods which includes dromaeosaurs and oviraptorids, among others; as more non-avian theropods that are related to birds are discovered, the clear distinction between non-birds and birds becomes less so. This was noted in the 19th century, with Thomas Huxley writing: We have had to stretch the definition of the class of birds so as to include birds with teeth and birds with paw-like fore limbs and long tails. There is no evidence. Discoveries in northeast China demonstrate that many small theropod dinosaurs did indeed have feathers, among them the compsognathid Sinosauropteryx and the microraptorian dromaeosaurid Sinornithosaurus; this has contributed to this ambiguity of where to draw the line between reptiles. Cryptovolans, a dromaeosaurid found in 2002 was capable of powered flight, possessed a sternal keel and had ribs with uncinate processes.
Cryptovolans seems to make a better "bird" than Archaeopteryx which lacks some of these modern bird features. Because of this, some paleontologists have suggested that dromaeosaurs are basal birds whose larger members are secondarily flightless, i.e. that dromaeosaurs evolved from birds and not the other way around. Evidence for this theory is inconclusive, but digs continue to unearth fossils of feathered dromaeosaurs. At any rate, it is certain that flight utilizing feathered wings existed in the mid-Jurassic theropods; the Cretaceous unenlagiine Rahonavis possesses features suggesting it was at least capable of powered flight. Although ornithischian dinosaurs share the same hip structure as birds, birds originated from the saurischian dinosaurs if the dinosaurian origin theory is correct, they thus arrived at their hip structure condition independently. In fact, a bird-like hip structure developed a third time among a peculiar group of theropods, the Therizinosauridae. An alternate theory to the dinosaurian origin of birds, espoused by a few scientists, notably Larry Martin and Alan Feduccia, states that birds evolved from early archosaurs like Longisquama.
This theory is contested by most other paleontologists and experts in feather development and evolution. The basal bird Archaeopteryx, from the Jurassic, is well known as one of the first "missing links" to be found in support of evolution in the late 19th century. Though it is not considered a direct ancestor of modern birds, it gives a fair representation of how flight evolved and how the first bird might have looked, it may be predated by Protoavis texensis, though the fragmentary nature of this fossil leaves it open to considerable doubt whether this was a bird ancestor. The skeleton of all early bird candidates is that of a small theropod dinosaur with long, clawed hands, though the exquisite preservation of the Solnhofen Plattenkalk shows Archaeopteryx was covered in feathers and had wings. While Archaeopteryx and its relatives may not have been good fliers, they would at least have been competent gliders, setting the stage for the evolution of life on the wing; the evolutionary trend among birds has been the reduction of anatomical elements to save weight.
The first element to disappear was the bony tail, being reduced to a pygostyle and the tail function taken over by feathers. Confuciusornis is an example of their trend. While keeping the clawed fingers for climbing, it had a pygostyle tail, though longer than in modern birds. A large group of birds, the Enantiornithes, evolved into ecological niches similar to those of modern birds and flourished throughout the Mesozoic. Though their wings resembled those of many modern bird groups, they retained the clawed wings and a snout with teeth rather than a beak in most forms; the loss of a long tail was followed by a rapid evolution of their legs which evolved to become versatile and adaptable tools that opened up new ecological niches. The Cretaceous saw the rise of more modern birds with a more rigid ribcage with a carina and shoulders able to allow for a powerful upstroke, essential to sustained powered flight. Another improve
Evolution of cetaceans
The evolutionary history of cetaceans is thought to have occurred in the Indian subcontinent from even-toed ungulates 50 million years ago, over a period of at least 15 million years. Cetaceans are aquatic marine mammals belonging to the order Artiodactyla, branched off from other artiodactyls around 50 mya. Cetaceans are thought to have evolved during the Eocene or earlier, sharing a closest common ancestor with hippopotamuses. Being mammals, they surface to breathe air. Discoveries starting in the late 1970s in Pakistan revealed several stages in the transition of cetaceans from land to sea; the two modern parvorders of cetaceans – Mysticeti and Odontoceti – are thought to have separated from each other around 28-33 million years ago in a second cetacean radiation, the first occurring with the archaeocetes. The adaptation of animal echolocation in toothed whales distinguishes them from aquatic archaeocetes and early baleen whales; the presence of baleen in baleen whales occurred with earlier varieties having little baleen, their size is linked to baleen dependence.
