The definition of the term life is controversial between different branches of science. In 1999, the Israeli chemist Noam Lahav listed 48 different definitions from experts over the past 100 years.
In today's natural sciences, life largely refers to a form of organization that is characterized by certain processes. In modern biology ( synthetic biology ), what life or a living being is is not defined in terms of individual properties, a certain state or a specific materiality, but rather a set of processes that, taken together, are characteristic and specific for life or living beings . These processes typically include:
- Energy and metabolism and thus the interaction of living beings with their environment .
- Organization and self-regulation ( homeostasis ).
- Irritability , that is, living things are able to react to chemical or physical changes in their environment .
- Reproduction , that is, living beings are capable of reproduction .
- Inheritance , i.e. living beings can transmit information (genetic material) to their descendants.
- Growth and thus the ability to develop.
These criteria define a system that, as basic equipment, must have the following properties. It needs:
Furthermore, it must be in a steady state between an influx of energy or energy-containing matter and an outflow of metabolic end products and other unneeded substances.
Such a system of high complexity can only be implemented in the context of organic chemistry . It consists of units that build on one another and interact with one another, which functionally condition and maintain one another.
The simplest system known today that fulfills all these requirements is a cell ( unicellular as an independent organism ). The extent to which structures (including viruses ) that only meet part of the above requirements are to be regarded as life is discussed.
All of the processes listed above are coordinated and regulated by RNAs. These RNAs are usually parts of co-opted infectious agents such as viruses and mobile genetic elements. No cellular organism would be able to organize itself without these RNAs.
The Biology examines and describes the manifestations of living systems, their relationships to each other and to their environment and the processes that take place in them. These include the exchange of energy and materials, growth, reproduction, reaction to changes in the environment and the possibility of coordinating via communication processes. Some of these features are also found in technical, physical and chemical systems, other features are only inherent in biological organisms. However, the minimum property of all living systems is considered to be autopoiesis : the ability to maintain and reproduce itself.
So far only life based on ribonucleic acid and deoxyribonucleic acid (RNA and DNA) is known, which began on earth about 3.5 to 3.9 billion years ago. The known life forms, bacteria , archaea , fungi , plants and animals with humans , use - with a few exceptions - the same, universally valid genetic code and generate from the same chemical building blocks, namely four different nucleotides and about 20 different amino acids , the nucleic acids and proteins typical of earthly life . In principle, it cannot be ruled out on the part of natural science that life in the universe can also be based on other chemical substances (see so-called carbon chauvinism ).
According to the theory of biological evolution, over billions of years, comparatively simple forms of life became increasingly complex.
Origin of Life
Today's knowledge of the natural sciences is insufficient to explain how life came about. If a genetic program, its functionality and its development is assumed to be essential for living beings, then the point in time at the beginning of life when molecules as carriers of the program and other auxiliary molecules for the realization, reproduction and adaptation of this program come together for the first time a system arises that supports the characteristic properties of life.
The currently most popular ( autotrophic ) theory about the origin of life postulates the development of a primitive metabolism on iron-sulfur surfaces under reducing conditions, such as those found in the vicinity of volcanic evaporation. During this phase of evolution on Earth, which took place in the Hadean ages , 4.6 to 4.0 billion years ago, the Earth's atmosphere was likely rich in gases, especially carbon dioxide , hydrogen, and carbon monoxide , while the hot oceans were relatively high in concentrations of ions of transition metals such as iron (Fe 2+ ) or nickel (Ni 2+ ). Similar conditions can be found today in the vicinity of hydrothermal vents, which were and are still being created in plate tectonic fault zones on the sea floor. In the vicinity of such chimneys, known as black smokers , thermophilic methanogenic archaea thrive on the basis of the oxidation of hydrogen and the reduction of carbon dioxide (CO 2 ) to methane (CH 4 ). This extreme habitat shows that life regardless of sun light can thrive as an energy source, a fundamental prerequisite for the development and maintenance of life before the advent of photosynthesis .
The phylogenetic perspective on the origin of life contains the question of whether life originated on earth or on another celestial body, and how the first living systems in an inanimate environment came into being (see the previous section).
