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Friday, November 19, 2021

Paleoanthropology

From Wikipedia, the free encyclopedia

Paleoanthropology or paleo-anthropology is a branch of paleontology and anthropology which seeks to understand the early development of anatomically modern humans, a process known as hominization, through the reconstruction of evolutionary kinship lines within the family Hominidae, working from biological evidence (such as petrified skeletal remains, bone fragments, footprints) and cultural evidence (such as stone tools, artifacts, and settlement localities).

The field draws from and combines primatology, paleontology, biological anthropology, and cultural anthropology. As technologies and methods advance, genetics plays an ever-increasing role, in particular to examine and compare DNA structure as a vital tool of research of the evolutionary kinship lines of related species and genera.

Etymology

The term paleoanthropology derives from Greek palaiós (παλαιός) "old, ancient", ánthrōpos (ἄνθρωπος) "man, human" and the suffix -logía (-λογία) "study of".

Hominoid taxonomies

Hominoids are a primate superfamily, the hominid family is currently considered to comprise both the great ape lineages and human lineages within the hominoid superfamily. The "Homininae" comprise both the human lineages and the African ape lineages. The term "African apes" refers only to chimpanzees and gorillas. The terminology of the immediate biological family is currently in flux. The term "hominin" refers to any genus in the human tribe (Hominini), of which Homo sapiens (modern humans) is the only living specimen.





Suborder Hominoids




















Family Hominids




















Subfamily Homininae
































Tribe Gorillini




Tribe Hominini













































Genus Ardipithecus
Genus Australopithecus
Genus Paranthropus
Genus Kenyanthropus
Genus Homo

History

18th century

In 1758 Carl Linnaeus introduced the name Homo sapiens as a species name in the 10th edition of his work Systema Naturae although without a scientific description of the species-specific characteristics. Since the great apes were considered the closest relatives of human beings, based on morphological similarity, in the 19th century, it was speculated that the closest living relatives to humans were chimpanzees (genus Pan) and gorilla (genus Gorilla), and based on the natural range of these creatures, it was surmised that humans shared a common ancestor with African apes and that fossils of these ancestors would ultimately be found in Africa.

19th century

Charles Darwin

The science arguably began in the late 19th century when important discoveries occurred that led to the study of human evolution. The discovery of the Neanderthal in Germany, Thomas Huxley's Evidence as to Man's Place in Nature, and Charles Darwin's The Descent of Man were all important to early paleoanthropological research.

The modern field of paleoanthropology began in the 19th century with the discovery of "Neanderthal man" (the eponymous skeleton was found in 1856, but there had been finds elsewhere since 1830), and with evidence of so-called cave men. The idea that humans are similar to certain great apes had been obvious to people for some time, but the idea of the biological evolution of species in general was not legitimized until after Charles Darwin published On the Origin of Species in 1859.

Though Darwin's first book on evolution did not address the specific question of human evolution—"light will be thrown on the origin of man and his history," was all Darwin wrote on the subject—the implications of evolutionary theory were clear to contemporary readers.

Debates between Thomas Huxley and Richard Owen focused on the idea of human evolution. Huxley convincingly illustrated many of the similarities and differences between humans and apes in his 1863 book Evidence as to Man's Place in Nature. By the time Darwin published his own book on the subject, Descent of Man, it was already a well-known interpretation of his theory—and the interpretation which made the theory highly controversial. Even many of Darwin's original supporters (such as Alfred Russel Wallace and Charles Lyell) balked at the idea that human beings could have evolved their apparently boundless mental capacities and moral sensibilities through natural selection.

Asia

Five of the seven known fossil teeth of Homo luzonensis found in Callao Cave, the Philippines.

Prior to the general acceptance of Africa as the root of genus Homo, 19th-century naturalists sought the origin of humans in Asia. So-called "dragon bones" (fossil bones and teeth) from Chinese apothecary shops were known, but it was not until the early 20th century that German paleontologist, Max Schlosser, first described a single human tooth from Beijing. Although Schlosser (1903) was very cautious, identifying the tooth only as "?Anthropoide g. et sp. indet?," he was hopeful that future work would discover a new anthropoid in China.

Eleven years later, the Swedish geologist Johan Gunnar Andersson was sent to China as a mining advisor and soon developed an interest in "dragon bones". It was he who, in 1918, discovered the sites around Zhoukoudian, a village about 50 kilometers southwest of Beijing. However, because of the sparse nature of the initial finds, the site was abandoned.

