A phylogenetic tree or evolutionary tree is a branching diagram or "tree" showing the evolutionary relationships among various biological species or other entities—their phylogeny —based upon similarities and differences in their physical or genetic characteristics. All life on Earth is part of a single phylogenetic tree. In a rooted phylogenetic tree, each node with descendants represents the inferred most recent common ancestor of those descendants, the edge lengths in some trees may be interpreted as time estimates; each node is called a taxonomic unit. Internal nodes are called hypothetical taxonomic units, as they cannot be directly observed. Trees are useful in fields of biology such as bioinformatics and phylogenetics. Unrooted trees illustrate only the relatedness of the leaf nodes and do not require the ancestral root to be known or inferred; the idea of a "tree of life" arose from ancient notions of a ladder-like progression from lower into higher forms of life. Early representations of "branching" phylogenetic trees include a "paleontological chart" showing the geological relationships among plants and animals in the book Elementary Geology, by Edward Hitchcock.
Charles Darwin produced one of the first illustrations and crucially popularized the notion of an evolutionary "tree" in his seminal book The Origin of Species. Over a century evolutionary biologists still use tree diagrams to depict evolution because such diagrams convey the concept that speciation occurs through the adaptive and semirandom splitting of lineages. Over time, species classification has become more dynamic; the term phylogenetic, or phylogeny, derives from the two ancient greek words φῦλον, meaning "race, lineage", γένεσις, meaning "origin, source". A rooted phylogenetic tree is a directed tree with a unique node — the root — corresponding to the most recent common ancestor of all the entities at the leaves of the tree; the root node serves as the parent of all other nodes in the tree. The root is therefore a node of degree 2 while other internal nodes have a minimum degree of 3; the most common method for rooting trees is the use of an uncontroversial outgroup—close enough to allow inference from trait data or molecular sequencing, but far enough to be a clear outgroup.
Unrooted trees illustrate the relatedness of the leaf nodes without making assumptions about ancestry. They do not require the ancestral root to be inferred. Unrooted trees can always be generated from rooted ones by omitting the root. By contrast, inferring the root of an unrooted tree requires some means of identifying ancestry; this is done by including an outgroup in the input data so that the root is between the outgroup and the rest of the taxa in the tree, or by introducing additional assumptions about the relative rates of evolution on each branch, such as an application of the molecular clock hypothesis. Both rooted and unrooted phylogenetic trees can be either bifurcating or multifurcating, either labeled or unlabeled. A rooted bifurcating tree has two descendants arising from each interior node, an unrooted bifurcating tree takes the form of an unrooted binary tree, a free tree with three neighbors at each internal node. In contrast, a rooted multifurcating tree may have more than two children at some nodes and an unrooted multifurcating tree may have more than three neighbors at some nodes.
A labeled tree has specific values assigned to its leaves, while an unlabeled tree, sometimes called a tree shape, defines a topology only. The number of possible trees for a given number of leaf nodes depends on the specific type of tree, but there are always more multifurcating than bifurcating trees, more labeled than unlabeled trees, more rooted than unrooted trees; the last distinction is the most biologically relevant. For labeled bifurcating trees, there are:!! =! 2 n − 2! for n ≥ 2 total rooted trees and!! =! 2 n − 3! for n ≥ 3 total unrooted trees, where n represents the number of leaf nodes. Among labeled bifurcating trees, the number of unrooted trees with n leaves is equal to the number of rooted trees with n − 1 leaves. A dendrogram is a general name for a tree, whether phylogenetic or not, hence for the diagrammatic representation of a phylogenetic tree. A cladogram only represents a branching pattern.
