Archaea

Group of micro-organisms that are without a nucleus and have a single chromosome. They are now known to constitute a separate domain in the tree of life, next to and equally distant from bacteria and eukaryotes. All are strict anaerobes, that is, they are killed by oxygen. This is thought to be a primitive condition and to indicate that Archaea are related to the earliest life forms, which appeared about 4 billion years ago, when there was little oxygen in the Earth's atmosphere. They are found in undersea vents, hot springs, the Dead Sea, and salt pans, and have even adapted to refuse tips.

The recognition of archaea as a separate domain unrelated to ordinary bacteria goes back to the work of Carl Woese, who investigated the family tree of methanogenic bacteria using fragments of ribosomal RNA. He concluded that these organisms should be set apart from ordinary bacteria. Although subsequent research revealed many differences between archaea and bacteria (for example in the composition of the cell membranes), the new classification remained controversial until 1996, when the complete genome sequencing of Methanococcus jannaschii (an archaeaon that lives in undersea vents at temperatures around 100°C/212°F) revealed that 56% of its genes were unlike those of any other organism, making Archaea unique.