The History of Life
The history of life on Earth traces the evolutionary history of life and the changes life has undergone since life appeared on the planet, the study of which is a branch of palaeontology. The history of life on Earth traces back to the emergence of life itself, some 3.5 billion years ago, and even earlier in the case of certain key evolutionary convergence events. Life arose independently at several locations throughout the early Earth. Some of these were hydrothermal vents located near the mid-ocean ridges where hot, mineral-rich water flowed to the surface, but there is no evidence that life ever existed at any of these locations. Over the course of time the continents formed from the crust of the Earth and the oceans gathered water into large bodies. During this time, the early atmosphere of the Earth was made up of nitrogen, carbon dioxide, and tiny amounts of other gases, but with traces of oxygen. The emergence of life in the early oceans is thought to have been an essential prerequisite for later evolutionary developments on the land. The history of life on Earth may be divided into three distinct periods: the early history of life on Earth (3.5–2.5 billion years ago), the middle history of life on Earth (2.5–0.5 billion years ago), and the late history of life on Earth (0.5–present day). The early history of life on Earth is divided into the Archaean and Proterozoic eons, which are also known as the Precambrian, with the earliest known life appearing towards the end of the Precambrian and the first multicellular life appearing in the Cambrian explosion. The late history of life on Earth is divided into the Phanerozoic eon, which features the rise of multicellular life, and a series of periods from 541 million years ago to the present, which includes the Paleozoic, Mesozoic, and Cenozoic eras. The following timeline of selected major events in the history of life on Earth represents a broad overview of the major developments in the biosphere: The study of prehistoric life, and the development of a fossil record, is the science of paleontology. The first evidence of life on Earth was in the form of microfossils of bacteria and algae found in the Black Sea sediment in 1783, and later in 1785 by Georges Cuvier. The record of life forms on Earth has been studied since the beginning of the 19th century. Since then, paleontology has steadily expanded to include the study of evidence of biotic change through time. It is divided into two main branches: archaeology and geology. The history of life on Earth has been affected by the evolution of life itself and its interactions with the environment and the effects of plate tectonics. During the Precambrian, the atmosphere of the Earth was composed primarily of carbon dioxide (CO) with a much smaller amount of nitrogen (N) and much less oxygen (O). The atmosphere contained approximately 0.03% oxygen by volume, and no free oxygen was in the atmosphere. The first life forms on land (eukaryotes) appeared about 1 billion years ago, and they began to produce oxygen as a byproduct of photosynthesis, and eventually oxygen became the principal gas in the atmosphere. The following timeline shows the most recent 500 million years of Earth's history. The Phanerozoic Eon spans from 541 million years ago to the present day. It is the time period in Earth's history where the great bulk of the planet's species lived. The term was coined in 1870 by the German geologist Friedrich Albrecht Karl Krause, and it is derived from the Greek words ??????? ("phanerós"), meaning visible, and ??? ("z??"), meaning life. In this time, hard-shelled animals and plants evolved, and the first vertebrates, amphibians, and arthropods appeared on the scene. The Phanerozoic is divided into three eras: the Paleozoic, Mesozoic, and Cenozoic (see table below). The Paleozoic Era began with the Cambrian Period, and lasted until the great Permian extinction event, which wiped out approximately 90% of marine species and 70% of land species. The Paleozoic is followed by the Mesozoic Era, which featured the rise of the dinosaurs, followed by the Cenozoic Era, which features the rise of mammals, including the emergence of humans. The following table presents a simplified outline of the major events of the Phanerozoic Eon: "Timeline of the History of Life on Earth" Most multicellular organisms, or animals, are composed of many cells. The cells work together to