The aquatic lifestyle of cetaceans first began in the Indian subcontinent from even-toed ungulates 50 million years ago, over a period of at least 15 million years, however a jawbone discovered in Antarctica may reduce this to 5 million years. Archaeoceti is an extinct parvorder of Cetacea containing ancient whales; the traditional hypothesis of cetacean evolution, first proposed by Van Valen in 1966, was that whales were related to the mesonychids, an extinct order of carnivorous ungulates that resembled wolves with hooves and were a sister group of the artiodactyls. This hypothesis was proposed due to similarities between the unusual triangular teeth of the mesonychids and those of early whales. However, molecular phylogeny data indicates that whales are closely related to the artiodactyls, with hippopotamuses as their closest living relative; because of this and hippopotamuses are placed in the same suborder, Whippomorpha. Cetartiodactyla is a proposed name for an order containing both artiodactyls.
However, the earliest anthracotheres, the ancestors of hippos, do not appear in the fossil record until the Middle Eocene, millions of years after Pakicetus, the first known whale ancestor, appeared during the Early Eocene, implying the two groups diverged well before the Eocene. Since molecular analysis identifies artiodactyls as being closely related to cetaceans, mesonychids are an offshoot from Artiodactyla, cetaceans did not derive directly from them, but that the two groups may share a common ancestor; the molecular data are supported by the discovery of the earliest archaeocete. The skeletons of Pakicetus show. Instead, they are artiodactyls that began to take to the water soon after artiodactyls split from mesonychids. Archaeocetes retained aspects of their mesonychid ancestry which modern artiodactyls, modern whales, have lost; the earliest ancestors of all hoofed mammals were at least carnivorous or scavengers, today's artiodactyls and perissodactyls became herbivores in their evolution.
Whales, retained their carnivorous diet because prey was more available and they needed higher caloric content in order to live as marine endotherms. Mesonychids became specialized carnivores, but this was a disadvantage because large prey was uncommon; this may be why they were out-competed by better-adapted animals like the hyaenodontids and Carnivora. Indohyus was a small chevrotain-like animal that lived about 48 million years ago in what is now Kashmir, it belongs to the artiodactyl family Raoellidae, is believed to be the closest sister group of Cetacea. Indohyus is identified as an artiodactyl because it has two trochlea hinges, a trait unique to artiodactyls; the size of a raccoon or domestic cat, this omnivorous creature shared some traits of modern whales, most notably the involucrum, a bone growth pattern, the diagnostic characteristic of any cetacean. It showed signs of adaptations to aquatic life, including dense limb bones that reduce buoyancy so that they could stay underwater, which are similar to the adaptations found in modern aquatic mammals such as the hippopotamus.
This suggests a similar survival strategy to the African mousedeer or water chevrotain which, when threatened by a bird of prey, dives into water and hides beneath the surface for up to four minutes. The pakicetids were digitigrade hoofed mammals that are thought to be the earliest known cetaceans, with Indohyus being the closest sister group, they lived in the early Eocene, around 50 million years ago. Their fossils were first discovered in North Pakistan in 1979, located at a river not far from the shores of the former Tethys Sea. After the initial discovery, more fossils were found in the early Eocene fluvial deposits in northern Pakistan and northwestern India. Based on this discovery, pakicetids most lived in an arid environment with ephemeral streams and moderately developed floodplains millions of years ago. By using stable oxygen isotopes analysis, they were shown to drink fresh water, implying that they lived around freshwater bodies, their diet included land animals that approached water for drinking or some freshwater aquatic organi
Evolution of fish
The evolution of fish began about 530 million years ago during the Cambrian explosion. It was during this time that the early chordates developed the skull and the vertebral column, leading to the first craniates and vertebrates; the first fish lineages belong to the Agnatha, or jawless fish. Early examples include Haikouichthys. During the late Cambrian, eel-like jawless fish called the conodonts, small armoured fish known as ostracoderms, first appeared. Most jawless fish are now extinct. Lampreys belong to the Cyclostomata, which includes the extant hagfish, this group may have split early on from other agnathans; the earliest jawed vertebrates developed during the late Ordovician period. They are first represented in the fossil record from the Silurian by two groups of fish: the armoured fish known as placoderms, which evolved from the ostracoderms; the jawed fish that are still extant in modern days appeared during the late Silurian: the Chondrichthyes and the Osteichthyes. The bony fish evolved into two separate groups: the Actinopterygii and Sarcopterygii.
