Domain is used by many scientists as the largest unit of biological classification. Traditionally, kingdoms were the largest classification units. But in 1990, the American biologist Carl R. Woese and others proposed that life on Earth could be divided into three domains. They developed this proposal largely from research on the genetic (hereditary) structure of living things. Today, many biologists recognize three domains: (1) Bacteria, (2) Archaea, and (3) Eukaryota, also called Eucarya or Eukarya.