__Life__ is a characteristic that distinguishes physical body that have biological processes, such as Cell signaling and self-sustainability processes, from those that do not, either because such functions have ceased (they have Death), or because they never had such functions and are classified as inanimate. Biology is the science concerned with the study of life - wikipedia
Various forms of life exist, such as plants, animals, fungus, protists, archaea, and bacteria. The criteria can at times be ambiguous and may or may not define viruses, viroids, or potential Synthetic life as "living".
The definition of life is controversial. The current definition is that organisms are Open system (thermodynamics) that maintain homeostasis, are composed of Cell (biology), have a Biological life cycle, undergo metabolism, can cell growth, Adaptation to their environment, respond to stimulus (physiology), reproduction and Evolution. However, several other biological definitions have been proposed, and there are some borderline cases of life, such as viruses or viroids. In the past, there have been many attempts to define what is meant by "life" through obsolete concepts such as odic force, hylomorphism, spontaneous generation and vitalism, that have now been disproved by Biological research. 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 Earliest known life forms as early as 4.28 billion years ago, soon after Origin of water on Earth#Water in the development of Earth, and not long after the Age 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 abiogenesis 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 taxonomy (biology) organisms. Later, Carl Linnaeus introduced Linnean taxonomy 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. The cell is considered the structural and functional unit of life. There are two kinds of cells, prokaryote and eukaryote, 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 process 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 trace fossil of organisms.
# See also