John Gould FRS was an English ornithologist and bird artist. He published a number of monographs on birds, illustrated by plates that he produced with the assistance of his wife, Elizabeth Gould, several other artists including Edward Lear, Henry Constantine Richter, Joseph Wolf and William Matthew Hart, he has been considered the father of bird study in Australia and the Gould League in Australia is named after him. His identification of the birds now nicknamed "Darwin's finches" played a role in the inception of Darwin's theory of evolution by natural selection. Gould's work is referenced On the Origin of Species. Gould was born in Lyme Regis the first son of a gardener, he and the boy had a scanty education. Shortly afterwards his father obtained a position on an estate near Guildford, in 1818 Gould became foreman in the Royal Gardens of Windsor, he was for some time under the care of the Royal Gardens of Windsor. The young Gould started training as a gardener, being employed under his father at Windsor from 1818 to 1824, he was subsequently a gardener at Ripley Castle in Yorkshire.
He became an expert in the art of taxidermy. In 1824 he set himself up in business in London as a taxidermist, his skill helped him to become the first Curator and Preserver at the museum of the Zoological Society of London in 1827. Gould's position brought him into contact with the country's leading naturalists; this meant that he was the first to see new collections of birds given to the Zoological Society of London. In 1830 a collection of birds arrived from the Himalayas, many not described. Gould published these birds in A Century of Birds from the Himalaya Mountains; the text was by Nicholas Aylward Vigors and the illustrations were drawn and lithographed by Gould's wife Elizabeth Coxen Gould. Most of Gould's work were rough sketches on paper from which other artists created the lithographic plates; this work was followed by four more in the next seven years, including Birds of Europe in five volumes. It was completed in 1837; the plates were lithographed by Elizabeth Coxen Gould. A few of the illustrations were made by Edward Lear as part of his Illustrations of the Family of Psittacidae in 1832.
Lear, was in financial difficulty, he sold the entire set of lithographs to Gould. The books were published in a large size, imperial folio, with magnificent coloured plates. 41 of these volumes were published, with about 3000 plates. They appeared in parts at £3 3s. A number, subscribed for in advance, in spite of the heavy expense of preparing the plates, Gould succeeded in making his ventures pay, realising a fortune; this was a busy period for Gould who published Icones Avium in two parts containing 18 leaves of bird studies on 54 cm plates as a supplement to his previous works. No further monographs were published as in 1838 he and his wife moved to Australia to work on the Birds of Australia. Shortly after their return to England, his wife died in 1841. Elizabeth Gould completed 84 plates for Birds of Australia before her death; when Charles Darwin presented his mammal and bird specimens collected during the second voyage of HMS Beagle to the Zoological Society of London on 4 January 1837, the bird specimens were given to Gould for identification.
He set aside his paying work and at the next meeting on 10 January reported that birds from the Galápagos Islands which Darwin had thought were blackbirds, "gross-bills" and finches were in fact "a series of ground Finches which are so peculiar" as to form "an new group, containing 12 species." This story made the newspapers. In March, Darwin met Gould again, learning that his Galápagos "wren" was another species of finch and the mockingbirds he had labelled by island were separate species rather than just varieties, with relatives on the South American mainland. Subsequently, Gould advised that the smaller southern Rhea specimen, rescued from a Christmas dinner was a separate species which he named Rhea darwinii, whose territory overlapped with the northern rheas. Darwin had not bothered to label his finches by island, but others on the expedition had taken more care, he now sought specimens collected by crewmen. From them he was able to establish that the species were unique to islands, an important step on the inception of his theory of evolution by natural selection.
Gould's work on the birds was published between 1838 and 1842 in five numbers as Part 3 of Zoology of the Voyage of H. M. S. Beagle, edited by Charles Darwin. Elizabeth Gould illustrated all the plates for Part 3. In 1838 the Goulds sailed to Australia, intending to study the birds of that country and be the first to produce a major work on the subject, they took with them the collector John Gilbert. They arrived in Tasmania in September, making the acquaintance of the governor Sir John Franklin and his wife. Gould and Gilbert collected on the island. In February 1839 Gould sailed to Sydney, he travelled to his brother-in-law's station at Yarrundi, spending his time searching for bowerbirds in the Liverpool Range. In April he returned to Tasmania for the birth of his son. In May he sailed to Adelaide to meet Charles Sturt, preparing to lead an expedition to the Murray River. Gould collected in the Mount Lofty range, the Murray Scrubs and Kangaroo Island, returning again to Hobart in July, he travelled with his wife to Yarrundi.
