Body fossils and trace fossils are the principal types of evidence about ancient life, and geochemical evidence has helped to decipher the evolution of life before there were organisns large enogh to leave fossils. Estimating the dates of these remains is essential but difficult: sometimes adjacent rock layers allow radiometric dating, which provide absolute dates that are accurate to within 0.5%, but more often paleontologists have to rely on relative dating by solving the "jigsaw puzzles" of biostratigraphy. Classifying ancient organisms is also difficult, as many do not fit well into the Linnean taxonomy that is commonly used for classifying living organisms, and paleontologists more often use cladistics to draw up evolutionary "family trees". The final quarter of the 20th century saw the develpoment of molecular phylogenetics, which investigates evolutionary "family trees" by applying the analystic techniques of cladistics to aspects of organisms's biochemistry, mainy DNA and RNA. These techniques have also been used to estimate the dates at which branches of "family trees" diverged, but there is controversy about the reliability of the molecular clock on which such estimates depend.
Use of all these techniques has enabled paleontologists to discover much of the evolutionary history of life, almost all the way back to when Earth became capable of supporting life, about . For about half of that time the only life was single-celled micro-organisms, mostly in microbial mats that formed ecosystems only a few millimeters thick. Earth's atmosphere originally contained virtually no oxygen, and its oxygenation began about . This caused an accelerating increase in the diversity and complexity of life, and early multicellular plants and fungi have been found in rocks dated from . The earliest multicellular animal fossils are much later, from about , but animals diversified very rapidly and there is a lively debate about whether most of this happened in a relatively short Cambrian explosion or started earlier but has been hidden by lack of fossils. All of these organisms lived in water, but plants and invertebrates started colonizing land from about and vertebrates followed them about . The first dinosaurs appeared about and birds evolved from one dinosaur group about . During the time of the dinosaurs, mammals' ancestors could survive only as small, mainly nocturnal insectivores, but after the dinosaurs became extinct in the Cretaceous–Tertiary extinction event mammals diversified rapidly. Flowering plants appeared and rapidly diversified between 130 million years ago and 90 million years ago, possibly helped by coevolution with pollinating insects. Social insects appeared around the same time and, although they have relatively few species, now form over 50% of the total mass of all insects. Humans evolved from a lineage of upright-walking apes that appeared , and anatomically modern humans appeared under 200,000 years ago. The course of evolution has been changed several times by mass extinctions that wiped out previously dominant groups and allowed other to rise from obscurity to become major components of ecosystems.
The simplest definition is "the study of ancient life" Paleontology seeks information about several aspects of past organisms: "their identity and origin, their environment and evolution, and what they can tell us about the Earth's organic and inorganic past".
When trying to explain past phenomena, paleontologists and other historical scientists often construct a set of hypotheses about the causes and then look for a "smoking gun", a piece of evidence which indicates that one of the hypotheses is a better explanation than the others. Sometimes the "smoking gun" is discovered by a fortunate accident during other research, for example the discovery by Luis Alvarez and Walter Alvarez of an iridium-rich layer at the Cretaceous-Tertiary boundary made asteroid impact and volcanism the most favored explanations for the Cretaceous–Tertiary extinction event.
The other main type of science is experimental science, which is often said to work by conducting experiments to disprove hypotheses about the workings and causes of natural phenomena – note that this approach cannot prove a hypothesis is correct, since some later experiment may disprove it. However when confronted with totally unexpected phenomena, such as the first evidence for invisible radiation, experimental scientists often use the same approach as historical scientists: construct a set of hypotheses about the causes and then look for a "smoking gun".
Paleontology also has some overlap with archaeology, which primarily works with objects made by humans and with human remains, while paleontologists are interested in the characteristics and evolution of humans as organisms. When dealing with evidence about humans, archaeologists and paleontologists may work together – for example paleontologists might identify animal or plant fossils around an archaeological site, to discover what the people who lived there ate; or they might analyze the climate at the time when site was inhabited by humans.
In addition paleontology often uses techniques derived from other sciences, including biology, ecology, chemistry, physics and mathematics. For example geochemical signatures from rocks may help to discover when life first arose on Earth, and analyses of carbon isotope ratios may help to identify climate changes and even to explain major transitions such as the Permian–Triassic extinction event. A relatively recent discipline, molecular phylogenetics, often helps by using comparisons of different modern organisms' DNA and RNA to re-construct evolutionary "family trees"; it has also been used to estimate the dates of important evolutionary developments, although this approach is controversial because of doubts about the reliability of the "molecular clock". Techniques developed in engineering have been used to analyse how ancient organisms might have worked, for example how fast Tyrannosaurus could move and how powerful its bite was.
