Until the late 1970s, there was no "domain" as the broadest category in taxonomy. instead, there were five kingdoms: protista, plantae, fungi, animalia, and monera, which included all bacteria-like organisms. researchers proposed the domains to distinguish between the bacteria-like organisms. which information supported the domain level, and subsequent domains of bacteria, archaea, and eukarya?

The answer is: Differences in DNA among the existing kingdoms showed a need for a broader naming category

There is a significant difference in the genetic material (primarily DNA) between the kingdoms and that imposed the need for a broader, more generalized classification system. The domain system are based on the differences in rRNA (ribosomal RNA) found in archaea, bacteria, and eukarya (the three domains). Bacteria and archaea are both prokaryotes, meaning they lack a true nucleus as their genetic material freely floats in their cytoplasm as a nucleoid; but they differ in rRNA sequences significantly and needed to be classified separately. On the other hand, eukarya are eukaryotes that has a true nucleus.