Flexi Says: Prokaryotes are the most numerous organisms on Earth. Most are too small to be seen with the naked eye. Because of their tiny size, the existence of prokaryotes was unknown until Anton van Leeuwenhoek first saw tiny cells with his microscope in the late 17th century. Since then, prokaryotes have been found almost everywhere on Earth. They are found in sea water, in animals' intestines, on your skin, and even in rocks deep beneath the Earth's crust.