There are wars raging inside all of us. An organism’s identity can be simplified to a cellular compatibility between two genomes — one in a cell’s nucleus, and small copies of another in mitochondria. Researchers have found that they don’t always match, sparking “mitonuclear conflict” and competitive friction that they believe spawns new species. They’ve tested their theory on different populations of tiny crustaceans, where incompatible nuclear and mitochondrial DNA explained their genetic differences. The evidence isn’t irrefutable, but further study could reveal the evolutionary bridge between one species and the next.