During the Devonian period a great increase in fish variety occurred among the ostracoderms and placoderms, among the lobe-finned fish and early sharks. This has led to the Devonian being known as the age of fishes, it was from the lobe-finned fish that the tetrapods evolved, the four-limbed vertebrates, represented today by amphibians, reptiles and birds. Transitional tetrapods first appeared during the early Devonian, by the late Devonian the first tetrapods appeared; the diversity of jawed vertebrates may indicate the evolutionary advantage of a jawed mouth. Fish do not represent a paraphyletic one, as they exclude the tetrapods. Fish, like many other organisms, have been affected by extinction events throughout natural history; the Ordovician–Silurian extinction events led to the loss of many species. The late Devonian extinction led to the extinction of the ostracoderms and placoderms by the end of the Devonian, as well as other fish; the spiny sharks became extinct at the Permian–Triassic extinction event.
The Cretaceous–Paleogene extinction event, the present day Holocene extinction, have affected fish variety and fish stocks. Fish may have evolved from an animal similar to a coral-like sea squirt, whose larvae resemble early fish in important ways; the first ancestors of fish may have kept the larval form into adulthood, although this path cannot be proven. Vertebrates, among them the first fishes, originated about 530 million years ago during the Cambrian explosion, which saw the rise in organism diversity; the first ancestors of fish, or animals that were closely related to fish, were Pikaia and Myllokunmingia. These three genera all appeared around 530 Ma. Pikaia had a primitive notochord, a structure that could have developed into a vertebral column later. Unlike the other fauna that dominated the Cambrian, these groups had the basic vertebrate body plan: a notochord, rudimentary vertebrae, a well-defined head and tail. All of these early vertebrates lacked jaws in the common sense and relied on filter feeding close to the seabed.
These were followed by indisputable fossil vertebrates in the form of armoured fishes discovered in rocks from the Ordovician Period 500–430 Ma. The first jawed vertebrates appeared in the late Ordovician and became common in the Devonian known as the "Age of Fishes"; the two groups of bony fishes, the actinopterygii and sarcopterygii and became common. The Devonian saw the demise of all jawless fishes, save for lampreys and hagfish, as well as the Placodermi, a group of armoured fish that dominated much of the late Silurian; the Devonian saw the rise of the first labyrinthodonts, a transitional between fishes and amphibians. The colonisation of new niches resulted in diversification of body plans and sometimes an increase in size; the Devonian Period brought in such giants as the placoderm Dunkleosteus, which could grow up to seven meters long, early air-breathing fish that could remain on land for extended periods. Among this latter group were ancestral amphibians; the reptiles appeared from labyrinthodonts in the subsequent Carboniferous period.
The anapsid and synapsid reptiles were common during the late Paleozoic, while the diapsids became dominant during the Mesozoic. In the sea, the bony fishes became dominant; the radiations, such as those of fish in the Silurian and Devonian periods, involved fewer taxa with similar body plans. The first animals to venture onto dry land were arthropods; some fish could crawl onto the land also. Jawless fishes belong to the superclass Agnatha in subphylum Vertebrata. Agnatha comes from the Greek, means "no jaws", it excludes all vertebrates with jaws, known as gnathostomes. Although a minor element of modern marine fauna, jawless fish were prominent among the early fish in the early Paleozoic. Two types of Early Cambrian animal having fins, vertebrate musculature, gills are known from the early Cambrian Maotianshan shales of China: Haikouichthys and Myllokunmingia, they have been tentatively assigned to Agnatha by Janvier. A third possible agnathid from the same region is Haikouella. A possible agnathid that has not
Evolution of cephalopods
The cephalopods have a long geological history, with the first nautiloids found in late Cambrian strata, purported stem-group representatives present in the earliest Cambrian lagerstätten. The class developed during the middle Cambrian, underwent pulses of diversification during the Ordovician period to become diverse and dominant in the Paleozoic and Mesozoic seas. Small shelly fossils such as Tommotia were once interpreted as early cephalopods, but today these tiny fossils are recognized as sclerites of larger animals, the earliest accepted cephalopods date to the Middle Cambrian Period. During the Cambrian, cephalopods are most common in shallow near-shore environments, but they have been found in deeper waters too. Cephalopods were thought to have "undoubtedly" arisen from within the tryblidiid monoplacophoran clade; however genetic studies suggest that they are more basal, forming a sister group to the Scaphopoda but otherwise basal to all other major mollusc classes. The internal phylogeny of Mollusca, however, is wide open to interpretation – see mollusc phylogeny.