They returned home to England in May 1840. The result of the trip was The Birds of Australia, it included a total of 600 plates in seven volumes.
Human evolution is the evolutionary process that led to the emergence of anatomically modern humans, beginning with the evolutionary history of primates—in particular genus Homo—and leading to the emergence of Homo sapiens as a distinct species of the hominid family, the great apes. This process involved the gradual development of traits such as human bipedalism and language, as well as interbreeding with other hominins, which indicate that human evolution was not linear but a web; the study of human evolution involves several scientific disciplines, including physical anthropology, archaeology, neurobiology, linguistics, evolutionary psychology and genetics. Genetic studies show that primates diverged from other mammals about 85 million years ago, in the Late Cretaceous period, the earliest fossils appear in the Paleocene, around 55 million years ago. Within the Hominoidea superfamily, the Hominidae family diverged from the Hylobatidae family some 15–20 million years ago. Human evolution from its first separation from the last common ancestor of humans and chimpanzees is characterized by a number of morphological, developmental and behavioral changes.
The most significant of these adaptations are bipedalism, increased brain size, lengthened ontogeny, decreased sexual dimorphism. The relationship between these changes is the subject of ongoing debate. Other significant morphological changes included the evolution of a power and precision grip, a change first occurring in H. erectus. Bipedalism is the basic adaptation of the hominid and is considered the main cause behind a suite of skeletal changes shared by all bipedal hominids; the earliest hominin, of primitive bipedalism, is considered to be either Sahelanthropus or Orrorin, both of which arose some 6 to 7 million years ago. The non-bipedal knuckle-walkers, the gorilla and chimpanzee, diverged from the hominin line over a period covering the same time, so either of Sahelanthropus or Orrorin may be our last shared ancestor. Ardipithecus, a full biped, arose 5.6 million years ago. The early bipeds evolved into the australopithecines and still into the genus Homo. There are several theories of the adaptation value of bipedalism.
It is possible that bipedalism was favored because it freed the hands for reaching and carrying food, saved energy during locomotion, enabled long distance running and hunting, provided an enhanced field of vision, helped avoid hyperthermia by reducing the surface area exposed to direct sun. A new study provides support for the hypothesis that walking on two legs, or bipedalism, evolved because it used less energy than quadrupedal knuckle-walking. However, recent studies suggest that bipedality without the ability to use fire would not have allowed global dispersal; this change in gait saw a lengthening of the legs proportionately when compared to the length of the arms, which were shortened through the removal of the need for brachiation. Another change is the shape of the big toe. Recent studies suggest that Australopithecines still lived part of the time in trees as a result of maintaining a grasping big toe; this was progressively lost in Habilines. Anatomically, the evolution of bipedalism has been accompanied by a large number of skeletal changes, not just to the legs and pelvis, but to the vertebral column and ankles, skull.
The femur evolved into a more angular position to move the center of gravity toward the geometric center of the body. The knee and ankle joints became robust to better support increased weight. To support the increased weight on each vertebra in the upright position, the human vertebral column became S-shaped and the lumbar vertebrae became shorter and wider. In the feet the big toe moved into alignment with the other toes to help in forward locomotion; the arms and forearms shortened relative to the legs making it easier to run. The foramen magnum migrated under more anterior; the most significant changes occurred in the pelvic region, where the long downward facing iliac blade was shortened and widened as a requirement for keeping the center of gravity stable while walking. A drawback is that the birth canal of bipedal apes is smaller than in knuckle-walking apes, though there has been a widening of it in comparison to that of australopithecine and modern humans, permitting the passage of newborns due to the increase in cranial size but this is limited to the upper portion, since further increase can hinder normal bipedal movement.
