Several lineages of bacteria and archaea are extremophiles: they thrive under extreme conditions that would kill most other organisms. The hadobacteria, for example, are thermophiles (Greek, “heat lovers”). The group’s name is derived from Hades, the ancient Greek name for the underworld. Hadobacteria of the genus Deinococcus are resistant to radiation and can degrade nuclear waste and other toxic materials. They can also survive extremes of cold as well as hot temperatures. Another hadobacterium, Thermus aquaticus, was the source of the thermally stable DNA polymerase that was critical for the development of the polymerase chain reaction. Thermus aquaticus was originally isolated from a hot spring, but it can be found wherever hot water occurs (including in many residential hot-water heaters).

The hyperthermophilic bacteria are another major group of extremophiles. Genera such as Aquifex live near volcanic vents and in hot springs, sometimes at temperatures near the boiling point of water. Some species of Aquifex need only hydrogen, oxygen, carbon dioxide, and mineral salts to live and grow. Species of the genus Thermotoga live deep underground in oil reservoirs as well as in other high-temperature environments.

Biologists have hypothesized that high temperatures characterized the ancestral conditions for life, given that most environments on early Earth were much hotter than those of today. Reconstructions of ancestral bacterial genes have supported this hypothesis by showing that the ancestral sequences functioned best at elevated temperatures. The presence of multiple lineages of extremophiles at the base of the bacterial tree (see Figure 25.1) also provides support for the origin of life in a high-temperature environment.