Why Scientists Think Completely Unclassifiable and Undiscovered Life Forms Exist

In high school biology, we are taught that there are three types of life: eukaryotes (that's us, and most everything else we often think of as life), bacteria, and archaea (extremophiles and other very primitive life forms). But some scientists are pretty sure that there are entirely different, undiscovered lifeforms that could be prevalent on Earth, and they remain undescribed because we're not good at looking for them.

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