During development, an organism's embryo passes through stages resembling its evolutionary ancestors , 'ontogeny recapitulates phylogeny' , showing a fish-like stage, amphibian stage, and so forth.
Ernst Haeckel's recapitulation theory captured evolutionary insight but was an oversimplification that became misleading when taken literally. Modern developmental biology shows that embryos resemble ancestral embryos, not ancestral adults, and embryonic development involves both conserved and highly modified processes. While comparative embryology reveals evolutionary relationships, the idea that human embryos literally 'replay' evolutionary history is false. Contemporary evo-devo biology provides a more nuanced understanding of evolutionary development.