Paleontology even contributes to astrobiology, the investigation of possible life on other planets, by developing models of how life may have arisen and by providing techniques for detecting evidence of life.
Instead of focusing on individual organisms, paleoecology examines the interactions between different organisms, for example their places in food chains, and the two-way interaction between organisms and their environment – for example the development of oxygenic photosynthesis by bacteria hugely increased the productivity and diversity of ecosystems, and also caused the oxygenation of the atmosphere, which in turn was a prerequisite for the evolution of the most complex eucaryotic cells, from which all multicellular organisms are built. Paleoclimatology, although sometimes treated as part of paleoecology, focuses more on the history of Earth's climate and the mechanisms which have changed it – which have sometimes included evolutionary developments, for example the rapid expansion of land plants in the Devonian period removed more carbon dioxide from the atmosphere, reducing the greenhouse effect and thus causing an ice age in the Carboniferous period.
Biostratigraphy, the use of fossils to work out the chronological order in which rocks were formed, is useful to both paleontologists and geologists. Biogeography studies the spatial distribution of organisms, and is also linked to geology, which explains how Earth's geography has changed over time. > > > > > > > > > >
Fossils of organisms' bodies are usually the most informative type of evidence. The most common types are wood, bones, and shells. Fossilisation is a rare event, and most fossils are destroyed by erosion or metamorphism before they can be observed. Hence the fossil record is very incomplete, increasingly so further back in time. Despite this, they are often adequate to illustrate the broader patterns of life's history. There are also biases in the fossil record: different environments are more favourable to the preservation of different types of organism or parts of organisms. Further, only the parts of organisms that were already mineralised are usually preserved, such as the shells of molluscs. Since most animal species are soft-bodied, they decay before they can become fossilised. As a result, although there are 30-plus phyla of living animals, two-thirds have never been found as fossils.
Occasionally, unusual environments may preserve soft tissues. These lagerstätten allow paleontologists to examine the internal anatomy of animals which in other sediments are only represented by shells, spines, claws, etc – if they are preserved at all. However even lagerstätten present an incomplete picture of life at the time. The majority of organisms living at the time are probably not represented because lagerstätten are restricted to a narrow range of environments, e.g. where soft-bodied organisms can be preserved very quickly by events such as mudslides; and the exceptional events that cause quick burial make it difficult to study the normal environments of the animals.
The sparseness of the fossil record means that organisms are expected to exist long before and after they are found in the fossil record - this is known as the Signor-Lipps effect.
Trace fossils consist mainly of tracks and burrows, but also include coprolites (fossil feces) Trace fossils are particularly significant because they represent a data source that is not limited to animals with easily-fossilized hard parts, and which reflects organisms' behaviour. Also many traces date from significantly earlier than the body fossils of animals that are thought to have been capable of making them. Whilst exact assignment of trace fossils to their makers is generally impossible, traces may for example provide the earliest physical evidence of the appearance of moderately complex animals (comparable to earthworms).
Naming groups of organisms in a way that is clear and widely agreed is important, as some apparent scientific disputes have arisen from misunderstandings over names. Linnean taxonomy is commonly used for classifying living organisms, but runs into difficulties when dealing with newly-discovered organisms that are significantly different from known ones, for example: it is hard to decide at what level to place a higher-level grouping, e.g. genus or family or order; the Linnean rules for naming groups are tied to their levels, and hence if a group is moved to a different level it has to be renamed.
Paleontologists generally use approaches based on cladistics, a technique for working out the evolutionary "family tree" of a set of organisms. It works by the logic that, if groups B and C have more similarities to each other than either has to group A, then B and C are more closely related to each other than either is to A. Characters which are compared may be anatomical, such as the presence of a notochord, or molecular, by comparing sequences of DNA or protein. The result of a successful analysis is a hierarchy of clades – groups whose members are believed to share a common ancestor. Ideally the "family tree" has only two branches leading from each node ("junction"), but sometimes there is insufficient information to achieve this and paleontologists have to make do with junctions that have several branches. The cladistic technique is sometimes fallible, as some features, such as wings or camera eyes, evolved more than once, convergently – this must be taken into account in analyses.