Work did not resume until 1921, when the Austrian paleontologist, Otto Zdansky, fresh with his doctoral degree from Vienna, came to Beijing to work for Andersson. Zdansky conducted short-term excavations at Locality 1 in 1921 and 1923, and recovered only two teeth of significance (one premolar and one molar) that he subsequently described, cautiously, as "?Homo sp." (Zdansky, 1927). With that done, Zdansky returned to Austria and suspended all fieldwork.

News of the fossil hominin teeth delighted the scientific community in Beijing, and plans for developing a larger, more systematic project at Zhoukoudian were soon formulated. At the epicenter of excitement was Davidson Black, a Canadian-born anatomist working at Peking Union Medical College. Black shared Andersson’s interest, as well as his view that central Asia was a promising home for early humankind. In late 1926, Black submitted a proposal to the Rockefeller Foundation seeking financial support for systematic excavation at Zhoukoudian and the establishment of an institute for the study of human biology in China.

The Zhoukoudian Project came into existence in the spring of 1927, and two years later, the Cenozoic Research Laboratory of the Geological Survey of China was formally established. Being the first institution of its kind, the Cenozoic Laboratory opened up new avenues for the study of paleogeology and paleontology in China. The Laboratory was the precursor of the Institute of Vertebrate Paleontology and Paleoanthropology (IVPP) of the Chinese Academy of Science, which took its modern form after 1949.

The first of the major project finds are attributed to the young Swedish paleontologist, Anders Birger Bohlin, then serving as the field advisor at Zhoukoudian. He recovered a left lower molar that Black (1927) identified as unmistakably human (it compared favorably to the previous find made by Zdansky), and subsequently coined it Sinanthropus pekinensis. The news was at first met with skepticism, and many scholars had reservations that a single tooth was sufficient to justify the naming of a new type of early hominin. Yet within a little more than two years, in the winter of 1929, Pei Wenzhong, then the field director at Zhoukoudian, unearthed the first complete calvaria of Peking Man. Twenty-seven years after Schlosser’s initial description, the antiquity of early humans in East Asia was no longer a speculation, but a reality.

The Zhoukoudian site

Excavations continued at the site and remained fruitful until the outbreak of the Second Sino-Japanese War in 1937. The decade-long research yielded a wealth of faunal and lithic materials, as well as hominin fossils. These included 5 more complete calvaria, 9 large cranial fragments, 6 facial fragments, 14 partial mandibles, 147 isolated teeth, and 11 postcranial elements—estimated to represent as least 40 individuals. Evidence of fire, marked by ash lenses and burned bones and stones, were apparently also present, although recent studies have challenged this view. Franz Weidenreich came to Beijing soon after Black’s untimely death in 1934, and took charge of the study of the hominin specimens.

Following the loss of the Peking Man materials in late 1941, scientific endeavors at Zhoukoudian slowed, primarily because of lack of funding. Frantic search for the missing fossils took place, and continued well into the 1950s. After the establishment of the People’s Republic of China in 1949, excavations resumed at Zhoukoudian. But with political instability and social unrest brewing in China, beginning in 1966, and major discoveries at Olduvai Gorge and East Turkana (Koobi Fora), the paleoanthropological spotlight shifted westward to East Africa. Although China re-opened its doors to the West in the late 1970s, national policy calling for self-reliance, coupled with a widened language barrier, thwarted all the possibilities of renewed scientific relationships. Indeed, Harvard anthropologist K. C. Chang noted, "international collaboration (in developing nations very often a disguise for Western domination) became a thing of the past" (1977: 139).

Africa

Skull of Australopithecus africanus

1920s – 1940s

The first paleoanthropological find made in Africa was the 1921 discovery of the Kabwe 1 skull at Kabwe (Broken Hill), Zambia. Initially, this specimen was named Homo rhodesiensis; however, today it is considered part of the species Homo heidelbergensis.

In 1924 in a limestone quarry at Taung, Professor Raymond Dart discovered a remarkably well-preserved juvenile specimen (face and brain endocast), which he named Australopithecus africanus (Australopithecus meaning "Southern Ape"). Although the brain was small (410 cm³), its shape was rounded, unlike the brain shape of chimpanzees and gorillas, and more like the shape seen in modern humans. In addition, the specimen exhibited short canine teeth, and the anterior placement of the foramen magnum was more like the placement seen in modern humans than the placement seen in chimpanzees and gorillas, suggesting that this species was bipedal.

