Question

Embryos of different species look very similar, which shows that the organisms share a
ancestor.

Answer 1

Your claim is essentially right if you mean “share a common ancestor.” Embryonic similarities are an important line of evidence for common descent, because developing embryos often reveal homologous structures and conserved developmental programs that are less altered by later specialization.

Key points:
- Many vertebrate embryos (fish, birds, mammals) show similar early features — pharyngeal arches (sometimes called gill slits), a tail, and segmented somites — indicating those features were present in an ancestral vertebrate.
- Developmental genes and pathways (for example Hox genes) are highly conserved across very different animals, which supports the idea of shared ancestry and deep homology.
- Historical caveat: the 19th‑century idea “ontogeny recapitulates phylogeny” (Haeckel) is oversimplified and parts of Haeckel’s embryo drawings were misleading. Modern evo‑devo shows a more nuanced picture: embryos can be conserved at particular stages (the “phylotypic” stage) and later diverge.
- Embryonic similarity is one line of evidence — together with comparative anatomy, molecular genetics, and the fossil record it builds a strong case for common ancestry.

So: yes — embryonic resemblance supports the organisms sharing a common ancestor, but it isn’t the only evidence and the historical oversimplification should be avoided.