Evolutionary developmental biology, commonly abbreviated to "Evo Devo", also helps paleontologists to produce "family trees", for example the embryological development of some modern brachiopods suggests that brachiopods may be descendants of the halkieriids, which became extinct in the Cambrian period.
Paleontology seeks to map out how the character of life has changed through time. A substantial hurdle to this aim is the difficulty of working out how old fossils are. Beds which preserve fossils typically lack radioactive elements on which radiometric dating techniques can be used. This technique is our only means of giving rocks greater than about 50 million years old an absolute age, and can be very accurate - to within 0.5% or better. Although radiometric dating requires very careful laboratory work, its basic principle is simple: the rates at which various radioactive elements decay are known, and so the ratio of the radioactive element to the element into which it decays can be used to calculate how long has passed since the radioactive element was incorporated into the rock. Radioactive elements are only common in rocks with a volcanic origin; in rare cases ash layers may contain fossils.
In general, palaeontologists must rely on stratigraphy to date fossils. Stratigraphy is the science of deciphering the "layer-cake" that is the sedimentary record, and has been compared to a jigsaw puzzle. Because rocks are laid down by deposition, they form relatively flat-lying layers, with each layer younger than the one underneath it. If a fossil is found between two layers whose ages are known, the age of the fossil can be constrained to between the two known ages. Because rock sequences are not continuous, but may be broken up by faults or periods of erosion, it is very difficult to correlate between rock beds which are not directly next to one another. Fossils of species with a short lifespan can be used to link up isolated rocks. For instance, the conodont Eoplacognathus pseudoplanus has a short range in the Middle Ordovician period. If rocks of unknown age are found to have traces of E. pseudoplanus, they must have a mid-Ordovician age. There are some difficulties with biostratigraphy, though - chosen index fossils must be distinctive, globally distributed and have a short life span to be useful. They may also turn out to have longer fossil ranges than first thought. Stratigraphy and biostratigraphy can in general provide only relative dating (A was before B), which is often sufficient for studying processes of evolution. However this is difficult for some time periods, because of the problems involved in matching up rocks of the same age across different continents.
Family tree relationships may also help to narrow down the date on which lineages first appeared. For instance, if fossils of B or C date to X million years ago and the calculated "family tree" says A was an ancestor of B and C, then A must have evolved more than X million years ago.
It is also possible to estimate how long ago two living clades diverged – i.e. approximately how long ago their last common ancestor must have lived – by assuming that DNA mutations accumulate at a constant rate. These "molecular clocks", however, are fallible, and provide only a very approximate timing: for example they are not sufficiently precise and reliable for estimating when the groups that feature in the Cambrian explosion first evolved, and estimates produced by different techniques vary by a factor of two.
For about 2,000 million years microbial mats, multi-layered colonies of different types of bacteria, were the dominant life on Earth. The evolution of oxygenic photosynthesis enabled them to play the major role in the oxygenation of the atmosphere,. for which there is geological evidence from about , and increased their effectiveness as nurseries of evolution. While eucaryotes, cells with complex internal structures, may have been present earlier, their evolution speeded up when they acquired the ability to transform oxygen from a poison to a powerful "fuel" for their metabolisms, a development that may have started with their capturing oxygen-powered bacteria as endosymbionts. The earliest evidence of complex eucaryotes with organelles, organs within a cell, dates from .
Multicellular life is composed only of eucaryotic cells, and the earliest evidence for it is from , although specialization of cells for different functions first appears between (a possible fungus) and (a probable red alga). Sexual reproduction may be a prerequisite for specialization of cells, as an asexual multicellular organism would be at risk of being taken over by rogue cells which retain the ability to reproduce.
The earliest known animals are cnidarians from about , but these are so modern-looking that the earliest animals must have appeared before then. Early fossils of animals are rare because they did not develop mineralized hard parts that fossilize easily until about . The earliest modern-looking bilaterian animals appear in the Early Cambrian, along with several "weird wonders" that bear little obvious resemblance to any modern animals. There is a long-running debate about whether this Cambrian explosion was truly a very rapid period of evolutionary experimentation; alternative views are that modern-looking animals began evolving earlier but fossils of their precursors have not yet been found, or that the "weird wonders" are evolutionary "aunts" and "cousins" of modern groups. Vertebrates remained an obscure group until the first fish with jaws appeared in the Late Ordovician.