All of these traits convinced Dart that the Taung child was a bipedal human ancestor, a transitional form between ape and human. However, Dart's conclusions were largely ignored for decades, as the prevailing view of the time was that a large brain evolved before bipedality. It took the discovery of additional australopith fossils in Africa that resembled his specimen, and the rejection of the Piltdown Man hoax, for Dart's claims to be taken seriously.

In the 1930s, paleontologist Robert Broom discovered and described a new species at Kromdraai, South Africa. Although similar in some ways to Dart's Australopithecus africanus, Broom's specimen had much larger cheek teeth. Because of this difference, Broom named his specimen Paranthropus robustus, using a new genus name. In doing so, he established the practice of grouping gracile australopiths in the genus Australopithecus and robust australopiths in the genus Paranthropus. During the 1960s, the robust variety was commonly moved into Australopithecus. A more recent consensus has been to return to the original classification of Paranthropus as a separate genus.

1950s – 1990s

The second half of the twentieth century saw a significant increase in the number of paleoanthropological finds made in Africa. Many of these finds were associated with the work of the Leakey family in eastern Africa. In 1959, Mary Leakey's discovery of the Zinj fossin (OH 5) at Olduvai Gorge, Tanzania, led to the identification of a new species, Paranthropus boisei. In 1960, the Leakeys discovered the fossil OH 7, also at Olduvai Gorge, and assigned it to a new species, Homo habilis. In 1972, Bernard Ngeneo, a fieldworker working for Richard Leakey, discovered the fossil KNM-ER 1470 near Lake Turkana in Kenya. KNM-ER 1470 has been interpreted as either a distinct species, Homo rudolfensis, or alternatively as evidence of sexual dimorphism in Homo habilis. In 1967, Richard Leakey reported the earliest definitive examples of anatomically modern Homo sapiens from the site of Omo Kibish in Ethiopia, known as the Omo remains. In the late 1970s, Mary Leakey excavated the famous Laetoli footprints in Tanzania, which demonstrated the antiquity of bipedality in the human lineage. In 1985, Richard Leakey and Alan Walker discovered a specimen they called the Black Skull, found near Lake Turkana. This specimen was assigned to another species, Paranthropus aethiopicus. In 1994, a team led by Meave Leakey announced a new species, Australopithecus anamensis, based on specimens found near Lake Turkana.

Numerous other researchers have made important discoveries in eastern Africa. Possibly the most famous is the Lucy skeleton, discovered in 1973 by Donald Johanson and Maurice Taieb in Ethiopia's Afar Triangle at the site of Hadar. On the basis of this skeleton and subsequent discoveries, the researchers came up with a new species, Australopithecus afarensis. In 1975, Colin Groves and Vratislav Mazák announced a new species of human they called Homo ergaster. Homo ergaster specimens have been found at numerous sites in eastern and southern Africa. In 1994, Tim D. White announced a new species, Ardipithecus ramidus, based on fossils from Ethiopia.

In 1999, two new species were announced. Berhane Asfaw and Tim D. White named Australopithecus garhi based on specimens discovered in Ethiopia's Awash valley. Meave Leakey announced a new species, Kenyanthropus platyops, based on the cranium KNM-WT 40000 from Lake Turkana.

21st century

In the 21st century, numerous fossils have been found that add to current knowledge of existing species. For example, in 2001, Zeresenay Alemseged discovered an Australopithecus afarensis child fossil, called Selam, from the site of Dikika in the Afar region of Ethiopia. This find is particularly important because the fossil included a preserved hyoid bone, something rarely found in other paleoanthropological fossils but important for understanding the evolution of speech capacities.

Two new species from southern Africa have been discovered and described in recent years. In 2008, a team led by Lee Berger announced a new species, Australopithecus sediba, based on fossils they had discovered in Malapa cave in South Africa. In 2015, a team also led by Lee Berger announced another species, Homo naledi, based on fossils representing 15 individuals from the Rising Star Cave system in South Africa.

New species have also been found in eastern Africa. In 2000, Brigitte Senut and Martin Pickford described the species Orrorin tugenensis, based on fossils they found in Kenya. In 2004, Yohannes Haile-Selassie announced that some specimens previously labeled as Ardipithecus ramidus made up a different species, Ardipithecus kadabba. In 2015, Haile-Selassie announced another new species, Australopithecus deyiremeda, though some scholars are skeptical that the associated fossils truly represent a unique species.

