Carbon of interconnecting carbon–carbon bonds, a property that is called catenation. Carbon-carbon bonds are strong and stable. Through catenation, carbon forms a countless number of compounds. A tally of unique compounds shows that more contain carbon that those that do not. A similar claim can be made for hydrogen because most organic compounds also contain hydrogen. The simplest form of an organic molecule is the hydrocarbon—a large family of organic molecules that are composed of hydrogen atoms bonded to a chain of carbon atoms. A hydrocarbon backbone can be substituted by other atoms, known as heteroatoms. Common heteroatoms that appear in
Hydrogen sulfide which uses water as electron donor and liberates oxygen.) The biochemistry of hydrogen sulfide is a key part of the chemistry of the iron-sulfur world. In this model of the origin of life on Earth, geologically produced hydrogen sulfide is postulated as an electron donor driving the reduction of carbon dioxide. Hydrogen sulfide is lethal to most animals, but a few highly specialized species (extremophiles) do thrive in habitats that are rich in this compound. In the deep sea, hydrothermal vents and cold seeps with high levels of hydrogen sulfide are home to a number of extremely specialized lifeforms, ranging