The newly described microbe is so stripped down that it strains the definition of a cell. It has lost so many genes that it cannot make many of the molecules it needs, and it survives only by living attached to another organism, stealing supplies from its host. Yet it is alive, and it reproduces, and it is rewriting assumptions about how little a protein.">genome must contain.
Organisms like this belong to a group of bacteria that have been shedding genes for hundreds of millions of years, evolving toward simplicity rather than complexity. Genome sequencing has revealed that they are not rare curiosities but a widespread and ancient strategy, especially in the microbial world, where dependence on a neighbour can be more efficient than self-sufficiency.
Their existence complicates the search for a minimal set of genes required for life. Every candidate list is drawn up from organisms that were already independent. These cells suggest that the true minimum depends on context: with the right partner, the requirements drop dramatically. They also raise the possibility that other, even more reduced forms remain undiscovered.
