The Three Domain System

I have noticed more lightening bugs in the evening air than I can remember seeing.  June bugs are out, and cicadas are on the way.  The abundance of life is, I find, always remarkable.  

“Life, like Caesar’s Gaul, is divided into three parts.  The Linnaean system of classification, with a prescriptive hierarchy of species, genus, family, order, class, phylum and kingdom, ultimately lumps everything alive into one of three giant groups known as domains.” 

Eukaryotes are the most familiar domain, though perhaps not the most important to the Earth’s overall biosphere.  Eukaryotes include all animals, plants, fungi and many single-celled creatures, all of which are grouped together because they have complex cell nuclei divided into linear chromosomes. 

Then there are the bacteria—familiar as agents of disease, but actually ecologically crucial.  Some feed on dead organic matter.  Some oxidize minerals.  And some photosynthesize, providing a significant fraction (around a quarter) of the world’s oxygen.  Bacteria, rather than having complex nuclei, carry their genes on simple rings of DNA which float inside their cells.          

The third domain of life, the archaea, look, under a microscope, like bacteria.  For that reason, their distinctiveness was recognized only in the 1970s.  Their biochemistry, however, is very different from that of bacteria (they are, for example, the only organisms that give off methane as a waste product), and their separate history seems to stretch back billions of years.      

The Three Domain System was developed by Carl Woese following the identification of archaea as a third domain to classify organisms.  Until the late 1960's when Woese made his discovery, organisms had been classified according to a six kingdom system. Genetic sequencing has given researchers a whole new way of analyzing relationships among organisms and classes.  The current system, the three domain system, combines organisms primarily based on differences in ribosomal RNA structure.  Ribosomal RNA is a molecular building block for ribosomes.  Ribosomes serve to “create” proteins from amino acids.  With Woese’s modern taxonomy in use, organisms are now classified into three domains and six kingdoms that capture the entire web of life in one organic system.

from Threes, Chapter 3, “Threes in Science”