A living fossil is an extant taxon that cosmetically resemble ancestral species known only from the fossil record. To be considered a living fossil, the fossil species must be old relative to the time of origin of the extant clade. Living fossils commonly are of species-poor lineages, but they need not be. While the body plan of a living fossil remains superficially similar, it is never the same species as the remote ancestors it resembles, because genetic drift would inevitably change its chromosomal structure.