A Cell So Minimal That It Challenges Definitions of Life

Life’s fundamental structure is the cell, and so the main things that a cell does — processing biomolecules, growing, replicating its genetic material and producing a new body — are considered hallmarks of life. But earlier this year, scientists discovered a cell so severely stripped of essential functions that it challenges biologists’ definitions of what counts as a living thing. The species is a single-celled organism known only by the mysterious sequence of its genetic code. Its genome is fantastically small: Along the organism’s evolutionary journey, it seems to have gotten rid of most of it. According to the shocked researchers who published the discovery in a preprint uploaded to biorxiv.org in May, the lost genes include those central to cell metabolism, meaning it can neither process nutrients nor grow on its own. Other cells with highly reduced genomes still encode proteins to create amino acids, break down carbohydrates for energy or synthesize vitamins. All this appears to be absent from the cell, which seems to be a parasite entirely dependent on a host or cellular community to meet its nutritional needs. Until now, these genetic pathways were considered fundamental for the survival of any cell. The organism’s “replicative core” — the genetic components needed to reproduce itself — remains, making up more than half of its genome.