Life
Scientists have compiled at least 123 different definitions of life, yet no single description satisfies every case. This challenge arises because life functions as a process rather than a static substance. Philosophers and biologists struggle to distinguish living entities from non-living matter without knowing what exists beyond Earth. Legal systems face similar difficulties when deciding the exact moment a human being dies. These debates often focus on the decision to declare death and its legal consequences. A lack of knowledge regarding extraterrestrial characteristics complicates the search for universal traits. Most current biological definitions remain descriptive rather than prescriptive. They list characteristics like homeostasis, organization, metabolism, growth, adaptation, response to stimuli, and reproduction. Homeostasis involves regulating an internal environment to maintain a constant state, such as sweating to reduce body temperature. Organization requires structural composition of one or more cells, which serve as the basic units of life. Metabolism transforms energy to convert chemicals into cellular components or decompose organic matter. Growth maintains a higher rate of building up than breaking down, increasing size and structure over time. Adaptation allows organisms to become better able to live in their specific habitats through evolutionary processes. Response to stimuli includes reactions ranging from unicellular movement away from chemicals to complex sensory actions in multicellular beings. Reproduction enables the production of new individual organisms either asexually from a single parent or sexually from two parents.
Life on Earth has existed for at least 3.5 billion years according to physical traces found in ancient rocks. The oldest evidence includes biogenic graphite discovered in 3.7 billion-year-old metasedimentary rocks from Western Greenland. Microbial mat fossils dating back 3.48 billion years were found in sandstone from Western Australia. More recent discoveries in 2015 revealed remains of biotic life within 4.1 billion-year-old rocks also located in Western Australia. Scientists announced putative fossilized microorganisms in hydrothermal vent precipitates from the Nuvvuagittuq Belt of Quebec, Canada in 2017. These fossils are as old as 4.28 billion years, suggesting an almost instantaneous emergence of life after ocean formation. Molecular clocks place the origin of life around 4.0 billion years ago based on data from the TimeTree public database. A set of 355 genes from the last universal common ancestor was tentatively identified in 2016. Evolution describes the change in heritable characteristics of biological populations over successive generations. Natural selection and genetic drift act on genetic variation to increase or decrease certain characteristics within a population. This process has given rise to biodiversity at every level of biological organization. Fossils preserve remains or traces of organisms older than 10,000 years ago, ranging from the Holocene Epoch to the Archaean Eon. Over 99% of all species that have ever lived are now extinct. Mass extinctions may have accelerated evolution by providing opportunities for new groups of organisms to diversify.
All life forms require core chemical elements including carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur. Carbon is the most abundant element in organisms and forms multiple stable covalent bonds. These bonds allow organic molecules to create immense variety described in organic chemistry. DNA carries most genetic instructions used in growth, development, functioning, and reproduction of known living organisms. DNA and RNA are nucleic acids alongside proteins and complex carbohydrates as major macromolecules essential for life. Most DNA molecules consist of two biopolymer strands coiled around each other to form a double helix. Each strand contains all information needed to recreate the other strand during cell division. Nucleotides join together via covalent bonds between sugar and phosphate groups creating an alternating backbone. Hydrogen bonds bind nitrogenous bases of separate polynucleotide strands according to base pairing rules. Cells serve as the basic unit of structure in every living thing, with all cells arising from pre-existing cells. Prokaryote cells lack a nucleus and membrane-bound organelles but contain circular DNA and ribosomes. Eukaryote cells possess a distinct nucleus bound by a nuclear membrane and various membrane-bound organelles. Mitochondria, chloroplasts, lysosomes, rough and smooth endoplasmic reticulum, and vacuoles exist within eukaryotic cells. Proteins synthesize through enzyme-catalyzed processes called protein biosynthesis based on gene expression. Ribosomes assemble sequences of amino acids joined into chains that determine cellular function.
Empedocles argued in 430 BC that everything in the universe consists of four eternal elements: earth, water, air, and fire. Democritus believed life possessed a soul composed of fiery atoms around 460 BC. Plato held that permanent forms organized the world imperfectly reflected in matter. René Descartes revived mechanistic materialism in the 17th century, viewing animals and humans as assemblages functioning like machines. Gottfried Wilhelm Leibniz emphasized hierarchical organization of living machines in his book Monadology published in 1714. Julien Offray de La Mettrie expanded these ideas in his book L'Homme Machine during the early 18th century. Aristotle expressed hylomorphism in 322 BC, asserting that living things embody both form and matter. He proposed three kinds of souls: vegetative for plants, animal for movement and sensation, and rational for consciousness found only in humans. Georg Ernst Stahl originated vitalism in the 17th century, believing a non-material life-principle existed. Friedrich Wöhler disproved vitalism in 1828 by preparing urea from inorganic materials. Hermann von Helmholtz demonstrated no energy loss in muscle movement during the 1850s. Eduard Buchner showed alcoholic fermentation could occur in cell-free extracts of yeast later in the 19th century. Louis Pasteur decisively dispelled spontaneous generation through experiments conducted in 1859.
