Final answer:
Scientific developments in the understanding of cellular structures and genetic relationships led to the establishment of Monera and Protista kingdoms, expanding from the traditional two-kingdom system to include a more nuanced classification of life forms.
Step-by-step explanation:
Early classification systems, which included only the kingdoms Plantae and Animalia, were expanded due to scientific advancements in understanding cellular structures and genetic relationships among organisms. The proposal to establish the kingdoms Monera and Protista was initially suggested by Ernst Haeckel. The kingdom Monera included unicellular organisms without a nucleus (prokaryotes), and the Protista included a diverse group of eukaryotic organisms that weren't clearly plants, animals, or fungi.
The real shift in taxonomic classification occurred with the work of Robert Whittaker and later Carl Woese. Whittaker proposed a five-kingdom system, which also included the kingdoms Fungi and Protista, while the pioneering work of Woese compared small-subunit ribosomal RNA (SSU rRNA) sequences, leading to the establishment of three domains (Bacteria, Archaea, and Eukarya), thus reorganizing the tree of life more fundamentally.
These scientific developments allowed taxonomists to recognize the limitation of the two-kingdom system and establish a more nuanced understanding of the diversity of life forms, leading to the establishment of the Monera and Protista kingdoms.