Horizontal Gene Transfer and the History of Life

Microbes acquire DNA from a variety of sources. The last decades, which have seen the development of genome sequencing, have revealed that horizontal gene transfer has been a major evolutionary force that has constantly reshaped genomes throughout evolution. However, because the history of life must ultimately be deduced from gene phylogenies, the lack of methods to account for horizontal gene transfer has thrown into confusion the very concept of the tree of life. As a result, many questions remain open, but emerging methodological developments promise to use information conveyed by horizontal gene transfer that remains unexploited today.The discovery of the existence of prokaryotic microbes dates back more than 300 years. Since then, our picture of our distant microscopic relatives has undergone several revolutions: from being the living “proofs” of the existence of spontaneous generation, they became later the “archaic” representatives of our distant ancestors, to finally be legitimately recognized as exceptionally diverse organisms, keystone to any ecosystem, including the most familiar and the most hostile environments on Earth. Similarly, although they were first seen as elementary and unbreakable bricks of life, they are now seen as genetically composite bodies, heavyweight champions of “gene robbery.” The most recent of these revolutions has indeed been the realization of their unparalleled ability to integrate genetic material coming from more or less evolutionarily distant organisms. This mechanism is called “horizontal gene transfer” as opposed to vertical transmission from mother to daughter cell.