Archaeons found on lower branches of the archaeal tree exhibit a remarkable ability to grow and reproduce at temperatures of 80°C or more (Fig. 26.17). Called hyperthermophiles, these organisms do not simply tolerate high temperature; they require it. At present, the world record for high temperature growth is held by Methanopyrus kandleri, which has been shown to grow at 122°C among hydrogen- and CO₂-rich hydrothermal vents on the seafloor beneath the Gulf of California. Most of these heat-loving archaeons are anaerobic, living as anaerobic respirers or as chemoautotrophs that obtain energy from the oxidation of hydrogen gas.

Why do anaerobic hyperthermophiles sit on the lowest branches of the archaeal tree? Many biologists believe that these features characterized the first Archaea and therefore imply that the Archaea evolved in hot environments with no oxygen. As we discussed in Chapter 23, geologists agree that oxygen gas was not present on the early Earth, consistent with the inference we can make from the archaeal tree that early Archaea were anaerobic. A hot environment for early Archaea can be understood in two ways. Either the entire ocean was hot—for which there is little evidence—or Archaea first evolved in hot springs where local supplies of hydrogen and metals enabled them to live as chemoautotrophs. Because some versions of the bacterial tree also show hyperthermophilic species on their lowest branches, it has been suggested that the last common ancestor of all living organisms lived in a hot environment.

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Quick Check 6 If early branches on the bacterial and archaeal trees are dominated by hyperthermophilic microorganisms, does this mean that the early oceans were very hot?