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False-color Cassini radar mosaic of Titan's north polar region; the blue areas are lakes of liquid hydrocarbons. "The existence of lakes of liquid hydrocarbons on Titan opens up the possibility for solvents and energy sources that are alternatives to those in our biosphere and that might support novel life forms altogether different from those on Earth."—NASA Astrobiology Roadmap 2008 [1]
A life form (also spelled life-form or lifeform) is an entity that is living, [1] [2] such as plants , animals , and fungi . It is estimated that more than 99% of all species that ever existed on Earth, amounting to over five billion species, [3] are extinct. [4] [5] Earth is the only celestial body known to harbor life forms. No form of ...
Basic chemical ingredients of life – the carbon-hydrogen molecule (CH), the carbon-hydrogen positive ion (CH+) and the carbon ion (C+) – can be produced by ultraviolet light from stars. [93] Complex molecules, including organic molecules, form naturally both in space and on planets. [ 94 ]
All life forms require certain core chemical elements for their biochemical functioning. These include carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur—the elemental macronutrients for all organisms. [130] Together these make up nucleic acids, proteins and lipids, the bulk of living matter. Five of these six elements comprise the ...
These first organisms are believed to have been chemoautotrophs, using carbon dioxide as a carbon source and oxidizing inorganic materials to extract energy. 3800 Ma Formation of a greenstone belt of the Isua complex in western Greenland, whose isotope frequencies suggest the presence of life. [28]
A scenario is a set of related concepts pertinent to the origin of life (abiogenesis), such as the iron-sulfur world.Many alternative abiogenesis scenarios have been proposed by scientists in a variety of fields from the 1950s onwards in an attempt to explain how the complex mechanisms of life could have come into existence.
All earlier forms of life preceding this divergence and all extant organisms are generally thought to share common ancestry. On the basis of a formal statistical test, this theory of a universal common ancestry (UCA) is supported versus competing multiple-ancestry hypotheses.
Some computer models suggest life began as early as 4.5 billion years ago. [36] [37] The oldest evidence of life is indirect in the form of isotopic fractionation. Microorganisms will preferentially use the lighter isotope of an atom to build biomass, as it takes less energy to break the bonds for metabolic processes. [38]