When you were a 5 week old embryo, you looked like a fish embryo and had a long tail and gill slits. It’s true!
What would you think if we told you that, when you were an embryo only 5 weeks after conception, you looked like a fish embryo and had a long tail and gill slits? It’s true! All embryos of backboned animals look fish-like at first, with long tails and gill slits and other fishy features. But then fish embryos grow and develop to keep these features, while the embryos of amphibians, reptiles, birds, and mammals modify these features, so the gills are replaced by lungs, and in some animals (like apes and humans) the tail disappears entirely. This powerful line of evidence is proof that all backboned animals evolved from a fish-like ancestor in the distant past, and have transformed by changing the later stages of their embryonic development. So you re-enact your long evolution from the fish in every part of your development as an embryo!