The embryology of different vertebrates provides very strong evidence of different vertebrates show striking similarities. There is an obvious similarity between embryos of fish, amphibians, reptiles, birds and mammals. A comparison of embryos of vertebrates shows that all have gill slits, even though they do not remain later in life, except in fish. This indicates a fundamental step that is common to all vertebrates and supports the idea of a common ancestor. Other features that that do not exist in the adult form but appear in the embryos include limb buds in Dolphins and human tail buds.
This shows that these species share a common ancestor, so their developmental processes occur in a similar way, regardless of what other changes have taken place since their divergence.