Paleontology or Palaeontology is the scientific study of prehistoric life, including the study of fossils to determine the organisms evolution and interactions with each other and their environments. Paleontological observations have been documented as far back as 5th century BC. The science became established in the 18th century as a result of Georges Cuvier’s work on comparative anatomy, and it developed quickly within the 19th century. The term itself comes from Greek palaios, meaning “old, ancient”.

It lies on the border between biology and geology, but differs from archaeology in that it does not include the study of morphologically modern humans. It now utilizes techniques drawn from a wide range of sciences, including biochemistry, engineering, and mathematics. The usage of all these techniques has allowed paleontologists to discover much of the evolutionary history of life, almost all the way back to when the Earth became capable of supporting life, about 3,800 million years ago. As the knowledge increased, paleontology has developed specialized sub-divisions, some of which focus on different types of fossil organisms, while other study environmental history and ecology, such as ancient climates.

Trace fossils and body fossils are the principle types of evidence about ancient life, and geochemical evidence has aided in deciphering the evolution of life before there were organisms large enough to leave fossils. Estimating the dates of these remains is crucial but hard: sometimes adjacent rock layers permit radiometric dating, which offers absolute dates that are accurate to within 0.5 percent, but more often paleontologist have to rely on relative dating. Classifying ancient organisms is also hard, as many do not fit well into the Linnean taxonomy that is most commonly used for classifying living organisms, and the paleontologists more often times 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 how similar the DNA is in the genomes. Molecular phylogenetics has also been utilized to estimate the dates when the species diverged, but there is controversy about how reliable the molecular clocks on which such estimates depend.

This study is one of the historical sciences, in addition to archaeology, biology, geology, cosmology, philology and history itself. This means that its goal is to describe phenomena of the past and reconstruct their causes. Therefore, it has three main elements: description of the phenomena; developing an over-all theory about the causes of a variety of types of change; and applying those theories to specific facts.

When attempting to describe past phenomena, paleontologists and other historical sciences frequently construct a set of hypotheses about the causes and then look for a smoking gun which is a piece of evidence that indicates that one hypotheses is a better explanation than others. Occasionally, the smoking gun is discovered by a fortunate accident during some other research.

The other main type of science is experimental science, which is often said to function by conducting experiments to disprove hypotheses about the workings and causes of natural phenomena. However, when confronted with totally unexpected phenomena, for example, the first evidence for invisible radiation, experimental scientists frequently use the same approach as historical scientists: construct a set of hypotheses about the causes, and then look for a “smoking gun”.

Paleontology lies on the boundary between biology and geology since paleontology focuses on the record of past life but its main source of evidence is fossils, which can be found in rocks. For historical reasons, paleontology is a part of the geology departments of many universities, because in the 19th century and early 20th century, geology departments found paleontological evidence significant for estimating the ages of rocks while biology departments showed little interest.

It also has some overlap with archaeology, which mostly works with objects made by humans and with human remains, while paleontologists are interested in the traits and evolution of humans as organisms. When dealing with evidence about humans, the archaeologists and paleontologists might work together. Additionally, paleontology frequently uses techniques that come from other sciences, including ecology, biology, chemistry, physics, and mathematics.

A combination of paleontology, biology, and archaeology, paleoneurology is the study of endocranial casts of species that are related to humans to learn about the evolution of the human brain.

It even contributes to astrobiology, which is the investigation of possible life on other plants, by developing models of how life might have arisen and by providing techniques for detecting evidence of life.

As knowledge increased, paleontology has established specialized subdivisions. Vertebrate paleontology concentrates on fossils of vertebrates, from the earliest fish to the direct ancestors of modern mammals. Invertebrate paleontology studies fossils of invertebrates such as arthropods, mollusks, annelid worms, and echinoderms. Paleobotany focuses on the study of fossil plants, but usually includes the study of fossil algae and fungi. Palynology, the study of pollen and spores that are produced by land plants and protists, straddles the border between paleontology and botany, as it deals with both the living and fossilized organisms. Micropaleontology deals with all microscopic fossil organisms with no regard of the group in which they belong.

Instead of focusing on individual organisms, paleoecology observes the interactions between different organisms, such as their places in food chains, and the two-way interaction between organisms and their environment. Paleoclimatology, although occasionally treated as a part of paleoecology, focuses more on the history of the Earth’s climate and the mechanisms that have altered it, which have sometimes included evolutionary developments.

Biostratigraphy, the use of fossils to work out the chronological order in which rocks were created, is useful to both geologists and paleontologists. Biogeography studies the spatial distribution of organisms, and is also linked to geology, which explains how the Earth’s geography has altered over time.

The evolutionary history of life stretches back to over 3,000 million years ago, maybe as far as 3,800 million years ago. The Earth formed about 4,570 million years ago and, after a collision that formed the Moon about 40 million years later, might have cooled quickly enough to have oceans and an atmosphere about 4,400 million years ago. However, there is evidence on the Moon of a Late Heavy Bombardment from 4,000 to 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 argued, 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 suggested that life on Earth was “seeded” from somewhere else, but the majority of research concentrates on a variety of explanations of how life could have arisen independently on Earth.