Although most hominin fossils from Africa have been found in eastern and southern Africa, there are a few exceptions. One is Sahelanthropus tchadensis, discovered in the central African country of Chad in 2002. This find is important because it widens the assumed geographic range of early hominins.

Renowned paleoanthropologists

Fossil hominid skull display at The Museum of Osteology in Oklahoma City, USA

 

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Paleontology

From Wikipedia, the free encyclopedia
https://en.wikipedia.org/wiki/Paleontology

Paleontology
Palais de la Decouverte Tyrannosaurus rex p1050042.jpg
A paleontologist at work at John Day Fossil Beds National Monument

Paleontology (/ˌpeɪliɒnˈtɒlədʒi, ˌpæli-, -ən-/), also spelled palaeontology or palæontology, is the scientific study of life that existed prior to, and sometimes including, the start of the Holocene epoch (roughly 11,700 years before present). It includes the study of fossils to classify organisms and study their interactions with each other and their environments (their paleoecology). Paleontological observations have been documented as far back as the 5th century BCE. The science became established in the 18th century as a result of Georges Cuvier's work on comparative anatomy, and developed rapidly in the 19th century. The term itself originates from Greek παλα ('palaios', "old, ancient"), ὄν ('on', (gen. 'ontos'), "being, creature"), and λόγος ('logos', "speech, thought, study").

Paleontology lies on the border between biology and geology, but differs from archaeology in that it excludes the study of anatomically modern humans. It now uses techniques drawn from a wide range of sciences, including biochemistry, mathematics, and engineering. 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, almost 4 billion years ago. As knowledge has increased, paleontology has developed specialised sub-divisions, some of which focus on different types of fossil organisms while others study ecology and environmental history, such as ancient climates.

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 organisms large enough to leave body fossils. Estimating the dates of these remains is essential but difficult: sometimes adjacent rock layers allow radiometric dating, which provides 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 (arrangement of rock layers from youngest to oldest). Classifying ancient organisms is also difficult, as many do not fit well into the Linnaean taxonomy classifying living organisms, and paleontologists more often use cladistics to draw up evolutionary "family trees". The final quarter of the 20th century saw the development of molecular phylogenetics, which investigates how closely organisms are related by measuring the similarity of the DNA in their genomes. Molecular phylogenetics has also been used to estimate the dates when species diverged, but there is controversy about the reliability of the molecular clock on which such estimates depend.

Overview

The simplest definition of "paleontology" is "the study of ancient life". The field 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".

Historical science

The preparation of the fossilised bones of Europasaurus holgeri

William Whewell (1794–1866) classified paleontology as one of the historical sciences, along with archaeology, geology, astronomy, cosmology, philology and history itself: paleontology aims to describe phenomena of the past and to reconstruct their causes. Hence it has three main elements: description of past phenomena; developing a general theory about the causes of various types of change; and applying those theories to specific facts. When trying to explain the past, paleontologists and other historical scientists often construct a set of one or more hypotheses about the causes and then look for a "smoking gun", a piece of evidence that strongly accords with one hypothesis over any others. Sometimes researchers discover a "smoking gun" by a fortunate accident during other research. For example, the 1980 discovery by Luis and Walter Alvarez of iridium, a mainly extraterrestrial metal, in the Cretaceous–Tertiary boundary layer made asteroid impact the most favored explanation for the Cretaceous–Paleogene extinction event – although debate continues about the contribution of volcanism.

A complementary approach to developing scientific knowledge, experimental science, is often said to work by conducting experiments to disprove hypotheses about the workings and causes of natural phenomena. This approach cannot prove a hypothesis, since some later experiment may disprove it, but the accumulation of failures to disprove is often compelling evidence in favor. 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".

Related sciences

Paleontology lies between biology and geology since it focuses on the record of past life, but its main source of evidence is fossils in rocks. For historical reasons, paleontology is part of the geology department at many universities: in the 19th and early 20th centuries, geology departments found fossil evidence important for dating rocks, while biology departments showed little interest.

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 a species. 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 the people who lived there, and what they ate; or they might analyze the climate at the time of habitation.

