|Life (Biota / Vitae / Eobionti)|
|Plants in the Rwenzori Mountains, Uganda|
|Domains and kingdoms|
Life on Earth:
|This article is one of a series on:|
|Life in the Universe|
|Life in the Solar System|
|Life on Venus|
|Life on Earth|
|Life on Mars|
|Life on Europa|
|Life on Titan|
Life outside the Solar System
|Circumstellar habitable zone|
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 (death), or because they lack 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.
Throughout history there have been many theories about life including materialism, hylomorphism and vitalism. Even today it is a challenge for scientists and philosophers to define life. The smallest contiguous unit of life is called an organism. Organisms are composed of one or more cells, undergo metabolism, maintain homeostasis, can grow, respond to stimuli, reproduce (either sexually or asexually) and, through evolution, adapt to their environment in successive generations. A diverse array of living organisms can be found in the biosphere of Earth, and the properties common to these organisms are a carbon- and water-based cellular form with complex organization and heritable genetic information.
Abiogenesis is the natural process of life arising from non-living matter, such as simple organic compounds. The earliest life on Earth arose at least 3.5 billion years ago, during the Eoarchean Era when sufficient crust had solidified following the molten Hadean Eon. The earliest physical evidence of life on Earth is biogenic graphite from 3.7 billion-year-old metasedimentary rocks found in Western Greenland and microbial mat fossils in 3.48 billion-year-old sandstone found in Western Australia. Some theories, such as the Late Heavy Bombardment theory, suggest that life on Earth may have started even earlier, as early as 4.1-4.4 billion years ago. According to one of the researchers, "If life arose relatively quickly on Earth ... then it could be common in the universe."
The mechanism by which life began on Earth is unknown, although many hypotheses have been formulated. Since emerging, life has evolved into a variety of forms, which have been classified into a hierarchy of taxa. Life can survive and thrive in a wide range of conditions. Nonetheless, it is estimated that 99 percent of all species, amounting to over five billion species, that ever lived on Earth are extinct. Estimates on the number of Earth's current species range from 10 million to 14 million, of which about 1.2 million have been documented and over 86 percent have not yet been described. The chemistry leading to life may have begun shortly after the Big Bang, 13.8 billion years ago, during a habitable epoch when the Universe was only 10–17 million years old. Though life is confirmed only on the Earth, many think that extraterrestrial life is not only plausible, but probable or inevitable. Other planets and moons in the Solar System and other planetary systems are being examined for evidence of having once supported simple life, and projects such as SETI are trying to detect radio transmissions from possible alien civilizations.
Some of the earliest theories of life were materialist, holding that all that exists is matter, and that life is merely a complex form or arrangement of matter. Empedocles (430 BC) argued that everything in the universe is made up of a combination of four eternal "elements" or "roots of all": earth, water, air, and fire. All change is explained by the arrangement and rearrangement of these four elements. The various forms of life are caused by an appropriate mixture of elements.
Democritus (460 BC) thought that the essential characteristic of life is having a soul (psyche). Like other ancient writers, he was attempting to explain what makes something a living thing. His explanation was that fiery atoms make a soul in exactly the same way atoms and void account for any other thing. He elaborates on fire because of the apparent connection between life and heat, and because fire moves.
Plato's world of eternal and unchanging Forms, imperfectly represented in matter by a divine Artisan, contrasts sharply with the various mechanistic Weltanschauungen, of which atomism was, by the fourth century at least, the most prominent ... This debate persisted throughout the ancient world. Atomistic mechanism got a shot in the arm from Epicurus ... while the Stoics adopted a divine teleology ... The choice seems simple: either show how a structured, regular world could arise out of undirected processes, or inject intelligence into the system.— R. J. Hankinson, Cause and Explanation in Ancient Greek Thought
The mechanistic materialism that originated in ancient Greece was revived and revised by the French philosopher René Descartes, who held that animals and humans were assemblages of parts that together functioned as a machine. In the 19th century, the advances in cell theory in biological science encouraged this view. The evolutionary theory of Charles Darwin (1859) is a mechanistic explanation for the origin of species by means of natural selection.