For roughly 2,000 million years microbial mats, multi-layered colonies of different types of bacteria, where the dominant life on Earth. The evolution of oxygenic photosynthesis enabled them to play the major role in the oxygenation of the atmosphere from roughly 2,400 million years ago. Their effectiveness as nurseries of evolution increased as a result of the change in the atmosphere. While eukaryotes, cells with complicated internal structures, may have been present earlier, their evolution sped up when they acquired the ability to transform oxygen from a poison to a powerful source of energy in their metabolism. This revolution might have come from primitive eukaryotes capturing oxygen-powered bacteria as endosymbionts and transforming them into organelles that are called mitochondria. The earliest evidentiary support of complex eukaryotes with organelles such as mitochondria dates from 1,850 million years ago.

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

The earliest known animals are cnidarians from roughly 580 million years ago, but these look so modern that the earliest animals must have appeared before then. Early fossils of animals are rare due to them not developing mineralized hard parts that fossilize easily until roughly 548 million years ago. The earliest modern looking bilaterian animals come about in the Early Cambrian, along with several “weird wonders” that have little obvious resemblance to any modern animals. There is a long-running debate about whether or not this Cambrian explosion was truly a very rapid period of evolutionary experimentation; alternative views are that modern looking animals began evolving at an earlier time but fossils of their precursors haven’t 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 that had jaws came about in the Late Ordovician.

The spread of life from water to land needed organisms to solve several issues, including protection against drying out and supporting themselves against the gravity. The earliest evidentiary support of land plants and land invertebrates date back about 476 million years ago and 490 million years ago respectively. The lineage that offered land vertebrates evolved later but very rapidly between 370 million years ago and 360 million years ago, recent discoveries have overturned some earlier ideas about the history and driving forces behind their evolution. Land plants were so successful that they caused an ecological crisis within the Late Devonian, until the evolution and spread of fungi that was able to digest dead wood.

In the Permian period synapsids, including the ancestors of mammals, might have dominated land environments, but the Permian-Triassic extinction event that happened roughly 251 million years ago came very close to wiping out complex life. The extinctions were apparently sudden, at least among the 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 leading 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 and mainly nocturnal insectivores, but this apparent set-back might have accelerated the development of mammalian characteristics such as endothermy and hair. After the Cretaceous-Paleogene extinction event 65 million years ago wiped off the non-avian dinosaurs, the mammals increased rapidly in size and diversity, and some took to the air and sea.

Some fossil evidence indicates that flowering plants appeared and rapidly diversified in the Early Cretaceous, between 130 and 90 million years ago. Their quick rise to dominance of terrestrial ecosystems is considered to have been propelled by coevolution with pollinating insects. Social insects came about around the same time and, although they account for only minute portions of the insect “family tree”, now form over 50 percent 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 percent as big as modern humans, there are signs of a steady increase in the size of the brains 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 around the world at the same time as a result of interbreeding.

Life on earth has grieved occasional mass extinctions at least since 542 million years ago. Although they are disasters at the time, mass extinctions have sometimes accelerated the evolution of life. When dominance of specific ecological niches passes from one group of organisms to another, it is rarely due to the new group being “superior” to the old and normally is because an extinction event eliminates the old dominant group and makes way for the new one.

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

Although paleontology became established around 1800, earlier thinkers had noticed some aspects of the fossil record. The ancient Greek philosopher Xenophanes concluded from fossil seas 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 suggested a theory of petrifying fluids on which Albert of Saxony elaborated in the 14th century. The Chinese naturalist Shen Kuo suggested a theory of climate change based on the presence of petrified bamboo in areas that in his time were too dry for bamboo.

In early modern Europe, the systematic study of fossils came about 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 fossil animals looked like 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, especially stratigraphy.

The first half of the 19th century experienced 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. The interest increased for reasons that weren’t purely scientific, as geology and paleontology helped industrialists to find and exploit natural resources such as coal.

This contributed to a quick increase in knowledge about the history of life on Earth and to progress the definition of the geologic time scale, largely based on fossil evidence. In 1882, Henri Marie Ducratay de Blanville, editor of Journal de Physique, coined the term paleontology to refer to the study of ancient living organisms via fossils. As knowledge of life’s history continued to get better, it became increasingly obvious that there had been some kind of successive order to the development of life. This initiated early evolutionary theories on the transmutation of species. After Charles Darwin published Origin of Species in 1859, much of the focus of paleontology altered to understanding evolutionary paths, including human evolution, and the evolutionary theory.

The last half of the 19th century experienced a tremendous expansion in paleontological activity, particularly in North America. The trend continued in the 20th century with additional areas of the Earth being opened to systematic fossil collection. Fossils that were found in China close to the end of the 20th century have been especially significant as they have provided new information about the earliest evolution of animals, dinosaurs, early fish, and the evolution of birds. The last few decades of the 20th century experienced a renewed interest in mass extinctions and the role they play 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 the majority of animal phyla. The discovery of fossils of the Ediacaran biota and developments in paleobiology stretched the knowledge about the history of life back far before the Cambrian.

Increasing awareness of Gregor Mendel’s pioneering work in genetics first led to the development of population genetics and then in the mid-20th century, led to the modern evolutionary synthesis, which describes 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 the 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 that come from biochemistry, began to make an impact, especially when it was suggested that the human lineage had diverged from apes much more recently than what was originally though at the time. Although this early study compared proteins from apes and humans, the majority of molecular phylogenetics research is now based on comparisons of RNA and DNA.

Image Caption: Tyrannosaurus rex, Palais de la Découverte, Paris. Credit: David Monniaux/Wikipedia (CC BY-SA 3.0)