What was before the three domain system?

Before the three-domain system, the prevalent classification system was the five-kingdom model proposed by Robert Whittaker in 1969. This system categorized life into five kingdoms: Monera (prokaryotes), Protista, Fungi, Plantae, and Animalia. The five-kingdom system was based on observable characteristics such as cell structure, mode of nutrition, and reproduction. However, this classification did not account for the genetic and molecular differences discovered later, particularly between prokaryotic groups, leading to the development of the three-domain system.