A unicellular organism is any life form that consists of just a single cell. This group includes most life on Earth, with bacteria serving as the majority. The main groups of single celled life are bacteria, archaea (both prokaryotes), and the eukaryota (eukaryotes). The differences between the prokaryota and eukaryota are significant: eukaryotes possess a nucleus, while prokaryotes lack it, and eukaryotes possess a range of subcellular organs called organelles, while prokaryotes are very minimal.
People can observe the larger unicellular organisms, such as amoebae, by using the higher settings on a light microscope. Bacteria are so small that they just appear as dots under such magnification. To gather them for observation, a person can place a cover slip on the surface of pond water, and leave it overnight. By the next morning, many organisms will have grown entire colonies on the bottom of the slip. They replicate fast: colonies can double their size in between 30 minutes and a few hours.
Unicellular organisms as diverse as they are ubiquitous. The oldest forms of life, they existed 3.8 billion years ago, if not longer. They pursue a variety of strategies for survival: photosynthesis (cyanobacteria), chemotrophy (many archaea), and heterotrophy (amoeba). Some have flagella, little tails they use for locomotion, or lobopods, extensions of the cellular skeleton (cytoskeleton), which appear as bloblike arms. The flagella of the unicellular ancestors of humans is retained all the way up into the animals, where it makes an appearance as flagellated sperm.
Of all the six eukaryote supergroups, four are exclusively composed of single cell organisms. Only the opisthokonts, consisting of animals, fungi, and close relatives, and the archaeplastids, consisting of both unicellular and multicellular plants, are exceptions. These organisms vary in size, with the smallest bacteria measuring 300 nanometers across, ranging up to the titantic plasmodial slime molds, which can grow to 8 inches (20 cm) across. The largest may have millions of nuclei scattered throughout the cellular envelope. To observe some of the smallest requires an expensive electron microscope, while the very largest can be seen with the naked eye.