Hylomorphism is a theory, originating with Aristotle (322 BC), that all things are a combination of matter and form. Biology was one of his main interests, and there is extensive biological material in his extant writings. In this view, all things in the material universe have both matter and form, and the form of a living thing is its soul (Greek psyche, Latin anima). There are three kinds of souls: the vegetative soul of plants, which causes them to grow and decay and nourish themselves, but does not cause motion and sensation; the animal soul, which causes animals to move and feel; and the rational soul, which is the source of consciousness and reasoning, which (Aristotle believed) is found only in man. Each higher soul has all the attributes of the lower one. Aristotle believed that while matter can exist without form, form cannot exist without matter, and therefore the soul cannot exist without the body.
This account is consistent with teleological explanations of life, which account for phenomena in terms of purpose or goal-directedness. Thus, the whiteness of the polar bear's coat is explained by its purpose of camouflage. The direction of causality (from the future to the past) is in contradiction with the scientific evidence for natural selection, which explains the consequence in terms of a prior cause. Biological features are explained not by looking at future optimal results, but by looking at the past evolutionary history of a species, which led to the natural selection of the features in question.
Vitalism is the belief that the life-principle is non-material. This originated with Georg Ernst Stahl (17th century), and remained popular until the middle of the 19th century. It appealed to philosophers such as Henri Bergson, Friedrich Nietzsche, Wilhelm Dilthey, anatomists like Marie François Xavier Bichat, and chemists like Justus von Liebig. Vitalism included the idea that there was a fundamental difference between organic and inorganic material, and the belief that organic material can only be derived from living things. This was disproved in 1828, when Friedrich Wöhler prepared urea from inorganic materials. This Wöhler synthesis is considered the starting point of modern organic chemistry. It is of historical significance because for the first time an organic compound was produced in inorganic reactions.
During the 1850s, Hermann von Helmholtz, anticipated by Julius Robert von Mayer, demonstrated that no energy is lost in muscle movement, suggesting that there were no "vital forces" necessary to move a muscle. These results led to the abandonment of scientific interest in vitalistic theories, although the belief lingered on in pseudoscientific theories such as homeopathy, which interprets diseases and sickness as caused by disturbances in a hypothetical vital force or life force.
It is a challenge for scientists and philosophers to define life. This is partially because life is a process, not a substance. Any definition must be general enough to both encompass all known life and any unknown life that may be different from life on Earth.
Since there is no unequivocal definition of life, the current understanding is descriptive. Life is considered a characteristic of something that exhibits all or most of the following traits:
- Homeostasis: Regulation of the internal environment to maintain a constant state; for example, sweating to reduce temperature.
- Organization: Being structurally composed of one or more cells — the basic units of life.
- Metabolism: Transformation of energy by converting chemicals and energy into cellular components (anabolism) and decomposing organic matter (catabolism). Living things require energy to maintain internal organization (homeostasis) and to produce the other phenomena associated with life.
- Growth: Maintenance of a higher rate of anabolism than catabolism. A growing organism increases in size in all of its parts, rather than simply accumulating matter.
- Adaptation: The ability to change over time in response to the environment. This ability is fundamental to the process of evolution and is determined by the organism's heredity, diet, and external factors.
- Response to stimuli: A response can take many forms, from the contraction of a unicellular organism to external chemicals, to complex reactions involving all the senses of multicellular organisms. A response is often expressed by motion; for example, the leaves of a plant turning toward the sun (phototropism), and chemotaxis.
- Reproduction: The ability to produce new individual organisms, either asexually from a single parent organism, or sexually from two parent organisms, "with an error rate below the sustainability threshold."
To reflect the minimum phenomena required, other biological definitions of life have been proposed, many of these are based upon chemical systems. Biophysicists have commented that living things function on negative entropy. In other words, living processes can be viewed as a delay of the spontaneous diffusion or dispersion of the internal energy of biological molecules towards more potential microstates. In more detail, according to physicists such as John Bernal, Erwin Schrödinger, Eugene Wigner, and John Avery, life is a member of the class of phenomena that are open or continuous systems able to decrease their internal entropy at the expense of substances or free energy taken in from the environment and subsequently rejected in a degraded form. At a higher level, living beings are thermodynamic systems that have an organized molecular structure. That is, life is matter that can reproduce itself and evolve as survival dictates. Hence, life is a self-sustained chemical system capable of undergoing Darwinian evolution.