The cephalopods were once thought to have evolved from a monoplacophoran-like ancestor with a curved, tapering shell, to be related to the gastropods. The similarity of the early shelled cephalopod Plectronoceras to some gastropods was used to support this view; the development of a siphuncle would have allowed the shells of these early forms to become gas-filled in order to support them and keep the shells upright while the animal crawled along the floor, separated the true cephalopods from putative ancestors such as Knightoconus, which lacked a siphuncle. Negative buoyancy would have come followed by swimming in the Plectronocerida and jet propulsion in more derived cephalopods. However, because chambered shells are found in a range of molluscs – monoplacophorans and gastropods as well as cephalopods – a siphuncle is essential to ally a fossil shell conclusively to the cephalopoda. Chambered gastropods can be distinguished from cephalopod shells by the absence of a siphuncle, the irregular spacing of septa, the layering of the shell and its microstructure, the thick width of the shell.
The earliest such shells do not have the muscle scars which would be expected if they had a monoplacophoran affinity. Additionally, the discovery that Nectocaris pteryx, which did not have a shell and appeared to possess jet propulsion in the manner of "derived" cephalopods, complicated the question of the order in which cephalopod features developed – provided it is in fact a cephalopod and not an arthropod. Understanding of early cephalopod origins is by necessity biased by the available fossil material, which on the whole consists of shelly fossils. Critical fossils are detailed below. With the exception of the shelly genera Ectenolites and Eoclarkoceras, none of the 30+ Cambrian cephalopod genera are known to have survived into the Ordovician. Cambrian cephalopods differ from their descendants by account of their small size. Tannuella is the oldest fossil to have been assigned to the cephalopods, dating from the Early Cambrian, ~522 million years ago, its position in this group is suggested based on the presence of chambers.
Under this hypothesis, it would be a precursor to the hypseloconids and genera such as Knightoconus that gave rise to the cephalopods. Knightoconus is a Late Cambrian monoplacophoran thought to represent an ancestor to the cephalopods, it lacked a siphuncle. Although earlier molluscan fossils are septate, Knightoconus is the latest septate mollusc before the first sipunculate cephalopods – a point, taken to prove its relevance to the Cephalopoda; the absence of this siphuncle has been taken as evidence against cephalopod ancestry – how, it is argued, could a siphuncle evolve to penetrate existing septa? The prevailing argument suggests that a strand of tissue remained attached to the previous septum as the mollusc moved forwards and deposited its next septum, producing an obstacle to the complete closure of the septum and becoming mineralised itself. 10 or more septa are found in mature individuals, occupying around a third of the shell – septa form early and have been found in specimens as small as 2 mm in length.
