Life is a characteristic distinguishing physical entities having 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 also some borderline cases, such as viruses. Biophysicists have also proposed some definitions, many being based on chemical systems. There are also some living systems theories, such as the Gaia hypothesis, the idea that the Earth is alive; the former first developed by James Grier Miller. Another one is that life is the property of ecological systems, and yet another is the complex systems biology, a branch or subfield of mathematical biology. Some other systemic definitions includes the theory involving the darwinian dynamic, and the operator theory. However, throughout history, there have been many other theories and definitions about life 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 emerge from non-life; and vitalism, a discredited scientific hypothesis that living organisms possess a "life force" or "vital spark". Abiogenesis is the natural process of life arising from non-living matter, such as simple organic compounds. Life on Earth arose 3.8–4.1 billion years ago. It is widely accepted that current life on Earth descended from an RNA world, but RNA based life may not have been the first. The mechanism by which life began on Earth is unknown, although many hypotheses have been formulated, most based on the Miller–Urey experiment. In July 2016, scientists reported identifying a set of 355 genes from the Last Universal Common Ancestor (LUCA) of all organisms living on Earth.
Since appearing, life on Earth has changed its environment on a geologic time scale. To survive in most ecosystems, life can adapt and thrive in a wide range of conditions. Some organisms, called extremophiles, can thrive in physically or geochemically extreme conditions that are detrimental to most other life on Earth. Properties common to all organisms are the need for certain core chemical elements needed for biochemical functioning. Aristotle was the first person to classify organisms. Later, Carl Linnaeus introduced his system of binomial nomenclature for the classification of species. Fungi was later classified as its own kingdom. Eventually new groups of life were revealed, such as cells and microorganisms, and even non-cellular reproducing agents, such as viruses and viroids. Cells are the smallest units of life, often called the "building blocks of life". There are two kind of cells, prokaryotic and eukaryotic. Cells consist of cytoplasm enclosed within a membrane, which contains 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 only known on Earth, many believe in the existence of extraterrestrial life. Artificial life is a computer simulation of any aspect of life, which is used to examine systems related to 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 a group of taxa, normally a species, dies out. Fossils are the preserved remains or traces of organisms.
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