The shortening of the pelvis and smaller birth canal evolved as a requirement for bipedalism and had significant effects on the process of human birth, much more difficult in modern humans than in other primates. During human birth, because of the variation in size of the pelvic region, the fetal head must be in a transverse position during entry into the birth canal and rotate about 90 degrees upon exit; the smaller birth canal became a limiting factor to brain size increases in early humans and prompted a shorter gestation period leading to the relative immaturity of human
Evolutionary medicine or Darwinian medicine is the application of modern evolutionary theory to understanding health and disease. Modern medical research and practice have focused on the molecular and physiological mechanisms underlying health and disease, while evolutionary medicine focuses on the question of why evolution has shaped these mechanisms in ways that may leave us susceptible to disease; the evolutionary approach has driven important advances in our understanding of cancer, autoimmune disease, anatomy. Medical schools have been slower to integrate evolutionary approaches because of limitations on what can be added to existing medical curricula. Utilizing the Delphi method, 56 experts from a variety of disciplines, including anthropology and biology agreed upon 14 core principles intrinsic to the education and practice of evolutionary medicine; these 14 principles can be further grouped into five general categories: question framing, evolution I and II, evolutionary trade-offs, reasons for vulnerability, culture.
Additional information regarding these principles may be found in the table below. Adaptation works within constraints, makes compromises and trade-offs, occurs in the context of different forms of competition. Adaptations can only occur; some adaptations which would prevent ill health are therefore not possible. DNA cannot be prevented from undergoing somatic replication corruption. Humans cannot biosynthesize vitamin C, so risk scurvy, vitamin C deficiency disease, if dietary intake of the vitamin is insufficient. Retinal neurons and their axon output have evolved to be inside the layer of retinal pigment cells; this creates a constraint on the evolution of the visual system such that the optic nerve is forced to exit the retina through a point called the optic disc. This, in turn, creates a blind spot. More it makes vision vulnerable to increased pressure within the eye since this cups and damages the optic nerve at this point, resulting in impaired vision. Other constraints occur as the byproduct of adaptive innovations.
One constraint upon selection is that different adaptations can conflict, which requires a compromise between them to ensure an optimal cost-benefit tradeoff. Running efficiency in women, birth canal size Encephalization, gut size Skin pigmentation protection from UV, the skin synthesis of vitamin D Speech and its use of a descended larynx, increased risk of choking Different forms of competition exist and these can shape the processes of genetic change. Mate choice and disease susceptibility genomic conflict between mother and fetus that results in pre-eclampsia Humans evolved to live as simple hunter-gatherers in small tribal bands. Contemporary humans now have a different environment and way of life; this change makes present humans vulnerable to a number of health problems, termed "diseases of civilization" and "diseases of affluence". Stone-age humans evolved to live off the land, taking advantage of the resources that were available to them. Evolution is slow, the rapid change from stone-age environments and practices to the world of today is problematic because we are still adapted to stone-age circumstances that no longer apply.
This misfit has serious implications for our health. "Modern environments may cause many diseases such as deficiency syndromes like scurvy and rickets".) In contrast to the diet of early hunter-gatherers, the modern Western diet contains high quantities of fat and simple carbohydrates, such as refined sugars and flours. These sudden dietary changes create health problems. Trans fat health risks Dental caries High GI foods Modern diet based on "common wisdom" regarding diets in the paleolithic era Examples of aging-associated diseases are atherosclerosis and cardiovascular disease, arthritis, osteoporosis, type 2 diabetes and Alzheimer's disease; the incidence of all of these diseases increases with aging. Of the 150,000 people who die each day across the globe, about two thirds—100,000 per day—die of age-related causes. In industrialized nations, the proportion is much higher, reaching 90%. Many contemporary humans engage in little physical exercise compared to the physically active lifestyles of ancestral hunter-gatherers.
Prolonged periods of inactivity may have only occurred in early humans following illness or injury, so a modern sedentary lifestyle may continuously cue the body to trigger life preserving metabolic and stress-related responses such as inflammation, some theorize that this causes chronic diseases. Contemporary humans in developed countries are free of parasites intestinal ones; this is due to frequent washing of clothing and the body, improved sanitation. Although such hygiene can be important when it comes to maintaining good health, it can be problematic for the proper development of the immune system; the hygiene hypothesis is that humans evolved to be dependent on certain microorganisms that help establish the immune system, modern hygiene practices can prevent necessary exposure to these microorganisms. "Microorganisms and macroorganisms such as helminths from mud and feces play a critical role in driving immunoregulation". Essential microorganisms play a crucial role in building and training immune functions that fight off and repel some diseases, protect against excessive inflammation, implicated in several diseases.
