What are the three major groups into which all living things are classified?

Living organisms are classified into three major domains based on their fundamental cellular structure and genetic organization: Bacteria, Archaea, and Eukarya (often referred to as Eukaryota). This classification is rooted in the work of Carl Woese, who proposed the three-domain system based on differences in ribosomal RNA sequences, genomic studies, and domain-specific characteristics.

Bacteria are single-celled prokaryotic organisms with no nucleus and are found in various environments. Archaea are also prokaryotic, but they have distinct biochemistry and genetic properties that allow them to thrive in extreme environments, such as hot springs and salt lakes. Eukarya consists of eukaryotic organisms, which have more complex cells with a nucleus and organelles, including animals, plants, fungi, and protists.

This three-domain system reflects major distinctions in the evolutionary history and cellular makeup of life forms, providing a clearer understanding of the diversity among living organisms than classifications that include options like viruses or specific groups within eukaryotes alone.