Types of Life

All living organisms can be classified based on their features. There is one special feature that divides all living organisms into two groups called empires, and that is the presence or absence of the cell nucleus. Take a look at this diagram of eukaryotic and prokaryotic cells.  

All of life is made out of cells, and all cells contain genetic information in the form of the molecule DNA. In eukaryotes, that DNA is held inside of the cell nucleus. The cell nucleus is enclosed by a fatty membrane, and contains DNA as well as a smaller structure called the nucleolus. In prokaryotes, DNA floats freely inside the cell. All multicellular organisms, including animals, plants, and fungi, are eukaryotes alongside other unicellular eukaryotes. As humans are multicellular animals whose cells contain nuclei, humans are eukaryotes. 

The prokaryota empire includes two distinct domains: bacteria and archaea. Prokaryotic cells are usually much smaller than eukaryotic cells. Both bacteria and archaea consist of small unicellular organisms, but they are extremely important to both the natural world and to human health (both bacteria and archaea can be found in your gut!). Because eukaryotes, bacteria, and archaea all perform starkly different chemistry inside of their cells, most biologists use the three domain system when classifying organisms: all organisms are either eukaryotes, bacteria, or archaea.