Initially, when scientists were first using the taxonomy system to classify living organisms there were only considered to be 2 kingdoms: plants and animals. However, when the microscope was invented, scientists were able to observe differences in organisms at the cellular level! They were also able to observe microscopic living organisms like bacteria that they had not been able to observe before. Scientists realized it was necessary to add 4 new kingdoms to account for the observations they had not seen before the invention of the microscope.
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