Others take a systemic viewpoint that does not necessarily depend on molecular chemistry. One systemic definition of life is that living things are self-organizing and autopoietic (self-producing). Variations of this definition include Stuart Kauffman's definition as an autonomous agent or a multi-agent system capable of reproducing itself or themselves, and of completing at least one thermodynamic work cycle.
Viruses are most often considered replicators rather than forms of life. They have been described as "organisms at the edge of life," since they possess genes, evolve by natural selection, and replicate by creating multiple copies of themselves through self-assembly. However, viruses do not metabolize and they require a host cell to make new products. Virus self-assembly within host cells has implications for the study of the origin of life, as it may support the hypothesis that life could have started as self-assembling organic molecules.
Living systems theories
The idea that the Earth is alive is found in philosophy and religion, but the first scientific discussion of it was by the Scottish scientist James Hutton. In 1785, he stated that the Earth was a superorganism and that its proper study should be physiology. Hutton is considered the father of geology, but his idea of a living Earth was forgotten in the intense reductionism of the 19th century. The Gaia hypothesis, proposed in the 1960s by scientist James Lovelock, suggests that life on Earth functions as a single organism that defines and maintains environmental conditions necessary for its survival.
The first attempt at a general living systems theory for explaining the nature of life was in 1978, by American biologist James Grier Miller. Such a general theory, arising out of the ecological and biological sciences, attempts to map general principles for how all living systems work. Instead of examining phenomena by attempting to break things down into component parts, a general living systems theory explores phenomena in terms of dynamic patterns of the relationships of organisms with their environment. Robert Rosen (1991) built on this by defining a system component as "a unit of organization; a part with a function, i.e., a definite relation between part and whole." From this and other starting concepts, he developed a "relational theory of systems" that attempts to explain the special properties of life. Specifically, he identified the "nonfractionability of components in an organism" as the fundamental difference between living systems and "biological machines."
A systems view of life treats environmental fluxes and biological fluxes together as a "reciprocity of influence", and a reciprocal relation with environment is arguably as important for understanding life as it is for understanding ecosystems. As Harold J. Morowitz (1992) explains it, life is a property of an ecological system rather than a single organism or species. He argues that an ecosystemic definition of life is preferable to a strictly biochemical or physical one. Robert Ulanowicz (2009) highlights mutualism as the key to understand the systemic, order-generating behavior of life and ecosystems.
Complex systems biology (CSB) is a field of science that studies the emergence of complexity in functional organisms from the viewpoint of dynamic systems theory. The latter is often called also systems biology and aims to understand the most fundamental aspects of life. A closely related approach to CSB and systems biology, called relational biology, is concerned mainly with understanding life processes in terms of the most important relations, and categories of such relations among the essential functional components of organisms; for multicellular organisms, this has been defined as "categorical biology", or a model representation of organisms as a category theory of biological relations, and also an algebraic topology of the functional organization of living organisms in terms of their dynamic, complex networks of metabolic, genetic, epigenetic processes and signaling pathways.
It has also been argued that the evolution of order in living systems and certain physical systems obey a common fundamental principle termed the Darwinian dynamic. The Darwinian dynamic was formulated by first considering how macroscopic order is generated in a simple non-biological system far from thermodynamic equilibrium, and then extending consideration to short, replicating RNA molecules. The underlying order generating process for both types of system was concluded to be basically similar.
Another systemic definition, called the Operator theory, proposes that 'life is a general term for the presence of the typical closures found in organisms; the typical closures are a membrane and an autocatalytic set in the cell', and also proposes that an organism is 'any system with an organisation that complies with an operator type that is at least as complex as the cell. Life can also be modeled as a network of inferior negative feedbacks of regulatory mechanisms subordinated to a superior positive feedback formed by the potential of expansion and reproduction.