The spread of life from water to land required organisms to solve several problems, including protection against drying out and supporting themselves against gravity. The earliest evidence of land plants and land invertebrates date back to about and respectively. The lineage that produced land animals evolved later but very rapidly between and ; recent discoveries have overturned earlier ideas about the history and driving forces behind their evolution. Land plants were so successful that they caused an ecological crisis in the Late Devonian, until the evolution and spread of fungi that could digest dead wood.
Fossil evidence indicates that flowering plants appeared and rapidly diversified in the Early Cretaceous, between and . Their rapid rise to dominance of terrestrial ecosystems is thought to have been propelled by coevolution with pollinating insects. Social insects appeared around the same time and, although they account for only small parts of the insect "family tree", now form over 50% of the total mass of all insects.
Humans evolved from a lineage of upright-walking apes whose earliest fossils date from over . Although early members of this lineage had chimp-sized brains, about 25% as big as modern humans', there are signs of a steady increase in brain size after about . There is a long-running debate about whether modern humans are descendants of a single small population in Africa, which then migrated all over the world less than 200,000 years ago and replaced previous hominine species, or arose worldwide at the same time as a result of interbreeding.
The fossil record appears to show that the gaps between mass extinctions are becoming longer and the average and background rates of extinction are decreasing. Both of these phenomena could be explained in one or more ways:
Biodiversity in the fossil record, which is
Although palaeontology become established around 1800, earlier thinkers had noticed aspects of the fossil record. The ancient Greek philosopher Xenophanes (570-480 BC) wrote about fossil sea shells indicating that land was once under water. During the Middle Ages the Persian naturalist Ibn Sina, (known as Avicenna in Europe, discussed fossils and proposed a theory of petrifying fluids that Albert of Saxony elaborated on in the 14th century. The Chinese naturalist Shen Kuo (1031-1095) proposed a theory of climate change based on the presence of petrified bamboo in regions that in his time were too dry for bamboo.
In early modern Europe, the systematic study of fossils emerged as an integral part of the changes in natural philosophy that occurred during the Age of Reason. At the end of the 18th century Georges Cuvier's work established comparative anatomy as a scientific discipline and, by proving that some fossils animals resembled no living ones, demonstrated that animals could become extinct and led to the emergence of paleontology. The expanding knowledge of the fossil record also played an increasing role in the development of geology, particularly stratigraphy.
The first half of the 19th century saw geological and paleontological activity become increasingly well organized with the growth of geologic societies and museums, and an increasing number of professional geologists and fossil specialists. Interest increased for reasons that were not purely scientific, as geology and palentology helped industrialists to find and exploit natural resources such as coal.
This contributed to a rapid increase in knowledge about the history of life on Earth and to progress in the definition of the geologic time scale, largely based on fossil evidence. In 1822 the word "paleontology" was invented by the editor of a French scientific journal to refer to the study of ancient living organisms through fossils. As knowledge of life's history continued to improve, it became increasingly obvious that there had been some kind of successive order to the development of life. This would encourage early evolutionary theories on the transmutation of species. After Charles Darwin published Origin of Species in 1859, much of the focus of paleontology shifted to understanding evolutionary paths, including human evolution, and evolutionary theory.
The last half of the 19th century saw a tremendous expansion in paleontological activity, especially in North America. The trend continued in the 20th century with additional regions of the Earth being opened to systematic fossil collection. Fossils found in China near the end of the 20th century have been particularly important as they have provided new information about the earliest evolution of animals, early fish, dinosaurs and the evolution of birds. The last few decades of the 20th century saw a renewed interest in mass extinctions and their role in the evolution of life on Earth. There was also a renewed interest in the Cambrian explosion that apparently saw the development of the body plans of most animal phyla. The discovery of fossils of the Ediacaran biota and developments in paleobiology extended knowledge about the history of life back far before the Cambrian.
Increasing awareness of Gregor Mendel's pioneering work in genetics led first to the development of population genetics and then in the mid-20th century to the modern evolutionary synthesis, in which factors such as mutations, speciation and genetic drift provide the variation in traits on which Darwin's natural selection operates. A few years later Watson and Crick discovered the structure of DNA and confirmed its role in genetic inheritance, which is now known as the "Central Dogma" of molecular biology. In the 1960s molecular phylogenetics, the investigation of evolutionary "family trees" by biochemistry techniques, began to make an impact, particularly when it was proposed that the human lineage had diverged from apes much more recently than was generally thought at the time. Although this early study compared proteins from apes and humans, most molecular phylogenetics research is now based on comparisons of RNA.
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