The ontogenetic perspective focuses on the development of an individual, not on the first emergence of living beings. It asks the question of how an organism develops (e.g. from a fertilized egg cell). One speaks wrongly here of the “beginning of life”, although it is a continuity of life over the course of generations and the emergence of an individual , not the first emergence of a living system. From ontogenesis there is a possibility of defining the beginning and end of an individual life: Life ends when the characteristic properties of living beings disappear, i.e. when death occurs. The beginning is defined differently, often in living beings with sexual reproduction the union of two gametes is seen as the beginning of the life of an individual.
Speculations about extraterrestrial life
In a meteorite from the planet Mars , traces were found that were initially interpreted as fossilized bacteria. Definitive proof of extraterrestrial life has not yet been found despite intensive research (see also Life on Mars , Astrobiology , Chemical Evolution and Cosmochemistry ).
In April 2007, Gliese 581 c, twenty light years from Earth, was discovered as the first planet with Earth-like conditions. It has been referred to as "second earth" and gave rise to vague speculations about life occurring there.
With the help of the Cassini-Huygens probe, scientists found evidence that a primitive life form could exist on Saturn's moon Titan . Measurements showed that there was less hydrogen and ethyne on titanium than the models predict. This could be explained by a methane-based way of life. (→ Life on Titan )
The theoretical physicist Gerald Feinberg and the chemist Robert Shapiro established the following definition, which covers all forms of life in the cosmos , in their book Life Beyond Earth, which was published in 1980 : Life arises through interactions between free energy and matter, which is able to create a greater order within of the common system. According to this, life in icy ammonia lakes would be just as conceivable as in oil seas; it could exist on the basis of electromagnetic or gravitational fields. There may be silicon beings in molten rock, plasma life inside stars, or radiating organisms in interstellar dust clouds. Possible living beings with scientific and technical organization in the sense of extraterrestrial civilizations on extrasolar planets are the subject of speculations and projections within astrobiology and exosociology . The probability of the existence and possible frequency of such civilizations is discussed primarily with the help of the Drake equation , which is also known as the Green Bank formula.
- Early theories about the origin of life from water, air, fire, earth or seeds
Thales postulated 2500 years ago that life emerged from the water and is closely linked to the question of the arché ( ἀρχή , "primordial reason") of all being and all that happens. As a versatile and widespread substance, water fulfills the claim to underlie everything and to be able to take on any shape.
Anaximander (around 610-547 BC) searched for the origin of life in water, as a spontaneous emergence from the damp environment. The first living things arose in the damp.
Anaximenes (around 585-528 / 524 BC) saw the air (ἀήρ aer ) as Arché (ἀρχή) and Apeiron (ἄπειρον, unlimited '). Even the divine either comes from the air or is the air. The invigorating principle lies in the material itself.
For Anaxagoras (499–428 BC) the seed (σπέρματα spermata ) was an infinitely small component of all things (e.g. meat, flowers) from the beginning.
Heraclitus (around 520-460 BC) saw the beginning of life in the primordial fire: “This world order, the same for all beings, did not create any god or man, but it was always there and is and will be his eternally living fire glowing in measure and dying out in measure. ”Everything is said to have emerged from all fire.
Empedocles (around 495–435 BC) advocated a biological theory of the origin of life and the evolution of living things. He introduced the doctrine of the four primary substances ( four-element theory ).
- Classic philosophy
Greek philosophy (e.g. Plato and Aristotle) conceptually distinguishes two aspects of life, which in medieval philosophy are both included under the term vita : ζωή ( zoḗ , see Zoe ) and βίος ( bíos , also in the sense of lifespan) . Ζωή means animatedness, which is common to animals and humans as a psycho-physical nature, whereas βίος means the way of life of the person distinguished by a rational soul. For Plato, self-movement is a characteristic of the living; Aristotle also attributes life to the first immobile mover. The later observations of the Neoplatonists build on this contradiction . For Plotinus , the movement moves away from the One and moves towards a state of diversity and dispersion.
In De anima, Aristotle explained the animate as the animated. He roughly differentiates between three different levels of life, which he hierarchically arranges according to their mental faculties: on the lowest level is the life of plants , which is determined solely by nutrition and reproduction , followed by the life of animals , which is additionally determined by sensory perception and locomotion , on the top level the life of man, determined by thought .
Aristotle also advocated the idea that life not only arose out of sexual and vegetative reproduction, but also formed anew from the inanimate. This theory was also advocated in the early modern period and called spontaneous generation . It even experienced an apparent confirmation by the invention of the microscope , through which microorganisms that were thought to be preliminary stages of higher forms of life became visible. Only Louis Pasteur and other natural scientists were able to experimentally refute this assumption definitively.
