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The Prokaryotic cells are the bacteria and archaea. The bacteria were formerly grouped together with archaea in the Kingdom of Monera, while newer cladistic systems taking advantage of information we have from gene sequencing divide the Prokaryotic cells into two kingdoms, the Eubacteria (or bacteria) and Archaea (formerly called “Archaebacteria”). The Prokaryotic cells don’t have a nucleus and they usually lack the other membrane-bounded organelles. In other words, they are very simple forms of life. They are also the oldest forms of life, according to fossil evidence. Fossils suggests earliest simple cells must have evolved by 3.5 billion years ago, but carbon isotopes in sedimentary rocks suggest life (and cells) may have existed a couple hundred million years earlier.
In fact, recent research from UCLA professors Mark Harrison and Elizabeth Bell strongly suggests the earliest life must have existed 4.1 billion years ago.
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