This article in Harvard magazine explains that plants, animals and fungi are just a tiny part of the tree of life.
The modern "tree of life," based on genetic analysis, shows that the bulk of Earth's biodiversity resides among the Archaea, Bacteria, and that portion of the Eukarya that does not include plants, animals, and fungi.
Scientists had known that there are more microbes in an ounce of soil than humans alive on Earth, but that was just a measure of abundance. Pace's discovery demonstrated something new, a previously unfathomed repository of biodiversity. Scientists began sequencing DNA from all sorts of environments. After looking at human gut microflora, they learned that each individual has his or her own characteristic set of a thousand species. "These represent three million genes that you carry," points out Kolter, "as compared to the estimated 18,000 genes of the human genome. So you are living and exchanging [metabolites] constantly with a diverse pool of some three million genes." Microbiologists continue to find new taxonomic divisions of microbes far faster than they can figure out how to culture them.
Link (Thanks, Thomas!)