In recent times, two opposing basic concepts have developed:
- Mechanicism : life can be fully explained solely from the laws of the movement of matter (see also: materialism and physicalism ).
- Vitalism : Life only belongs to the "organic manifestations" (the organic) and differs qualitatively from "inorganic manifestations" (the inorganic): Everything living is characterized by a purposefully shaping life force ( vis vitalis ) (see also: idealism ). Based on religious ideas it was assumed that there is animate and inanimate matter.
The organism represents a synthesis of both approaches: life processes can be explained by principles of physics and chemistry. But living beings would also have properties that inanimate matter does not have. These would be emergent properties that should result on the one hand from the complexity of living beings, on the other hand from the special role of their genetic program.
Wilhelm Dilthey (1833–1911) formulated in his later writings: “Life is now the basic fact that must form the starting point of philosophy. It is what is known from within; it is the one behind which one cannot go back. "
Karl Popper (1902–1994) put it: “I think we couldn't really appreciate life if it kept going. The very fact ... that it is finite and limited ... increases the value of life and thus even the value of death ... "
Almost all religions, especially the world religions , define at least two realities of life. The earthly-biological form, as it can be described in the natural sciences, and eternal life . In the latter, they see a state or place that is immortal, a form of being that cannot be explained by natural sciences, that is also distinguishable from matter and that has its basis in divine activity or a creation . The earthly life (at least the human) takes place in both forms of being at the same time, in the mortal earthly and in the divine eternal. The divine state of being inside and outside the human being, especially expressed through the frequently mentioned love of God , cannot be scientifically explained and understood, it is mostly called holy .
In the Judeo-Christian tradition, God is the creator of inanimate and animate nature and he is the origin of holy life (at least in man). Thus he is the reason for eternal life and “ruler” over mortal life and death. He had commanded man to love and not to make any decisions that contradict the love of God, not even about death; this equates to a voluntary intervention in the life and work of God in man. (See also Ex 20:13 “You shall not kill”).
In the Christian faith, eternal life through the resurrection of Jesus Christ has a special meaning. Jesus described himself as the source of eternal life ( Jn 4:14 ), in a parable as the "... bread of life , whoever comes to me will never hunger again, and whoever believes in me will never be thirsty again" ( Jn 6:35) . At the same time he proclaimed his “divinity” (in him) and the belief in it as access to eternal life (Jn 14: 6). The earthly life, which is “imperfect” from the Christian point of view, is only the preliminary stage to eternal life in glorified form, in the absence of nature that has become dust and its “products” (for example pain, suffering, death and mourning).
In Islam , there are six articles of faith, including the belief in the Day of Judgment and the Afterlife : The man is pulled in one day for his acts to justice and to the hells fire ( Hell , Koran 67: 7) punished or rewarded with paradise ( Janna , Koran: 13:35).
As in these religions, there is also in many other religions the idea of eternal life or of survival after death .
Artificial life means the production of a known living being in the laboratory as well as the production of new, also non-organic life forms from non-living starting elements. The breeding or genetic modification of living beings does not constitute the creation of artificial life.
The idea and manufacturing instructions for artificial life are thousands of years old. They are based on traditional cross-religious beliefs that at least simple forms of life can arise spontaneously. There were no ethical or religious objections. The 20th century was marked by a multitude of announcements that artificial life had been created in the laboratory or was about to be created. In 2010, researchers working with Craig Venter announced the manufacture of the artificial bacterium Mycoplasma mycoides JCVI-syn1.0 . Previously, they had successfully synthesized the 1.08 million base pair genome of a laboratory strain of Mycoplasma mycoides from chemical raw material and transferred it to a Mycoplasma capricolum bacterium that had previously been freed from DNA . In doing so, they did not create life artificially, but instead changed a naturally occurring living being so that its genetic information is largely artificial.
Fictional, d. H. Fiction and film depictions of artificial beings with artificial intelligence can be found in works such as Ich, der Robot , Ghost in the Shell , 2001: A Space Odyssey , Spaceship Enterprise: The Next Century , Do androids dream of electric sheep? (better known as Blade Runner ), Battlestar Galactica, and Mass Effect . Simulations of expressions of life:
- Conway's Game of Life is an example of the simulation of population development.
