We begin our survey of the different kinds of living things with the smallest organisms, the prokaryotes, also known as bacteria. Each prokaryotic organism consists of just a single cell. You don’t have to go to the forest to find prokaryotes. Our world is teeming with them! Any soil or puddle of water contains huge numbers of prokaryotic cells. In fact, prokaryotes occupy every habitat on the planet! Every person’s skin is covered with bacteria. There are more bacterial cells in your mouth than there are people on Earth, no matter how well you brush your teeth!

Since prokaryotic cells are some of the smallest in the living world, we need a microscope to see them. About 50 average-sized bacteria, lined up end-to-end, might just span the thickness of a human hair. The details within a prokaryotic cell can only be seen with a powerful electron microscope.

Although prokaryotes are small, and simple in some respects, they carry out the most complex biochemical processes of any life forms. Of all organisms, they show the greatest diversity in obtaining and using nutrients from their environment. All other living things on Earth ultimately depend on prokaryotes for their survival.

Prokaryotes break down dead organic matter and recycle its components, releasing compounds into the environment that other organisms require. Ancient prokaryotes living billions of years ago are even responsible for the oxygen-rich atmosphere we enjoy today, as well as for the ozone layer in the atmosphere that shields the planet from the sun’s ultraviolet rays! Prokaryotes have changed the Earth and the course of evolution.

Bacteria have had very important impacts on humanity. Even though they’re infamous as the cause of some deadly diseases, bacteria are also beneficial—for example, as the source of important medical drugs. They’ve been used for centuries for fermenting yogurt and cheese, and in recent times they’ve had a critical role as tiny workhorses in genetic engineering.

The study of prokaryotes at the molecular level has shed light on evolution at the root of the tree of life. A once-obscure group of prokaryotes, the archaebacteria, are now known to represent a clade distinct from the more familiar eubacteria, as well as from the eukaryotes. The result is a new concept about evolutionary history—the three domains of life. These domains are Bacteria, containing only the eubacteria; Archaea, containing only the archaebacteria; and Eukarya, containing all the eukaryotes.

We’ll begin our survey of prokaryotes by looking at their physical characteristics. Then we’ll consider prokaryotic lifestyles and activities. Finally, we’ll take a closer look at the two domains of prokaryotes, and find out what’s special about archaebacteria.

Copyright 2006 The Regents of the University of California and Monterey Institute for Technology and Education