In addition, paleontology often borrows techniques from other sciences, including biology, osteology, 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, compares the DNA and RNA of modern organisms to re-construct the "family trees" of their evolutionary ancestors. 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 from engineering have been used to analyse how the bodies of ancient organisms might have worked, for example the running speed and bite strength of Tyrannosaurus, or the flight mechanics of Microraptor. It is relatively commonplace to study the internal details of fossils using X-ray microtomography. Paleontology, biology, archaeology, and paleoneurobiology combine to study endocranial casts (endocasts) of species related to humans to clarify the evolution of the human brain.

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.

Subdivisions

As knowledge has increased, paleontology has developed specialised subdivisions. Vertebrate paleontology concentrates on fossils from the earliest fish to the immediate ancestors of modern mammals. Invertebrate paleontology deals with fossils such as molluscs, arthropods, annelid worms and echinoderms. Paleobotany studies fossil plants, algae, and fungi. Palynology, the study of pollen and spores produced by land plants and protists, straddles paleontology and botany, as it deals with both living and fossil organisms. Micropaleontology deals with microscopic fossil organisms of all kinds.

Analyses using engineering techniques show that Tyrannosaurus had a devastating bite, but raise doubts about its running ability.

Instead of focusing on individual organisms, paleoecology examines the interactions between different ancient organisms, such as their food chains, and the two-way interactions with their environments.  For example, the development of oxygenic photosynthesis by bacteria caused the oxygenation of the atmosphere and hugely increased the productivity and diversity of ecosystems. Together, these led to the evolution of complex eukaryotic 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 that 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 helping to cause 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.

Sources of evidence

Body fossils

Main article: Fossil
 
This Marrella specimen illustrates how clear and detailed the fossils from the Burgess Shale lagerstätte are

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, it is often adequate to illustrate the broader patterns of life's history. There are also biases in the fossil record: different environments are more favorable 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 that in other sediments are represented only 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

Cambrian trace fossils including Rusophycus, made by a trilobite
 
Climactichnites---Cambrian trackways (10–12 cm wide) from large, slug-like animals on a Cambrian tidal flat in what is now Wisconsin.
 
Main article: Trace fossil

Trace fossils consist mainly of tracks and burrows, but also include coprolites (fossil feces) and marks left by feeding. Trace fossils are particularly significant because they represent a data source that is not limited to animals with easily fossilised hard parts, and they reflect organisms' behaviours. 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).

Geochemical observations

Main article: Geochemistry

Geochemical observations may help to deduce the global level of biological activity at a certain period, or the affinity of certain fossils. For example, geochemical features of rocks may reveal when life first arose on Earth, and may provide evidence of the presence of eukaryotic cells, the type from which all multicellular organisms are built. Analyses of carbon isotope ratios may help to explain major transitions such as the Permian–Triassic extinction event.

Classifying ancient organisms

Main articles: Biological classification, Cladistics, Phylogenetic nomenclature, and Evolutionary taxonomy
 
Levels in the Linnaean taxonomy

Naming groups of organisms in a way that is clear and widely agreed is important, as some disputes in paleontology have been based just on misunderstandings over names. Linnaean 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 new higher-level grouping, e.g. genus or family or order; this is important since the Linnaean rules for naming groups are tied to their levels, and hence if a group is moved to a different level it must 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 that are compared may be anatomical, such as the presence of a notochord, or molecular, by comparing sequences of DNA or proteins. The result of a successful analysis is a hierarchy of clades – groups that share a common ancestor. Ideally the "family tree" has only two branches leading from each node ("junction"), but sometimes there is too little 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", and understand fossils. 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.

Estimating the dates of organisms

Common index fossils used to date rocks in the northeast United States

Paleontology seeks to map out how living things have changed through time. A substantial hurdle to this aim is the difficulty of working out how old fossils are. Beds that preserve fossils typically lack the radioactive elements needed for radiometric dating. This technique is our only means of giving rocks greater than about 50 million years old an absolute age, and can be 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 shows how long ago the radioactive element was incorporated into the rock. Radioactive elements are common only in rocks with a volcanic origin, and so the only fossil-bearing rocks that can be dated radiometrically are a few volcanic ash layers.

Consequently, paleontologists must usually 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. Rocks normally form relatively horizontal layers, with each layer younger than the one underneath it. If a fossil is found between two layers whose ages are known, the fossil's age must lie 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 match up rock beds that are not directly next to one another. However, fossils of species that survived for a relatively short time can be used to link up isolated rocks: this technique is called biostratigraphy. 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. Such index fossils must be distinctive, be globally distributed and have a short time range to be useful. However, misleading results are produced if the index fossils 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 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 when 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 may vary by a factor of two.

