Extremophilic organisms are adapted to harsh environmental conditions like high temperature, extremely acidic or alkaline pH, high salt, or a combination of those. With a few exceptions extremophilic bacteria are colonizing only moderately hot biotopes, whereas hyperthermophiles are found specifically among archaea (formerly 'archaebacteria') which can thrive at temperatures close to or even above the boiling point of water. It has been a challenging question whether the special properties of their proteins and membranes have been acquired by adaptation, or whether they might reflect early evolutionary states as suggested by their phylogenetic position at the lowest branches of the universal tree of life.