For instance, recent studies have found evidence supporting in
Evolution is change in the heritable characteristics of biological populations over successive generations. These characteristics are the expressions of genes that are passed on from parent to offspring during reproduction. Different characteristics tend to exist within any given population as a result of mutation, genetic recombination and other sources of genetic variation. Evolution occurs when evolutionary processes such as natural selection and genetic drift act on this variation, resulting in certain characteristics becoming more common or rare within a population, it is this process of evolution that has given rise to biodiversity at every level of biological organisation, including the levels of species, individual organisms and molecules. The scientific theory of evolution by natural selection was proposed by Charles Darwin and Alfred Russel Wallace in the mid-19th century and was set out in detail in Darwin's book On the Origin of Species. Evolution by natural selection was first demonstrated by the observation that more offspring are produced than can survive.
This is followed by three observable facts about living organisms: 1) traits vary among individuals with respect to their morphology and behaviour, 2) different traits confer different rates of survival and reproduction and 3) traits can be passed from generation to generation. Thus, in successive generations members of a population are more to be replaced by the progenies of parents with favourable characteristics that have enabled them to survive and reproduce in their respective environments. In the early 20th century, other competing ideas of evolution such as mutationism and orthogenesis were refuted as the modern synthesis reconciled Darwinian evolution with classical genetics, which established adaptive evolution as being caused by natural selection acting on Mendelian genetic variation. All life on Earth shares a last universal common ancestor that lived 3.5–3.8 billion years ago. The fossil record includes a progression from early biogenic graphite, to microbial mat fossils, to fossilised multicellular organisms.
Existing patterns of biodiversity have been shaped by repeated formations of new species, changes within species and loss of species throughout the evolutionary history of life on Earth. Morphological and biochemical traits are more similar among species that share a more recent common ancestor, can be used to reconstruct phylogenetic trees. Evolutionary biologists have continued to study various aspects of evolution by forming and testing hypotheses as well as constructing theories based on evidence from the field or laboratory and on data generated by the methods of mathematical and theoretical biology, their discoveries have influenced not just the development of biology but numerous other scientific and industrial fields, including agriculture and computer science. The proposal that one type of organism could descend from another type goes back to some of the first pre-Socratic Greek philosophers, such as Anaximander and Empedocles; such proposals survived into Roman times. The poet and philosopher Lucretius followed Empedocles in his masterwork De rerum natura.
In contrast to these materialistic views, Aristotelianism considered all natural things as actualisations of fixed natural possibilities, known as forms. This was part of a medieval teleological understanding of nature in which all things have an intended role to play in a divine cosmic order. Variations of this idea became the standard understanding of the Middle Ages and were integrated into Christian learning, but Aristotle did not demand that real types of organisms always correspond one-for-one with exact metaphysical forms and gave examples of how new types of living things could come to be. In the 17th century, the new method of modern science rejected the Aristotelian approach, it sought explanations of natural phenomena in terms of physical laws that were the same for all visible things and that did not require the existence of any fixed natural categories or divine cosmic order. However, this new approach was slow to take root in the biological sciences, the last bastion of the concept of fixed natural types.
John Ray applied one of the more general terms for fixed natural types, "species," to plant and animal types, but he identified each type of living thing as a species and proposed that each species could be defined by the features that perpetuated themselves generation after generation. The biological classification introduced by Carl Linnaeus in 1735 explicitly recognised the hierarchical nature of species relationships, but still viewed species as fixed according to a divine plan. Other naturalists of this time speculated on the evolutionary change of species over time according to natural laws. In 1751, Pierre Louis Maupertuis wrote of natural modifications occurring during reproduction and accumulating over many generations to produce new species. Georges-Louis Leclerc, Comte de Buffon suggested that species could degenerate into different organisms, Erasmus Darwin proposed that all warm-blooded animals could have descended from a single microorganism; the first full-fledged evolutionary scheme was Jean-Baptiste Lamarck's "transmutation" theory of 1809, which envisaged spontaneous generation continually producing simple forms of life that developed greater complexity in parallel lineages with an inherent progressive tendency, postulated that on a local level, these lineages adapted to the environment by inheriting changes caused by their use or disuse in parents.
