Prokaryotes: Archaea

Prokaryotes split into archaea and bacteria early in life's history. Archaea, once thought to exist only in extreme environments, are now known to inhabit diverse habitats. They differ from bacteria in cell wall composition, plasma membranes, and RNA. Archaea are categorized into methanogens, halophiles, and thermophiles based on their living conditions.

Name the three groups into which archaea are sometimes divided based on their living environments.

What are methanogens and where can they typically be found?