Enlarge / All life with complex cells, including us, is part of the little green rounding error in the lower right. (credit: Banfield/Nature Microbiology)
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.
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