Septa are uniformly spaced, inconsistent with a gastropod affinity. Unlike monoplacophoran fossils, there is no evidence of muscle scarring in Knightoconus fossils. Plectronoceras is arguably the earliest known crown-group cephalopod, its 14 known specimens hail from the basal Fengshan Formation of the earliest Fengshanian stage. None of the fossils are complete, none show the tip or opening of the shell. Half of its shell was filled with septa, its shell contains transverse septa separated by about half a millimetre, with a siphuncle on its concave side. Its morphology matches to that hypothesised for the last common ancestor of all cephalopods, the Plectronocerida have been said to be the ancestors of the Ellesmerocerids, the first "true cephalopods"; the Yochelcionellids have given rise to the "snorkel hypothesis". These fossils are aseptate helcionellids with a snorkel-like tube on one surface; the snorkel has been seized upon as characteristic of a ce
Palynology is the "study of dust" or of "particles that are strewn". A classic palynologist analyses particulate samples collected from the air, from water, or from deposits including sediments of any age; the condition and identification of those particles and inorganic, give the palynologist clues to the life and energetic conditions that produced them. The term is sometimes narrowly used to refer to a subset of the discipline, defined as "the study of microscopic objects of macromolecular organic composition, not capable of dissolution in hydrochloric or hydrofluoric acids", it is the science that studies contemporary and fossil palynomorphs, including pollen, orbicules, acritarchs and scolecodonts, together with particulate organic matter and kerogen found in sedimentary rocks and sediments. Palynology does not include diatoms, foraminiferans or other organisms with siliceous or calcareous exoskeletons. Palynology as an interdisciplinary science stands at the intersection of earth science and biological science plant science.
Stratigraphical palynology, a branch of micropalaeontology and paleobotany, studies fossil palynomorphs from the Precambrian to the Holocene. The earliest reported observations of pollen under a microscope are to have been in the 1640s by the English botanist Nehemiah Grew, who described pollen and the stamen, concluded that pollen is required for sexual reproduction in flowering plants. By the late 1870s, as optical microscopes improved and the principles of stratigraphy were worked out, Robert Kidston and P. Reinsch were able to examine the presence of fossil spores in the Devonian and Carboniferous coal seams and make comparisons between the living spores and the ancient fossil spores. Early investigators include Gideon Mantell and Henry Hopley White. Quantitative analysis of pollen began with Lennart von Post's published work. Although he published in the Swedish language, his methodology gained a wide audience through his lectures. In particular, his Kristiania lecture of 1916 was important in gaining a wider audience.
Because the early investigations were published in the Nordic languages, the field of pollen analysis was confined to those countries. The isolation ended with the German publication of Gunnar Erdtman's 1921 thesis; the methodology of pollen analysis became widespread throughout Europe and North America and revolutionized Quaternary vegetation and climate change research. Earlier pollen researchers include Früh, who enumerated many common tree pollen types, a considerable number of spores and herb pollen grains. There is a study of pollen samples taken from sediments of Swedish lakes by Trybom. Georg F. L. Sarauw studied fossil pollen of middle Pleistocene age from the harbour of Copenhagen. Lagerheim and C. A. Weber appear to be among the first to undertake'percentage frequency' calculations; the term palynology was introduced by Hyde and Williams in 1944, following correspondence with the Swedish geologist Ernst Antevs, in the pages of the Pollen Analysis Circular. Hyde and Williams chose palynology on the basis of the Greek words paluno meaning'to sprinkle' and pale meaning'dust'.
Pollen analysis in North America stemmed from Phyllis Draper, an MS student under Sears at the University of Oklahoma. During her time as a student, she developed the first pollen diagram from a sample that depicted the percentage of several species at different depths at Curtis Bog; this was the introduction of pollen analysis in North America. Pollen analysis advanced in this period due to advances in optics and computers. Much of the science was revised by Knut Fægri in their textbook on the subject. Palynomorphs are broadly defined as organic-walled microfossils between 5 and 500 micrometres in size, they are extracted from sedimentary rocks and sediment cores both physically, by ultrasonic treatment and wet sieving, chemically, by chemical digestion to remove the non-organic fraction. Palynomorphs may be composed of organic material such as chitin and sporopollenin. Palynomorphs that have a taxonomy description are sometimes referred to as palynotaxa. Palynomorphs form a geological record of importance in determining the type of prehistoric life that existed at the time the sedimentary formation was laid down.