These ideas were cond
Evolutionary history of life
The evolutionary history of life on Earth traces the processes by which living and fossil organisms evolved, from the earliest emergence of life to the present. Earth formed about 4.5 billion years ago and evidence suggests life emerged prior to 3.7 Ga. The similarities among all known present-day species indicate that they have diverged through the process of evolution from a common ancestor. 1 trillion species live on Earth of which only 1.75–1.8 million have been named and 1.6 million documented in a central database. These living species represent less than one percent of all species that have lived on earth; the earliest evidence of life comes from biogenic carbon signatures and stromatolite fossils discovered in 3.7 billion-year-old metasedimentary rocks from western Greenland. In 2015, possible "remains of biotic life" were found in 4.1 billion-year-old rocks in Western Australia. In March 2017, putative evidence of the oldest forms of life on Earth was reported in the form of fossilized microorganisms discovered in hydrothermal vent precipitates in the Nuvvuagittuq Belt of Quebec, that may have lived as early as 4.28 billion years ago, not long after the oceans formed 4.4 billion years ago, not long after the formation of the Earth 4.54 billion years ago.
Microbial mats of coexisting bacteria and archaea were the dominant form of life in the early Archean Epoch and many of the major steps in early evolution are thought to have taken place in this environment. The evolution of photosynthesis, around 3.5 Ga led to a buildup of its waste product, oxygen, in the atmosphere, leading to the great oxygenation event, beginning around 2.4 Ga. The earliest evidence of eukaryotes dates from 1.85 Ga, while they may have been present earlier, their diversification accelerated when they started using oxygen in their metabolism. Around 1.7 Ga, multicellular organisms began to appear, with differentiated cells performing specialised functions. Sexual reproduction, which involves the fusion of male and female reproductive cells to create a zygote in a process called fertilization is, in contrast to asexual reproduction, the primary method of reproduction for the vast majority of macroscopic organisms, including all eukaryotes; however the origin and evolution of sexual reproduction remain a puzzle for biologists though it did evolve from a common ancestor, a single celled eukaryotic species.
Bilateria, animals with a front and a back, appeared by 555 Ma. The earliest complex land plants date back to around 850 Ma, from carbon isotopes in Precambrian rocks, while algae-like multicellular land plants are dated back to about 1 billion years ago, although evidence suggests that microorganisms formed the earliest terrestrial ecosystems, at least 2.7 Ga. Microorganisms are thought to have paved the way for the inception of land plants in the Ordovician. Land plants were so successful that they are thought to have contributed to the Late Devonian extinction event. Ediacara biota appear during the Ediacaran period, while vertebrates, along with most other modern phyla originated about 525 Ma during the Cambrian explosion. During the Permian period, including the ancestors of mammals, dominated the land, but most of this group became extinct in the Permian–Triassic extinction event 252 Ma. During the recovery from this catastrophe, archosaurs became the most abundant land vertebrates. After the Cretaceous–Paleogene extinction event 66 Ma killed off the non-avian dinosaurs, mammals increased in size and diversity.
Such mass extinctions may have accelerated evolution by providing opportunities for new groups of organisms to diversify. The oldest meteorite fragments found on Earth are about 4.54 billion years old. The Moon has the same composition as Earth's crust but does not contain an iron-rich core like the Earth's. Many scientists think that about 40 million years after the formation of Earth, it collided with a body the size of Mars, throwing into orbit crust material that formed the Moon. Another hypothesis is that the Earth and Moon started to coalesce at the same time but the Earth, having much stronger gravity than the early Moon, attracted all the iron particles in the area; until 2001, the oldest rocks found on Earth were about 3.8 billion years old, leading scientists to estimate that the Earth's surface had been molten until then. Accordingly, they named this part of Earth's history the Hadean. However, analysis of zircons formed 4.4 Ga indicates that Earth's crust solidified about 100 million years after the planet's formation and that the planet acquired oceans and an atmosphere, which may have been capable of supporting life.
