Ever Since Darwin, 4: Patterns and Punctuations in the History of Life, 13: The Pentagon of Life

The division of life into plants and animals is not good enough (Plantae and Animalia).

Two new kingdoms must be created for single-celled organisms.

Bacteria and blue-green algae have no internal structures, they are called prokaryotes (literally, before nuclei, karyon is greek for "kernel"). Pre-cambrian has been called "the age of blue-green algae". For 2 billion years, blue-green algal mats were the most complicated forms of life on earth.

Cells with internal structures are called eukaryotes, truly nucleate. The eukaryotic cell might have arisen as a family of prokaryotic cells!

The two kingdoms of Monera and Protista are for prokaryotic unicells and eukaryotic unicells respectively. What about fungi? They are not considered animals but they don't photosynthesize as well. They form the fifth and final kingdom.

The two main events that resulted in the diversity of life are:

• Evolution of the eukaryotic cell
• Cambrian explosion - the filling of the ecological barrel by an explosive radiation of multicellular eukaryotes

Ever Since Darwin: Reflections in Natural History by Stephen Jay Gould