Despite the best efforts of Walt Disney and Elton John, it is the tree of life, not the circle, that remains the primary way that organisms are classified and by which their evolutionary relationships are depicted. The tree was initially made by categorizing life forms with similar features into groups; this method distinguished not only amphibians from reptiles but also protists from amoeba.

Genetic data expanded the tree by allowing us to use similarities in genetic sequences—we didn’t have to actually see anything in order to determine how everyone is related to each other. Now, genomic studies have expanded the tree still further, allowing us to place species we can’t even grow in the lab onto their proper branch.

It is hardly news that most life on Earth is unicellular. But the newest tree of life, published in Nature Microbiology, reveals that most of life's diversity is bacterial and that much of it belongs to a recently discovered branch of especially tiny bacteria that no one has ever grown or seen under a microscope. All we have is their DNA, mixed in with the DNA of everything else that inhabits the same ecosystem.

To classify organisms genetically, scientists have placed them in the appropriate “bins,” often by looking at the sequence of a specific gene (one that encodes the small subunit ribosomal RNA). This gene is required to make proteins, so all cells have it. The sequence of species’ rRNA gene is unique, like a fingerprint, so it has provided a tidy classification system. But these rRNA genes are still similar enough between species to be recognizable—or so it was thought.

In order to fish out a gene, scientists need to hook onto it using a complementary sequence, known as a primer. But these new bacteria, identified by the lab of Jillian Banfield at UC Berkeley, have rRNA genes that don’t interact with any previously used primers. That rendered them pretty much undetectable by genetic methods.

The new tree is made with genomic data in which researchers get all of the DNA of an organism rather than trying to fish out one or a few particular predetermined sequences. Dr. Banfield and her lab looked at thousands of genomes from the three traditional branches of life—Bacteria, Archaea, and Eukarya—to make their new tree. They were overwhelmed by the preponderance of life forms that have never been cultivated in a laboratory setting.

These bacteria are tinier than any we've studied to date; they fit through filters with holes that are only 0.1 microns in size. And their genomes reflect that, as they are about five times smaller than E. coli's. They lack genes for basic metabolic processes, like making amino acids, DNA and RNA nucleotides, respiration, and the Krebs cycle, used to generate energy from food. The researchers infer that they must be symbionts with a partner supplying all of their required materials.

It’s not clear if they were always that way or if they had their own metabolic capabilities once upon a time and lost them after establishing symbiosis with more complex life forms.

Eukarya, including all multicellular life, such as ladybugs and sunflowers and starfish and panda bears and us, comprise only a tiny fraction of the tree, which makes sense as we only developed a billion-and-a-half years into the whole evolutionary process. Ever since then, we’ve been swimming in a sea of bacteria. This tree just gives us a new way to visualize that fact.

Nature Microbiology, 2016. DOI: 10.1038/NMICROBIOL.2016/48 (About DOIs).