Life is a characteristic that distinguishes
physical entities that do have biological processes, such as signaling and self-sustaining processes, from those that do not, either because such functions have ceased, or because they never had such functions and are classified as inanimate. Various forms of life exist, such as plants, animals, fungi, protists, archaea, and bacteria. The criteria can at times be ambiguous and may or may not define viruses, viroids, or potential artificial life as "living". Biology is the primary science concerned with the study of life, although many other sciences are involved.
The definition of life is controversial. The current definition is that organisms maintain homeostasis, are composed of cells, undergo metabolism, can grow, adapt to their environment, respond to stimuli, and reproduce. However, many other biological definitions have been proposed, and there are some borderline cases of life, such as viruses. Throughout history, there have been many attempts to define what is meant by "life" and many theories on the properties and emergence of living things, such as materialism, the belief that everything is made out of matter and that life is merely a complex form of it; hylomorphism, the belief that all things are a combination of matter and form, and the form of a living thing is its soul; spontaneous generation, the belief that life repeatedly emerges from non-life; and vitalism, a now largely discredited hypothesis that living organisms possess a "life force" or "vital spark". Modern definitions are more complex, with input from a diversity of scientific disciplines. Biophysicists have proposed many definitions based on chemical systems; there are also some living systems theories, such as the Gaia hypothesis, the idea that the Earth itself is alive. Another theory is that life is the property of ecological systems, and yet another is elaborated in complex systems biology, a branch or subfield of mathematical biology. Abiogenesis describes the natural process of life arising from non-living matter, such as simple organic compounds. Properties common to all organisms include the need for certain core chemical elements to sustain biochemical functions.
Life on Earth first appeared as early as 4.28 billion years ago, soon after ocean formation 4.41 billion years ago, and not long after the formation of the Earth 4.54 billion years ago. Earth's current life may have descended from an RNA world, although RNA-based life may not have been the first. The mechanism by which life began on Earth is unknown, though many hypotheses have been formulated and are often based on the Miller–Urey experiment. The earliest known life forms are microfossils of bacteria. 3.45 billion year old Australian rocks are reported to have contained microorganisms. In 2016, scientists reported identifying a set of 355 genes thought to be present in the last universal common ancestor (LUCA) of all living organisms, already a complex organism and not the first living thing.
Since its primordial beginnings, life on Earth has changed its environment on a geologic time scale. To survive in most ecosystems, life must often adapt to a wide range of conditions. Some microorganisms, called extremophiles, thrive in physically or geochemically extreme environments that are detrimental to most other life on Earth. Aristotle was the first person to classify organisms. Later, Carl Linnaeus introduced his system of binomial nomenclature for the classification of species. Eventually new groups and categories of life were discovered, such as cells and microorganisms, forcing dramatic revisions of the structure of relationships between living organisms. Cells are sometimes considered the smallest units and "building blocks" of life. There are two kinds of cells, prokaryotic and eukaryotic, both of which consist of cytoplasm enclosed within a membrane and contain many biomolecules such as proteins and nucleic acids. Cells reproduce through a process of cell division, in which the parent cell divides into two or more daughter cells.
Though currently only known on Earth, life need not be restricted to it, and many scientists speculate in the existence of extraterrestrial life. Artificial life is a computer simulation or man-made reconstruction of any aspect of life, which is often used to examine systems related to natural life. Death is the permanent termination of all biological functions which sustain an organism, and as such, is the end of its life. Extinction is the process by which an entire group or taxon, normally a species, dies out. Fossils are the preserved remains or traces of organisms.