The classification system developed in the early 1700s divided living organisms into plant and animals. Today, that has been expanded into five kingdoms. What invention was most responsible for creating the need for the additional three kingdoms?
Up until van Leeuwenhoek showed the world that all life is made up of cells using his newly invented microscope, people were happy to divide organisms that moved (animals) from those that didn't (plants) and call it a day. The microscope revealed an exotic world of tiny organisms that often blurred the line between plants and animals. For the first time, a new kingdom was necessary to classify all these tiny bugs -- protists .
Some "plants" looked very different under a microscope. They reproduced with spores and grew large webs invisible to the naked eye. Savvy botanists could now tell that fungi were not plants, and they eventually got their own kingdom.
Later on, the electron microscope enabled scientists in the 1930s to see bacteria for what they were -- entirely new forms of super tiny life that didn't even have a nucleus. These were called prokaryotes , meaning "before+nucleus," since people figured they evolved first.
Today we are in the middle of a new revolution of understanding life. DNA sequencing technology is giving scientists a straight look at the genetic code, allowing for much more careful classification based on genetic inheritance and evolution. There have been lots of surprises thanks to DNA sequencing. For example, did you know whales and hippos are genetic cousins?
P.S. The most modern classification system actually has seven kingdoms: Bacteria, Archaea, Protozoa, Chromista, Plantae, Fungi, Animalia