Microbes once filed as bacteria turned out to be our distant ancestors
Archaea look like bacteria under a microscope, yet their machinery for reading genes resembles ours, and complex cells, including human ones, appear to have evolved from them. First found in boiling springs and salt lakes, they now turn up everywhere, from ocean plankton to the human gut.
Until 1977 these single cells were lumped with bacteria as archaebacteria. That year Carl Woese and George Fox separated them using ribosomal RNA genes, pointing to cell walls without peptidoglycan and two unusual coenzymes. Woese, Otto Kandler and Mark Wheelis then proposed three domains of life: Bacteria, Archaea and Eukarya. Because eukaryotes arose from within Archaea, the domain strictly includes them, though in everyday usage the word still means just the simple-celled members. The name comes from Greek for ancient things.
Their chemistry is distinctive. Membranes are built from ether-linked lipids found nowhere else, some make methane, and they swim with a unique structure called the archaellum. Most are bacteria-sized, but Haloquadratum walsbyi forms flat square cells. They draw energy from sugars, ammonia, metal ions or hydrogen; salt-loving Halobacteria harvest sunlight, and others fix carbon, though no known species does both. They reproduce by splitting, fragmenting or budding and never form endospores.
Early specimens were extremophiles, which fed the belief that archaea lived only in harsh places. By the close of the 20th century, copying ribosomal genes from soil and water samples with PCR revealed them almost everywhere, and planktonic archaea may be among the most abundant organisms on Earth. Methane-making species were grown in culture as early as the 1930s and 1940s but were then filed as bacteria. Tiny oddities keep appearing, such as Nanoarchaeum equitans, found in 2003, and the minute ARMAN group, found in 2006.
Archaea help cycle carbon and nitrogen and live in the mouths, guts and skin of people. Methanogens in the digestive tracts of humans and cattle aid digestion, and as of 2024 just one parasitic species was known. Industry uses methanogens for biogas and sewage treatment, and borrows heat-tolerant enzymes from extremophiles.
Source: Archaea
