Abstract
Metagenomics has opened up an entirely new perspective on life. It reveals that microorganisms colonize every environment on the planet and that the diversity studied in the laboratory represents only a tiny fraction of the total. The number of bacterial species remains unknown, with estimates varying by six orders of magnitude. These discoveries have profoundly reshaped the tree of life, which is now dominated by uncultured microbial lineages, such as the ultramicrobacteria of the Candidate Phyla Radiation . The Asgard archaea have also revolutionized our understanding of the origin of eukaryotes. Animals, plants, and humans thus appear on a small, relatively recent branch of a tree that is immensely older and profoundly microbial.
