Three Domains

All the components were now in place for the first cells to form. Current understanding of early evolution is that life (based on an analysis of the RNA in their ribosomes) split into two domains one of which later split into two. Hence there are now three domains, called Bacteria, Archaea and Eukaryotes, related as follows:

Possible relationships between three main domains of life

Note that the Bacteria and Archaea appear very similar to each other and very different from more advanced Eukaryotes, so Bacteria and Archaea are often grouped together under the heading prokaryotes. Biologists think that the Archaea are perhaps older than the Bacteria.