The Theory of Development of Complex Organisms in the Process of Evolution

Many of those who remember their high school biology saw a comparison of developmental stages among embryonic vertebrates. This concept of comparing embryonic development led Ernst Haeckel (1834-1919) to propose his famous principle "ontogeny recapitulates phylogeny." This argument while less scientific than intuitive, offering significant face validity and verifying only some predictive validity, non-the-less was offered as basic biological defense of evolution.
PBS.org

All vertebrate embryos follow a common developmental path due to their common ancestry. All have a set of very similar genes (the homeobox genes) that define their basic body plan.

Now however, our understanding of DNA and molecular dynamics has greatly advanced since then. We now have a cohesive understanding of development of complex life from the less complex.
Harvard Magazine

The molecular evidence shows incontrovertibly that species have come into being gradually throughout the history of life on Earth. MORE