Answer:
The invention of the microscope
Step-by-step explanation:
The invention of the microscope and the ability to see organisms that are otherwise invisible to the ordinary eye led to the establishment of Monera and Protista kingdoms.
Organisms in the Monera and Protista kingdom are microscopic and the invention of the microscope created and avenue for them to be studied extensively by scientists.
Through their studies, taxonomists discovered that these microscopic organisms have characteristics that are neither of plants nor of animals.
Some organisms were found to lack nucleus and membrane-bound organelles which is unlike plants and animals. Some were also found to posses most of the characteristics of animals except that they are unicellular.
These led to the establishment of the Monera and Protista kingdoms respectively.