Archaea is a microbe, one of the oldest organisms on earth, having evolved more than 3 billion years ago, single-celled without a nucleus, and part of the microbiota of all organisms (including humans). It is also one of the most plentiful organisms on earth, including in the gut biome of humans. Three major domains, Archaea, Bacteria, and Eukaryota, describe all living things on earth. Being in its own separate domain means Archaea is unlike anything else on earth. Each photo shows millions of tiny cells.