- Daisy World is a computer simulation of a hypothetical planet on which depends (Engl. From the sunlight daisies daisy whose growth than grow), feedback influenced process the radiation absorption.
- ELIZA by Joseph Weizenbaum simulates a conversation partner by mimicking behaviors that were originally developed by psychotherapists in order to animate their patients to react.
- Erwin Schrödinger : What is life? Piper Verlag, Munich 2001, ISBN 3-492-21134-8 . (Revision of the 2nd edition of the German-language edition from 1951)
- Manfred Eigen , W. Gardiner, P. Schuster, R. Winkler-Oswatitsch: The Origin of Genetic Information. In: Scientific American . 244, 1981, pp. 88-118. (Reliable and generally understandable presentation of the theory represented)
- Manfred Eigen : Steps to Life. Early evolution in the sights of molecular biology. 3. Edition. Piper Verlag, 1993, ISBN 3-492-10765-6 . (First edition 1987)
- Humberto Maturana , Francisco Varela : The Tree of Knowledge. The biological roots of human knowledge. Munich 1987. (With a detailed explanation of the concept of autopoiesis )
- C. De Duve: Blueprint for a Cell. The Nature and Origin of Life. Neil Patterson, Burlington, NC 1991. (with bibliography)
- Daniel E. Koshland : Special essay. The seven pillars of life. In: Science. Volume 295, Number 5563, March 2002, , pp. 2215-2216, doi: 10.1126 / science.1068489 . PMID 11910092 .
- Axel W. Bauer : Life in health and illness - tasks and riddles for medicine. In: What do we know about life? An approach from different perspectives, Evangelical Academy of the Palatinate in cooperation with the Catholic Academy Speyer, 26/27. November 2004 in Speyer. Speyer 2005, pp. 1-12.
- Peter Ward et al. Joe Kirschvink: A New Story of Life: How Disasters Changed the Course of Evolution. Munich: Deutsche Verlagsanstalt 2016 (table of contents under https://d-nb.info/1098326903/04 ).
- Noam Lahav: Biogenetics - Theories of Life's Origin. New York 1999 (table of contents at http://www.gbv.de/dms/hebis-darmstadt/toc/79183573.pdf ).
- Nick Lane: The Spark of Life: Energy and Evolution. Konrad Theiss Verlag: Darmstadt 2017 (table of contents at https://d-nb.info/1118389840/04 ).
- What do we know about life? An approach from different perspectives, Evangelical Academy of the Palatinate in cooperation with the Catholic Academy Speyer, 26/27. November 2004 in Speyer. Speyer 2005.
- Hans Rainer Sepp , Ichiro Yamaguchi (ed.): Life as a phenomenon. The Freiburg Phenomenology in the East-West Dialogue. (= Orbis Phaenomenologicus, Perspektiven NF. Volume 13). Königshausen & Neumann, Würzburg 2006, ISBN 3-8260-3213-6 .
- Mark A. Bedau et al. a .: The nature of life. Classical and contemporary perspectives from philosophy and science. Cambridge Univ. Press, Cambridge 2010, ISBN 978-0-521-51775-1 .
Philosophy, dictionary article (German):
- Pierre Hadot et al. a .: life. In: Historical Dictionary of Philosophy . Volume 5, pp. 52-103.
- Ferdinand Fellmann : Life. In: Christian Bermes , Ulrich Dierse (Hrsg.): Key terms of the philosophy of the 20th century . Meiner, Hamburg 2010, ISBN 978-3-7873-1916-9 , pp. 189-206.
- Theo Sundermeier et al. a .: life. In: Theological Real Encyclopedia . Volume 20, pp. 520-566.
Philosophy, dictionary article (English):
- Paul Edwards, Joseph Runzo: Life, Meaning and Value of. In: Encyclopedia of Philosophy . 2nd Edition. Volume 5, pp. 345-359.
- Leslie E. Organ: Life, Origin of. In: Encyclopedia of Philosophy. 2nd Edition. Volume 5, pp. 359-362.
- John Harris: Life and Death. Susan Wolf: Life, meaning of. and Lenny Moss: Life, origin of. In: Routledge Encyclopedia of Philosophy . (At Moss a short selection bibliography on the currently debated theories)
- Herbert Frohnhofen : Selected bibliography on the term life from v. a. theological point of view
- Nicole C. Karafyllis: Bios and Zoe. In: Basic Concepts of Natural Philosophy.