The biosphere encompasses all ecosystems including soil, hot springs, deep ocean subsurfaces, and atmospheric layers up to 77 kilometers high. Spores of Aspergillus niger have been detected in the mesosphere at altitudes between 48 and 77 kilometers. Life forms survive in the vacuum of space under test conditions. Organisms thrive in the deep Mariana Trench and inside rocks below the sea floor off the northwestern United States. Expeditions of the International Ocean Discovery Program found unicellular life in sediment reaching 120 degrees Celsius located 1.2 kilometers below the seafloor in the Nankai Trough subduction zone. Deinococcus radiodurans is an extremophile capable of resisting extremes of cold, dehydration, vacuum, acid, and radiation exposure. Some microorganisms withstand freezing, complete desiccation, starvation, and high levels of radiation for long periods. Cyanobacteria dramatically changed Earth's environment by releasing molecular oxygen as a photosynthesis by-product. This oxygen was toxic to most life on Earth at that time, posing novel evolutionary challenges. The range of tolerance defines conditions where survival and reproduction are possible but not optimal. Zones of physiological stress exist outside this range where survival occurs without optimization. Zones of intolerance lie beyond these areas where survival becomes unlikely or impossible. Organisms with wide ranges of tolerance distribute more widely than those with narrow tolerances.
Many scientists believe extraterrestrial life is plausible, probable, or inevitable despite confirmation only existing on Earth. Projects such as SETI attempt to detect radio transmissions from possible alien civilizations. Subsurface oceans on moons of giant planets may host microbial life alongside subsurface regions of Mars. Lichen could survive for a month in simulated Martian environments according to laboratory tests. The habitable zone refers to regions around main-sequence stars supporting Earth-like life on Earth-like planets. Stars more massive than the Sun possess larger habitable zones but remain on the main sequence for shorter intervals. Small red dwarfs have smaller habitable zones subject to higher magnetic activity and tidal locking effects. Stars in intermediate mass ranges like the Sun likely offer greater chances for complex life development. Regions with abundant heavier elements forming planets predict higher probabilities of hosting complex life if supernova events remain low. Variables of the Drake equation discuss conditions where civilization might exist within wide uncertainty bounds. A Confidence of Life Detection scale proposes reporting evidence of life beyond Earth. Synthetic biology combines science and biological engineering to design new functions not found in nature. Scientists simulate aspects of life through computers, robotics, or biochemistry to understand potential extraterrestrial forms.
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Common questions
What are the 123 different definitions of life compiled by scientists?
Scientists have compiled at least 123 different definitions of life, yet no single description satisfies every case. These definitions list characteristics like homeostasis, organization, metabolism, growth, adaptation, response to stimuli, and reproduction. Most current biological definitions remain descriptive rather than prescriptive.
When did life first appear on Earth according to physical traces in rocks?
Life on Earth has existed for at least 3.5 billion years according to physical traces found in ancient rocks. The oldest evidence includes biogenic graphite discovered in 3.7 billion-year-old metasedimentary rocks from Western Greenland. Molecular clocks place the origin of life around 4.0 billion years ago based on data from the TimeTree public database.
Which chemical elements do all life forms require to survive?
All life forms require core chemical elements including carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur. Carbon is the most abundant element in organisms and forms multiple stable covalent bonds. DNA carries most genetic instructions used in growth, development, functioning, and reproduction of known living organisms.
Who proposed that everything consists of four eternal elements earth water air and fire?
Empedocles argued in 430 BC that everything in the universe consists of four eternal elements: earth, water, air, and fire. Aristotle expressed hylomorphism in 322 BC, asserting that living things embody both form and matter. He proposed three kinds of souls: vegetative for plants, animal for movement and sensation, and rational for consciousness found only in humans.
Where does the biosphere extend in terms of altitude and depth?
The biosphere encompasses all ecosystems including soil, hot springs, deep ocean subsurfaces, and atmospheric layers up to 77 kilometers high. Spores of Aspergillus niger have been detected in the mesosphere at altitudes between 48 and 77 kilometers. Expeditions of the International Ocean Discovery Program found unicellular life in sediment reaching 120 degrees Celsius located 1.2 kilometers below the seafloor in the Nankai Trough subduction zone.