As a result, these microfossils give important clues to the prevailing climatic conditions of the time. Their paleontological utility derives from an abundance numbering in millions of cells per gram in organic marine deposits when such deposits are not fossiliferous. Palynomorphs, however have been destroyed in metamorphic or recrystallized rocks. Palynomorphs are dinoflagellate cysts, spores, fungi, arthropod organs and microforams. Palynomorph microscopic structures that are abundant in most sediments are resistant to routine pollen extraction including stron
Evolution of spiders
The evolution of spiders has been going on for at least 380 million years, since the first true spiders evolved from crab-like chelicerate ancestors. More than 45,000 extant species have been described, organised taxonomically in 3,958 genera and 114 families. There may be more than 120,000 species. Fossil diversity rates make up a larger proportion than extant diversity would suggest with 1,593 arachnid species described out of 1,952 recognized chelicerates. Both extant and fossil species are described yearly by researchers in the field. Major developments in spider evolution include the development of spinnerets and silk secretion. Among the oldest known land arthropods are Trigonotarbids, members of an extinct order of spider-like arachnids. Trigonotarbids share many superficial characteristics with spiders, including a terrestrial lifestyle, respiration through book lungs, walking on eight legs, with a pair of leg-like pedipalps near the mouth and mouth parts. Arguments still remain open as to.
This had been a popular thought for quite some time, until an unpublished fossil was described with distinct microtubercles on its hind legs, akin to those used by spiders to direct and manipulate their silk. Trigonotarbids are not true spiders, most Trigonotarbid species have no living descendants today. One lineage, led to the earliest tetrapulmonates, which evolved into spiders, whip scorpions, close relatives. At one stage the oldest fossil spider was believed to be Attercopus which lived 380 million years ago during the Devonian. Attercopus was placed as the sister-taxon to all living spiders, but has now been reinterpreted as a member of a separate, extinct order Uraraneida which could produce silk, but did not have true spinnerets; the oldest true spiders date to about 300 million years ago. Most of these early segmented fossil spiders from the Coal Measures of Europe and North America belonged to the Mesothelae, or something similar, a group of primitive spiders with the spinnerets placed underneath the middle of the abdomen, rather than at the end as in modern spiders.
They were ground-dwelling predators, living in the giant clubmoss and fern forests of the mid-late Palaeozoic, where they were predators of other primitive arthropods. Silk may have been used as a protective covering for the eggs, a lining for a retreat hole, perhaps for simple ground sheet web and trapdoor construction; as plant and insect life diversified so did the spider's use of silk. Spiders with spinnerets at the end of the abdomen appeared more than 250 million years ago promoting the development of more elaborate sheet and maze webs for prey capture both on ground and foliage, as well as the development of the safety dragline; the oldest mygalomorph, was described from the Triassic of France and belongs to the modern family Hexathelidae. Megarachne servinei from the Permo-Carboniferous was once thought to be a giant mygalomorph spider and, with its body length of 1 foot and leg span of above 20 inches, the largest known spider to have lived on Earth, but subsequent examination by an expert revealed that it was a small sea scorpion.
By the Jurassic, the sophisticated aerial webs of the orb-weaver spiders had developed to take advantage of the diversifying groups of insects. A spider web preserved in amber, thought to be 110 million years old, shows evidence of a perfect "orb" web, the most famous, circular kind one thinks of when imagining spider webs. An examination of the drift of those genes thought to be used to produce the web-spinning behavior suggests that orb spinning was in an advanced state as many as 136 million years ago. One of these, the araneid Mongolarachne jurassica, from about 165 million years ago, recorded from Daohuogo, Inner Mongolia in China, is the largest known fossil spider; the 110-million-year-old amber-preserved web is the oldest to show trapped insects, containing a beetle, a mite, a wasp's leg, a fly. The ability to weave orb webs is thought to have been "lost", sometimes re-evolved or evolved separately, in different breeds of spiders since its first appearance. Spider taxonomy Insect evolution Brunetta, Leslie.
Spider silk: evolution and 400 million years of spinning, waiting and mating. New Haven: Yale University Press. ISBN 978-0-300-14922-7. Penney, D.. Dominican Amber Spiders: a comparative neontological approach to identification faunistics ecology and biogeography. Manchester: Siri Scientific Press. ISBN 978-0-9558636-0-8. Penney, D.. A.. Fossil Spiders: the evolutionary history of a mega-diverse order. Manchester: Siri Scientific Press. ISBN 978-0-9558636-5-3. Picture of spider fossil Dunlop, J. A. Penney, D. & Jekel, D.. A summary list of fossil spiders and their relatives. World Spider Catalog. Natural History Museum Bern, online at http://wsc.nmbe.ch, version 16.5
Abiogenesis, or informally the origin of life, is the natural process by which life has arisen from non-living matter, such as simple organic compounds. While the details of this process are still unknown, the prevailing scientific hypothesis is that the transition from non-living to living entities was not a single event, but a gradual process of increasing complexity that involved molecular self-replication, self-assembly and the emergence of cell membranes. Although the occurrence of abiogenesis is uncontroversial among scientists, there is no single accepted model for the origin of life, this article presents several principles and hypotheses for how abiogenesis could have occurred. Researchers study abiogenesis through a combination of molecular biology, astrobiology, biophysics and biochemistry, aim to determine how pre-life chemical reactions gave rise to life; the study of abiogenesis can be geophysical, chemical, or biological, with more recent approaches attempting a synthesis of all three, as life arose under conditions that are strikingly different from those on Earth today.