- Bruce Weber: Life. In: Edward N. Zalta (Ed.): Stanford Encyclopedia of Philosophy .
- Spektrum .de: Fossils Shed New Light on Early Earth Times June 14, 2018
- Spektrum.de: The Hunt for the Origin of Life July 21, 2018
- LUCA : How life came to earth - Radio Academy: Evolution: Flow of Life - SWR2 Knowledge Manuscript (PDF file; 67 kB)
- Tom Marshall: Building blocks of life came from deep Earth. PhysOrg, August 7, 2012. (English)
- Kelly Potts: Critical role of granite in evolution of life on Earth revealed in new study. PhysOrg, June 13, 2012. (English)
- Chris Gorski: Research suggests that ancient granites made advanced life possible. PhysOrg, June 21, 2012. (English)
- Eckart Löhr: What is life? In: Re-Visionen, May 2016.
- Noam Lahav: Biogenesis - Theories of Life's Origin . New York 1999.
- Quoted from Robert Hazen: Was ist Leben ?, Spectrum essay, spectrum of science 10/2007. Heidelberg: Spektrum-der-Wissenschaft-Verlagsgesellschaft 2007.
- Toepfer, Georg 2017: Leben , in: Naturphilosophie. A text and study book. Tübingen, UTB / Mohr Siebeck: 159–164, here 161f .; Toepfer, Georg 2011: Life , in: Historical dictionary of biology. History and theory of basic biological concepts, Vol. 2. Stuttgart: 420–483.
- The Max Planck Society gives a slightly different list at https://www.synthetik-biologie.mpg.de/17480/was-ist-leben
- Johann Grolle: Competition for God . In: Der Spiegel . No. 1 , 2010, p. 115 ( online ).
- Luis P. Villarreal: Virolution can help us understand the origin of life . In: Vera Kolb (Ed.): Astrobiology. An evolutionary approach . CrC Press, Boka Raton 2015, ISBN 978-1-4665-8461-7 , pp. 421-440 .
- Olaf Fritsche : We were lucky! - Twelve reasons why we exist at all . Springer Spektrum, Heidelberg 2014, ISBN 978-3-642-41654-5 , p. 125.
- Günter Wächterhäuser: From Volcanic Origins of Chemoautotrophic Life to Bacteria, Archaea and Eukarya . In: Philosophical Transactions of the Royal Society . Vol. 361. Royal Society, London 2006, pp. 1787-1808 , PMID 17008219 .
- Speculations about life on the "second earth"
- Is Saturn's moon Titan home to some kind of exotic life form? ( Page no longer available , search in web archives ) Info: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice. June 4, 2010, accessed June 6, 2010
- Karl Vorländer : Philosophy of antiquity. History of Philosophy I . Rowohlt, 1963, p. 42.
- Martin G. Weiß (Ed.): Bios and Zoe. Human nature in the age of its technical reproducibility. Suhrkamp, Frankfurt am Main 2009; Nicole C. Karafyllis: Bios and Zoe. [Version 1.0]. In: Basic Concepts of Natural Philosophy. 2012. http://www.naturphilosophie.org/bios-und-zoe/ .
- Plotin: Enneaden , Vol. II, 4, 5, 29-34; Vol. V, 2, 1, 8.
- "From a physical point of view, it is particularly fun to talk about life because it represents the most extreme case of the emergence of laws." - Robert B. Laughlin : Basics of Life. In: Farewell to the universal formula. Piper Verlag, 2007, ISBN 978-3-492-04718-0 , 13th chapter.
- Karl Popper, John C. Eccles : The I and its brain. Piper, Munich 1982, ISBN 3-492-21096-1 , p. 654.
- Joachim Schummer. The work of God. The artificial production of life in the laboratory. Suhrkamp Berlin. Edition Unseld Volume 39. 2011. ISBN 978-3-518-26039-5 .
- Daniel G. Gibson et al. a .: Creation of a Bacterial Cell Controlled by a Chemically Synthesized Genome . In: Science . Vol. 329, No. 5987 , 2010, p. 52-56 , doi : 10.1126 / science.1190719 .
- Johann Grolle: Competition for God . In: Der Spiegel . No. 1 , 2010, p. 110-119 ( online ).