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Carbon is a primary component of all known life on Earth, and represents approximately 45–50% of all dry biomass. Carbon compounds occur naturally in great abundance on Earth. Complex biological molecules consist of carbon atoms bonded with other elements, especially oxygen and hydrogen and frequently also nitrogen, phosphorus, and sulfur (collectively known as CHNOPS). Because it is lightweight and relatively small in size, carbon molecules are easy for enzymes to manipulate. Carbonic anhydrase is part of this process. Carbon has an atomic number of 6 on the periodic table. The carbon cycle is a biogeochemical cycle that is important in maintaining life on Earth over a long time span. The cycle includes carbon sequestration and carbon sinks. Plate tectonics are needed for life over a long time span, and carbon based life is important in the plate tectonics process. An abundance of iron and sulfur based Anoxygenic photosynthesis life forms that lived from 3.80 to 3.85 billion years ago on Earth produces an abundance black shale deposits. These shale deposits increase heat flow and crust buoyancy, especially on the sea floor, helping to increase plate tectonics. Talc is another organic mineral that helps drive plate tectonics. Inorganic processes also help drive plate tectonics. Carbon based photosynthesis life caused a rise in oxygen on Earth. This increase of oxygen helped plate tectonics form the first continents. It is frequently assumed in astrobiology that if life exists elsewhere in the Universe, it will also be carbon based. Critics, like Carl Sagan in 1973, refer to this assumption as carbon chauvinism.

Carbon is capable of forming a vast number of compounds, more than any other element, with almost ten million compounds described to date, and yet that is but a fraction of the number of compounds that are theoretically possible under standard conditions. The enormous diversity of carbon compounds, known as organic compounds, has led to a distinction between them and the inorganic compounds that do not contain carbon. The branch of chemistry that studies organic compounds is known as organic chemistry. Carbon is the 15th most abundant element in the Earth's crust, and the fourth most abundant element in the universe by mass, after hydrogen, helium, and oxygen. Carbon's widespread abundance, its ability to form stable bonds with numerous other elements, and its unusual ability to form polymers at the temperatures commonly encountered on Earth enables it to serve as a common element of all known living organisms. In a 2018 study, carbon was found to compose approximately 550 billion tons of all life on Earth. It is the second most abundant element in the human body by mass (about 18.5%) after oxygen. The most important characteristics of carbon as a basis for the chemistry of cellular life are that each carbon atom is capable of forming up to four valence bonds with other atoms simultaneously, and that the energy required to make or break a bond with a carbon atom is at an appropriate level for building large and complex molecules which may be both stable and reactive. Carbon atoms bond readily to other carbon atoms; this allows the building of arbitrarily long macromolecules and polymers in a process known as catenation. "What we normally think of as 'life' is based on chains of carbon atoms, with a few other atoms, such as nitrogen or phosphorus", per Stephen Hawking in a 2008 lecture, "carbon [ ] has the richest chemistry." Norman Horowitz was the head of the Jet Propulsion Laboratory's bioscience section for the first U. S. mission, Viking Lander of 1976, to successfully land an unmanned probe on the surface of Mars. He considered that the great versatility of the carbon atom makes it the element most likely to provide solutions, even exotic solutions, to the problems of survival on other planets. However, the results of this mission indicated that Mars was presently extremely hostile to carbon based life. He also considered that, in general, there was only a remote possibility that non carbon life forms would be able to evolve with genetic information systems capable of self replication and adaptation.

A few other elements have been proposed as candidates for supporting biological systems and processes as fundamentally as carbon does, for example, processes such as metabolism. The most frequently suggested alternative is silicon. Silicon, atomic number of 14, more than twice the size of carbon, shares a group in the periodic table with carbon, can also form four valence bonds, and also bonds to itself readily, though generally in the form of crystal lattices rather than long chains. Despite these similarities, silicon is considerably more electropositive than carbon, and silicon compounds do not readily recombine into different permutations in a manner that would plausibly support lifelike processes. Silicon is abundant on Earth, but as it is more electropositive, it mainly forms Si–O bonds rather than Si–Si bonds. Boron does not react with acids and does not form chains naturally. Thus boron is not a candidate for life. Arsenic is toxic to life, and its possible candidacy has been rejected. In the past (1960s 1970s) other candidates for life were plausible, but with time and more research, only carbon as the complexity and stability for life, to make very large molecules, like polymers. Thus life must be carbon based.

Speculations about the chemical structure and properties of hypothetical non carbon based life have been a recurring theme in science fiction. Silicon is often used as a substitute for carbon in fictional lifeforms because of its chemical similarities. In cinematic and literary science fiction, when man made machines cross from non living to living, this new form is often presented as an example of non carbon based life. Since the advent of the microprocessor in the late 1960s, such machines are often classed as "silicon based life". Other examples of fictional "silicon based life" can be seen in the 1967 episode "The Devil in the Dark" from Star Trek: The Original Series, in which a living rock creature's biochemistry is based on silicon. In the 1994 The X Files episode "Firewalker", in which a silicon based organism is discovered in a volcano. In the 1984 film adaptation of Arthur C. Clarke's 1982 novel 2010: Odyssey Two, a character argues, "Whether we are based on carbon or on silicon makes no fundamental difference; we should each be treated with appropriate respect." In JoJolion, the eighth part of the larger JoJo's Bizarre Adventure series, a mysterious race of silicon based lifeforms "Rock Humans" serve as the primary antagonists.