History of life

This wrinkled "elephant skin" texture is a trace fossil of a non-stromatolite microbial mat. The image shows the location, in the Burgsvik beds of Sweden, where the texture was first identified as evidence of a microbial mat.
 
Main article: Evolutionary history of life
Further information: Timeline of evolutionary history of life

Earth formed about 4,570 million years ago and, after a collision that formed the Moon about 40 million years later, may have cooled quickly enough to have oceans and an atmosphere about 4,440 million years ago. There is evidence on the Moon of a Late Heavy Bombardment by asteroids from 4,000 to 3,800 million years ago. If, as seems likely, such a bombardment struck Earth at the same time, the first atmosphere and oceans may have been stripped away.

Paleontology traces the evolutionary history of life back to over 3,000 million years ago, possibly as far as 3,800 million years ago. The oldest clear evidence of life on Earth dates to 3,000 million years ago, although there have been reports, often disputed, of fossil bacteria from 3,400 million years ago and of geochemical evidence for the presence of life 3,800 million years ago. Some scientists have proposed that life on Earth was "seeded" from elsewhere, but most research concentrates on various explanations of how life could have arisen independently on Earth.

For about 2,000 million years microbial mats, multi-layered colonies of different 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 from about 2,400 million years ago. This change in the atmosphere increased their effectiveness as nurseries of evolution. While eukaryotes, 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 source of metabolic energy. This innovation may have come from primitive eukaryotes capturing oxygen-powered bacteria as endosymbionts and transforming them into organelles called mitochondria. The earliest evidence of complex eukaryotes with organelles (such as mitochondria) dates from 1,850 million years ago.

Opabinia sparked modern interest in the Cambrian explosion

Multicellular life is composed only of eukaryotic cells, and the earliest evidence for it is the Francevillian Group Fossils from 2,100 million years ago, although specialisation of cells for different functions first appears between 1,430 million years ago (a possible fungus) and 1,200 million years ago (a probable red alga). Sexual reproduction may be a prerequisite for specialisation of cells, as an asexual multicellular organism might be at risk of being taken over by rogue cells that retain the ability to reproduce.

The earliest known animals are cnidarians from about 580 million years ago, but these are so modern-looking that must be descendants of earlier animals. Early fossils of animals are rare because they had not developed mineralised, easily fossilized hard parts until about 548 million years ago. 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 a minor group until the first jawed fish appeared in the Late Ordovician.

At about 13 centimetres (5.1 in) the Early Cretaceous Yanoconodon was longer than the average mammal of the time

The spread of animals and plants 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 476 million years ago and 490 million years ago respectively. Those invertebrates, as indicated by their trace and body fossils, were shown to be arthropods known as euthycarcinoids. The lineage that produced land vertebrates evolved later but very rapidly between 370 million years ago and 360 million years ago; recent discoveries have overturned earlier ideas about the history and driving forces behind their evolution. Land plants were so successful that their detritus caused an ecological crisis in the Late Devonian, until the evolution of fungi that could digest dead wood.

Birds are the only surviving dinosaurs

During the Permian period, synapsids, including the ancestors of mammals, may have dominated land environments, but this ended with the Permian–Triassic extinction event 251 million years ago, which came very close to wiping out all complex life. The extinctions were apparently fairly sudden, at least among vertebrates. During the slow recovery from this catastrophe a previously obscure group, archosaurs, became the most abundant and diverse terrestrial vertebrates. One archosaur group, the dinosaurs, were the dominant land vertebrates for the rest of the Mesozoic, and birds evolved from one group of dinosaurs. During this time mammals' ancestors survived only as small, mainly nocturnal insectivores, which may have accelerated the development of mammalian traits such as endothermy and hair. After the Cretaceous–Paleogene extinction event 66 million years ago killed off all the dinosaurs except the birds, mammals increased rapidly in size and diversity, and some took to the air and the sea.

Fossil evidence indicates that flowering plants appeared and rapidly diversified in the Early Cretaceous between 130 million years ago and 90 million years ago. 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 6 million years ago. 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 3 million years ago. 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.