Evidence from the Moon indicates that from 4 to 3.8 Ga it suffered a Late Heavy Bombardment by debris, left over from the formation of the Solar System, the Earth should have experienced an heavier bombardment due
History of paleontology
The history of paleontology traces the history of the effort to understand the history of life on Earth by studying the fossil record left behind by living organisms. Since it is concerned with understanding living organisms of the past, paleontology can be considered to be a field of biology, but its historical development has been tied to geology and the effort to understand the history of Earth itself. In ancient times, Herodotus and Strabo wrote about fossils of marine organisms, indicating that land was once under water. During the Middle Ages, fossils were discussed by Persian naturalist Ibn Sina in The Book of Healing, which proposed a theory of petrifying fluids that Albert of Saxony would elaborate on in the 14th century; the Chinese naturalist Shen Kuo would propose a theory of climate change based on evidence from petrified 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.
The nature of fossils and their relationship to life in the past became better understood during the 17th and 18th centuries, at the end of the 18th century, the work of Georges Cuvier had ended a long running debate about the reality of extinction, leading to the emergence of paleontology- in association with comparative anatomy- as a scientific discipline. The expanding knowledge of the fossil record played an increasing role in the development of geology, stratigraphy in particular. In 1822, the word "paleontology" was used by the editor of a French scientific journal to refer to the study of ancient living organisms through fossils, the first half of the 19th century saw geological and paleontological activity become well organized with the growth of geologic societies and museums and an increasing number of professional geologists and fossil specialists; this contributed to a rapid increase in knowledge about the history of life on Earth, progress towards definition of the geologic time scale based on fossil evidence.
As knowledge of life's history continued to improve, it became 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, evolutionary theory; the last half of the 19th century saw a tremendous expansion in paleontological activity in North America. The trend continued in the 20th century with additional regions of the Earth being opened to systematic fossil collection, as demonstrated by a series of important discoveries in China near the end of the 20th century. Many transitional fossils have been discovered, there is now considered to be abundant evidence of how all classes of vertebrates are related, much of it in the form of transitional fossils; 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 a renewed interest in the Cambrian explosion that 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; as early as the 6th century BC, the Greek philosopher Xenophanes of Colophon recognized that some fossil shells were remains of shellfish, which he used to argue that what was at the time dry land was once under the sea. Leonardo da Vinci, in an unpublished notebook concluded that some fossil sea shells were the remains of shellfish. However, in both cases, the fossils were complete remains of shellfish species that resembled living species, were therefore easy to classify. In 1027, the Persian naturalist, Ibn Sina, proposed an explanation of how the stoniness of fossils was caused in The Book of Healing, he modified an idea of Aristotle's. Ibn Sina modified this into the theory of petrifying fluids, elaborated on by Albert of Saxony in the 14th century and was accepted in some form by most naturalists by the 16th century.
Shen Kuo of the Song Dynasty used marine fossils found in the Taihang Mountains to infer the existence of geological processes such as geomorphology and the shifting of seashores over time. Using his observation of preserved petrified bamboos found underground in Yan'an, Shanbei region, Shaanxi province, he argued for a theory of gradual climate change, since Shaanxi was part of a dry climate zone that did not support a habitat for the growth of bamboos; as a result of a new emphasis on observing and cataloging nature, 16th century natural philosophers in Europe began to establish extensive collections of fossil objects, which were stored in specially built cabinets to help organize them. Conrad Gesner published a 1565 work on fossils that contained one of the first detailed descriptions of such a cabinet and collection; the collection belonged to a member of the extensive network of correspondents that Gesner drew on for his works. Such informal correspondence networks among natural philosophers and collectors became important during the course of the 16th century and were direct forerunners of the scientific societies that would begin to form in the 17th century.
These cabinet collections and correspondence networks played an important role in the developme