Life functions through the specialized chemistry of carbon and water and builds upon four key families of chemicals: lipids, amino acids, nucleic acids. Any successful theory of abiogenesis must explain the origins and interactions of these classes of molecules. Many approaches to abiogenesis investigate how self-replicating molecules, or their components, came into existence. Researchers think that current life on Earth descends from an RNA world, although RNA-based life may not have been the first life to have existed; the classic 1952 Miller–Urey experiment and similar research demonstrated that most amino acids, the chemical constituents of the proteins used in all living organisms, can be synthesized from inorganic compounds under conditions intended to replicate those of the early Earth. Scientists have proposed various external sources of energy that may have triggered these reactions, including lightning and radiation. Other approaches focus on understanding how catalysis in chemical systems on the early Earth might have provided the precursor molecules necessary for self-replication.
Complex organic molecules occur in the Solar System and in interstellar space, these molecules may have provided starting material for the development of life on Earth. The biochemistry of life may have begun shortly after the Big Bang, 13.8 billion years ago, during a habitable epoch when the age of the universe was only 10 to 17 million years. The panspermia hypothesis suggests that microscopic life was distributed to the early Earth by space dust, meteoroids and other small Solar System bodies and that life may exist throughout the universe; the panspermia hypothesis proposes that life originated outside the Earth, but does not definitively explain its origin. Earth remains the only place in the universe known to harbour life, fossil evidence from the Earth informs most studies of abiogenesis; the age of the Earth is about 4.54 billion years. In May 2017 scientists found possible evidence of early life on land in 3.48-billion-year-old geyserite and other related mineral deposits uncovered in the Pilbara Craton of Western Australia.
However, a number of discoveries suggest that life may have appeared on Earth earlier. As of 2017, microfossils, or fossilised microorganisms, within hydrothermal-vent precipitates dated from 3.77 to 4.28 billion years old found in Quebec, Canadian rocks may harbour the oldest record of life on Earth, suggesting life started soon after ocean formation 4.4 billion years ago. According to biologist Stephen Blair Hedges, "If life arose quickly on Earth … it could be common in the universe." The Hadean Earth is thought to have had a secondary atmosphere, formed through degassing of the rocks that accumulated from planetesimal impactors. At first, it was thought that the Earth's atmosphere consisted of hydrogen compounds—methane and water vapour—and that life began under such reducing conditions, which are conducive to the formation of organic molecules. According to models, suggested by study of ancient minerals, the atmosphere in the late Hadean period consisted of water vapour and carbon dioxide, with smaller amounts of carbon monoxide and sulfur compounds.
During its formation, the Earth lost a significant part of its initial mass, with a nucleus of the heavier rocky elements of the protoplanetary disk remaining. As a consequence, Earth lacked the gravity to hold any molecular hydrogen in its atmosphere, lost it during the Hadean period, along with the bulk of the original inert gases; the solution of carbon dioxide in water is thought to have made the seas acidic, giving it a pH of about 5.5. The atmosphere at the time has been characterized as a "gigantic, productive outdoor chemical laboratory." It may have been similar to the mixture of gases released today by volcanoes, which still support some abiotic chemistry. Oceans may have appeared first in the Hadean Eon, as soon as two hundred million years after the Earth was formed, in a hot 100 °C reducing environment, the pH of about 5.8 rose towards neutral. This has been supported by the dating of 4.404 Ga-old zircon crystals from metamorphosed quartzite of Mount Narryer in the W