Dollo’s law, named for the 19th-century paleontologist Louis Dollo, states that evolution always moves forward — that an organism cannot redevelop an organ or attribute discarded by its ancestors. But new research on humble dust mites, the tiny arachnids that live in cushions and carpets everywhere, is challenging the law.

Using DNA data to construct an elaborate family tree, two University of Michigan biologists have shown that dust mites — which are not parasites but free-living organisms — evolved from parasites that in turn evolved from other free-living organisms. That would seem to contradict Dollo’s law, since the mites should be unable to readopt the free-living characteristics discarded by their ancestors.