In the late 1800s, a biologist studying animal embryos coined the phrase "ontogeny recapitulates phylogeny," meaning that the physical development of an animal embryo (ontogeny) seemed to retrace the changing form of the species during its evolutionary history (phylogeny). Why would embryonic development retrace evolutionary steps?

Embryo provides information about their ancestors.

Explanation:

Embryonic development retrace evolutionary steps because embryo provides evidence for evolution as embryo formation in widely-divergent groups of organisms are conserved. This embryo provides information that leads to their ancestors from which they evolve so we can say that embryonic development provides evidence for evolution of an organism.