Life
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| Diverse forms of life on a coral reef | |
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Life on Earth: |
Life is the capacity in matter, formed of one or more units called cells, for processes such as cell signaling, homeostasis, metabolism, cell growth, adaptation, response to stimuli, and reproduction. All life eventually reaches a state of death. Many philosophical definitions of living systems have been proposed, such as self-organizing systems. Defining life is further complicated by viruses, which replicate only in host cells, and the possibility of extraterrestrial life, which could be very different from life on Earth. Life exists all over the Earth in air, water, and soil, with many ecosystems forming the biosphere. Some of these are harsh environments occupied only by extremophiles. The life in a particular ecosystem is called its biota.
Life has been studied since ancient times, with theories such as Empedocles's materialism asserting that it was composed of four eternal elements, and Aristotle's hylomorphism asserting that living things have souls and embody both form and matter. Life originated at least 3.5 billion years ago, resulting in a universal common ancestor. This evolved into all the species that exist now, by way of many extinct species, some of which have left traces as fossils. Attempts to classify living things, too, began with Aristotle. Modern classification began with Carl Linnaeus's system of binomial nomenclature in the 1740s.
Living things are composed of biochemical molecules, formed mainly from a few core chemical elements. All living things contain two types of macromolecules, proteins and nucleic acids, the latter usually both DNA and RNA: these carry the information needed by each species, including the instructions to make each type of protein. The proteins, in turn, serve as the machinery which carries out the many chemical processes of life. The cell is the structural and functional unit of life. Smaller organisms, including prokaryotes (bacteria and archaea), consist of small single cells. Larger organisms, mainly eukaryotes, can consist of single cells or may be multicellular with more complex structure. Life is only known to exist on Earth, but extraterrestrial life is thought to be probable. Artificial life is being simulated and explored by scientists and engineers.
Definitions
Challenge
The definition of life has long been a challenge for scientists and philosophers.[5][6][7] This is partially because life is a process, not a substance.[8][9][10] This is complicated by a lack of knowledge of the characteristics of living entities, if any, that may have developed outside Earth.[11][12] Philosophical definitions of life have also been put forward, with similar difficulties in how to distinguish living things from the non-living.[13] Legal definitions of life have been debated, though these generally focus on the decision to declare a human dead, and the legal ramifications of this decision.[14] At least 123 definitions of life have been compiled.[15]
A biota is the assemblage of living things, especially the animals and plants, that inhabit a specific place and time,[16] such as an ecosystem or biome; thus, the goal of nature conservation is to preserve the biota of an ecosystem.[17]
Descriptive
Since there is no consensus for a definition of life, most current definitions in biology, the scientific study of life, are descriptive. Life is considered a characteristic of something that preserves, furthers, or reinforces its existence in the given environment. This implies all or most of the following traits:[7][18][19][20][21][22]
- Homeostasis: regulation of the internal environment to maintain a constant state; for example, sweating to reduce temperature.
- Organisation: being structurally composed of one or more cells – the basic units of life.
- Metabolism: transformation of energy, used to convert chemicals into cellular components (anabolism) and to decompose organic matter (catabolism). Living things require energy for homeostasis and other activities.
- Growth: maintenance of a higher rate of anabolism than catabolism. A growing organism increases in size and structure.
- Adaptation: the evolutionary process whereby an organism becomes better able to live in its habitat.[23][24][25]
- Response to stimuli: such as the contraction of a unicellular organism away from external chemicals, the complex reactions involving all the senses of multicellular organisms, or the motion of 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.
Physics
From a physics perspective, an organism is a thermodynamic system with an organised molecular structure that can reproduce itself and evolve as survival dictates.[26][27] Thermodynamically, life has been described as an open system which makes use of gradients in its surroundings to create imperfect copies of itself.[28] Another way of putting this is to define life as "a self-sustained chemical system capable of undergoing Darwinian evolution", a definition adopted by a NASA committee attempting to define life for the purposes of exobiology, based on a suggestion by Carl Sagan.[29][30] This definition, however, has been widely criticised because, according to it, a single sexually reproducing individual is not alive as it is incapable of evolving on its own.[29]
Living systems
Others take a living systems theory 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 include Stuart Kauffman's definition as an autonomous agent or a multi-agent system capable of reproducing itself and of completing at least one thermodynamic work cycle.[31] This definition is extended by the evolution of novel functions over time.[32] Living systems are characterized by a multiscale, hierarchical organization, spanning from molecular machines to cells, organs, tissues, organisms, populations, ecosystems, up to the whole biosphere.[33]
Death
Death is the termination of all vital functions or life processes in an organism or cell.[34][35] One of the challenges in defining death is distinguishing it from life. Death would seem to refer to either the moment life ends, or when the state that follows life begins.[35] However, determining when death has occurred is difficult, as cessation of life functions is often not simultaneous across organ systems.[36] Such determination, therefore, requires drawing conceptual lines between life and death. This is problematic because there is little consensus over how to define life. The nature of death has for millennia been a central concern of the world's religious traditions and of philosophical inquiry. Many religions maintain faith in either a kind of afterlife or reincarnation for the soul, or resurrection of the body at a later date.[37]
Viruses
Whether or not viruses should be considered as alive is controversial.[38][39] They are most often considered as just gene-coding replicators rather than forms of life.[40] They have been described as "organisms at the edge of life"[41] because they possess genes, evolve by natural selection,[42][43] and replicate by making multiple copies of themselves through self-assembly. However, viruses do not metabolise, 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.[44][45]
History of study
Materialism
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.[46] Democritus (460 BC) was an atomist; he thought that the essential characteristic of life was having a soul (psyche), and that the soul, like everything else, was composed of fiery atoms. He elaborated on fire because of the apparent connection between life and heat, and because fire moves.[47] Plato, in contrast, held that the world was organised by permanent forms, reflected imperfectly in matter; forms provided direction or intelligence, explaining the regularities observed in the world.[48]
The mechanistic materialism that originated in ancient Greece was revived and revised by the French philosopher René Descartes (1596–1650), who held that animals and humans were assemblages of parts that together functioned as a machine. Gottfried Wilhelm Leibniz emphasised the hierarchical organization of living machines, noting in his book Monadology (1714) that "...the machines of nature, that is living bodies, are still machines in their smallest parts, to infinity."[49] This idea was developed further by Julien Offray de La Mettrie (1709–1750) in his book L'Homme Machine.[50] 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.[51] At the beginning of the 20th century Stéphane Leduc (1853–1939) promoted the idea that biological processes could be understood in terms of physics and chemistry, and that their growth resembled that of inorganic crystals immersed in solutions of sodium silicate. His ideas, set out in his book La biologie synthétique,[52] were widely dismissed during his lifetime, but have incurred a resurgence of interest in the work of Russell, Barge and colleagues.[53]
Hylomorphism
Hylomorphism is a theory first expressed by the Greek philosopher Aristotle (322 BC). The application of hylomorphism to biology was important to Aristotle, and biology is extensively covered in his extant writings. In this view, everything in the material universe has both matter and form, and the form of a living thing is its