Biologists have found that savannah monitor lizards extract oxygen from the air when inhaling and when exhaling — a characteristic, known as unidirectional breathing, that is most associated with birds. The discovery of this trait in lizards, reported in the journal Nature, raises questions about when and why it evolved. Either it developed in the 270-million-year-old common ancestor of lizards, birds and alligators (which were also recently found to practice unilateral breathing) — much longer ago than originally thought — or it developed independently in each evolutionary branch. As for why it evolved, the long-held theory that it aided flight in birds appears to need updating.