Mass extinctions

Extinction intensity.svgCambrianOrdovicianSilurianDevonianCarboniferousPermianTriassicJurassicCretaceousPaleogeneNeogene
Marine extinction intensity during the Phanerozoi
%
Millions of years ago
Extinction intensity.svg
Apparent extinction intensity, i.e. the fraction of genera going extinct at any given time, as reconstructed from the fossil record (graph not meant to include recent epoch of Holocene extinction event)
Main article: Mass extinction

Life on earth has suffered occasional mass extinctions at least since 542 million years ago. Despite their disastrous effects, mass extinctions have sometimes accelerated the evolution of life on earth. When dominance of an ecological niche passes from one group of organisms to another, this is rarely because the new dominant group outcompetes the old, but usually because an extinction event allows new group to outlive the old and move into its niche.

The fossil record appears to show that the rate of extinction is slowing down, with both the gaps between mass extinctions becoming longer and the average and background rates of extinction decreasing. However, it is not certain whether the actual rate of extinction has altered, since both of these observations could be explained in several ways:

  • The oceans may have become more hospitable to life over the last 500 million years and less vulnerable to mass extinctions: dissolved oxygen became more widespread and penetrated to greater depths; the development of life on land reduced the run-off of nutrients and hence the risk of eutrophication and anoxic events; marine ecosystems became more diversified so that food chains were less likely to be disrupted.
  • Reasonably complete fossils are very rare: most extinct organisms are represented only by partial fossils, and complete fossils are rarest in the oldest rocks. So paleontologists have mistakenly assigned parts of the same organism to different genera, which were often defined solely to accommodate these finds – the story of Anomalocaris is an example of this. The risk of this mistake is higher for older fossils because these are often unlike parts of any living organism. Many "superfluous" genera are represented by fragments that are not found again, and these "superfluous" genera are interpreted as becoming extinct very quickly.
All genera
"Well-defined" genera
Trend line
"Big Five" mass extinctions
Other mass extinctions
Million years ago
Thousands of genera
Phanerozoic biodiversity as shown by the fossil record

Biodiversity in the fossil record, which is

"the number of distinct genera alive at any given time; that is, those whose first occurrence predates and whose last occurrence postdates that time"

shows a different trend: a fairly swift rise from 542 to 400 million years ago, a slight decline from 400 to 200 million years ago, in which the devastating Permian–Triassic extinction event is an important factor, and a swift rise from 200 million years ago to the present.

History

Main article: History of paleontology
Further information: Timeline of paleontology
 
This illustration of an Indian elephant jaw and a mammoth jaw (top) is from Cuvier's 1796 paper on living and fossil elephants.

Although paleontology became established around 1800, earlier thinkers had noticed aspects of the fossil record. The ancient Greek philosopher Xenophanes (570–480 BCE) concluded from fossil sea shells that some areas of land were 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 on which Albert of Saxony elaborated 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. In the Italian Renaissance, Leonardo da Vinci made various significant contributions to the field as well depicted numerous fossils. Leonardo's contributions are central to the history of paleontology because he established a line of continuity between the two main branches of paleontology – ichnology and body fossil paleontology.  He identified the following:

  1. The biogenic nature of ichnofossils, i.e. ichnofossils were structures left by living organisms;
  2. The utility of ichnofossils as paleoenvironmental tools – certain ichnofossils show the marine origin of rock strata;
  3. The importance of the neoichnological approach – recent traces are a key to understanding ichnofossils;
  4. The independence and complementary evidence of ichnofossils and body fossils – ichnofossils are distinct from body fossils, but can be integrated with body fossils to provide paleontological information

At the end of the 18th century Georges Cuvier's work established comparative anatomy as a scientific discipline and, by proving that some fossil animals resembled no living ones, demonstrated that animals could become extinct, leading to the emergence of paleontology. The expanding knowledge of the fossil record also played an increasing role in the development of geology, particularly stratigraphy.

First mention of the word palæontologie, as coined in January 1822 by Henri Marie Ducrotay de Blainville in his Journal de physique.

The first half of the 19th century saw geological and paleontological activity become increasingly well organised 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 paleontology 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 Henri Marie Ducrotay de Blainville, editor of Journal de Physique, coined the word "palaeontology" 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 encouraged 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.

Haikouichthys, from about 518 million years ago in China, may be the earliest known fish

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, which explains evolution as the outcome of events such as mutations and horizontal gene transfer, which provide genetic variation, with genetic drift and natural selection driving changes in this variation over time. Within the next few years the role and operation of DNA in genetic inheritance were discovered, leading to what is now known as the "Central Dogma" of molecular biology. In the 1960s molecular phylogenetics, the investigation of evolutionary "family trees" by techniques derived from biochemistry, 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 and DNA.

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