Archaea
Archaea are single-celled organisms that look simple under a microscope but form a major domain of life distinct from bacteria and eukaryotes.
What archaea are
Archaea are microscopic, usually single-celled organisms without a nucleus. They were once grouped with bacteria because both are prokaryotes, but molecular evidence showed that archaea form a separate branch of life with their own genetic, biochemical, and cellular traits.
A third domain
The recognition of archaea helped reshape the tree of life. Instead of placing all nucleus-free organisms together, biologists began separating cellular life into major domains: Bacteria, Archaea, and Eukarya. Some modern evolutionary models place eukaryotes especially close to archaeal lineages.
How they differ from bacteria
Archaea can resemble bacteria in size and shape, but their membranes, cell walls, information-processing enzymes, and many genes are different. Their membranes often use ether-linked lipids, and they do not have the same peptidoglycan cell wall that many bacteria have.
Where archaea live
Archaea are famous for extreme habitats such as boiling hot springs, acidic pools, deep-sea vents, and hypersaline lakes. That reputation is real but incomplete. Archaea are also common in ordinary-looking environments, including marine plankton communities, wetlands, soils, sediments, and digestive tracts.
Methane makers
Methanogens are archaea that produce methane as part of their metabolism. They live in oxygen-poor places such as marshes, rice paddies, sediments, landfills, and the guts of ruminants. Their activity matters for carbon cycling, biogas production, and climate science.
Archaea and extreme life
Many archaeal species are extremophiles, including heat-loving thermophiles and salt-loving halophiles. Their enzymes and membranes can remain stable where many organisms would fail, making archaea valuable for studying adaptation and for finding useful biological tools.
Why they are hard to notice
Archaea are often invisible in everyday life because they are small, many are hard to grow in the lab, and they rarely fit familiar disease-centered stories about microbes. DNA sequencing changed that picture by revealing archaeal diversity in places where scientists had not expected it.
Why it matters
Archaea help explain the deep history of life, the origin of complex cells, methane cycling, survival in extreme environments, and the hidden microbial systems that shape oceans, soils, and animal bodies. They are small organisms with a large evolutionary footprint.