Paleontology or palaeontology (//, // or //, //) is the scientific study of life existent prior to, and at times including, the start of the Holocene Epoch roughly 11,700 years before present. It includes the study of fossils to determine organisms' evolution and interactions with each additional and their environments (their paleoecology). Paleontological observations have been documented as far back as the 5th century BC. The science became established in the 18th century as a consequence of Georges Cuvier's work on comparative anatomy, and developed rapidly in the 19th century. The term itself originates from Greek παλαιός, palaios, i.e. "old, ancient", ὄν, on (gen. ontos), i.e. "being, creature" and λόγος, logos, i.e. "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 palaeontologists to discover much of the evolutionary history of life, almost all the way back to when Earth became capable of supporting life, about . As knowledge has increased, palaeontology has developed specialised sub-divisions, a few of which focus on different types of fossil organisms while others study ecology and environmental history, like 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: at times adjacent rock layers allow radiometric dating, which provides absolute dates that are accurate to within 0.5%, but more most often palaeontologists have to rely on relative dating by solving the "jigsaw puzzles" of biostratigraphy. Classifying ancient organisms is additionally difficult, as a large number of don't fit well into the Linnean taxonomy that's commonly used for classifying living organisms, and palaeontologists more most often use cladistics to draw up evolutionary "family trees". The final quarter of the twentieth century saw the development of molecular phylogenetics, which investigates how closely organisms are related by measuring how similar the DNA is in their genomes. Molecular phylogenetics has additionally been used to estimate the dates when species diverged, but there's controversy about the reliability of the molecular clock on which such estimates depend.
The simplest definition is "the study of ancient life". Paleontology seeks information about several aspects of past organisms: "their identity and origin, their environment and evolution, and what they can tell us about the Earth's organic and inorganic past".
A historical science
Paleontology is one of the historical sciences, along with archaeology, geology, astronomy, cosmology, philology and history itself. This means that it aims to describe phenomena of the past and reconstruct their causes. Hence it has three main elements: description of the phenomena; developing a general theory about the causes of numerous types of change; and applying those theories to specific facts.
When trying to explain past phenomena, palaeontologists and additional historical scientists most often construct a set of hypotheses about the causes and then look for a smoking gun, a piece of evidence that indicates that one hypothesis is a better explanation than others. Sometimes the smoking gun is detected by a fortunate accident throughout additional research. For instance, the discovery by Luis Alvarez and Walter Alvarez of an iridium-rich layer at the Cretaceous–Tertiary boundary made asteroid impact and volcanism the most favoured explanations for the Cretaceous–Paleogene extinction event.
The additional main type of science is experimental science, which is most often said to work by conducting experiments to disprove hypotheses about the workings and causes of natural phenomena – note that this approach can't confirm a hypothesis is correct, after a few later experiment might disprove it. Notwithstanding when confronted with totally unexpected phenomena, like the first evidence for invisible radiation, experimental scientists most often 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 after palaeontology focuses on the record of past life but its main source of evidence is fossils, which are found in rocks. For historical reasons palaeontology is part of the geology departments of a large number of universities, because in the nineteenth century and early twentieth century geology departments found paleontological evidence important for estimating the ages of rocks while biology departments showed little interest.
Paleontology additionally has a few overlap with archaeology, which primarily works with objects made by humans and with human remains, while palaeontologists are interested in the characteristics and evolution of humans as organisms. When dealing with evidence about humans, archaeologists and palaeontologists might work together – for example palaeontologists might identify animal or plant fossils around an archaeological site, to discover what the people who lived there ate; or they might analyse the climate at the time when the site was inhabited by humans.
In addition palaeontology most often uses techniques derived from additional sciences, including biology, osteology, ecology, chemistry, physics and mathematics. For instance, geochemical signatures from rocks might help to discover when life first arose on Earth, and analyses of carbon isotope ratios might help to identify climate changes and even to explain major transitions like the Permian–Triassic extinction event. A relatively recent discipline, molecular phylogenetics, most often helps by using comparisons of different modern organisms' DNA and RNA to re-construct evolutionary "family trees"; it has additionally been used to estimate the dates of important evolutionary developments, although this approach is controversial because of doubts about the reliability of the "molecular clock". Techniques developed in engineering have been used to analyse how ancient organisms might have worked, for example how fast Tyrannosaurus could move and how powerful its bite was. It is relatively commonplace to study fossils using X-ray microtomography A combination of paleontology, biology, and archaeology, paleoneurology is the study of endocranial casts (or endocasts) of species related to humans to learn about the evolution of human brains.
Paleontology even contributes to astrobiology, the investigation of possible life on additional planets, by developing models of how life might have arisen and by providing techniques for detecting evidence of life.
As knowledge has increased, palaeontology has developed specialised subdivisions. Vertebrate paleontology concentrates on fossils of vertebrates, from the earliest fish to the immediate ancestors of modern mammals. Invertebrate paleontology deals with fossils of invertebrates like molluscs, arthropods, annelid worms and echinoderms. Paleobotany focuses on the study of fossil plants, but traditionally includes the study of fossil algae and fungi. Palynology, the study of pollen and spores produced by land plants and protists, straddles the border between palaeontology and botany, as it deals with both living and fossil organisms. Micropaleontology deals with all microscopic fossil organisms, regardless of the group to which they belong.
Instead of focusing on individual organisms, paleoecology examines the interactions between different organisms, like their places in food chains, and the two-way interaction between organisms and their environment. One example is the development of oxygenic photosynthesis by bacteria, which hugely increased the productivity and diversity of ecosystems. This additionally caused the oxygenation of the atmosphere. Together, these were a prerequisite for the evolution of the most complex eukaryotic cells, from which all multicellular organisms are built.
Paleoclimatology, although at times treated as part of paleoecology, focuses more on the history of Earth's climate and the mechanisms that have changed it – which have at times 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 palaeontologists and geologists. Biogeography studies the spatial distribution of organisms, and is additionally linked to geology, which explains how Earth's geography has changed over time.