According to embryology, why do fish and birds have common ancestor?

Both fish and bird embryos possess gill slits and gill arches.

Explanation:

Embryology -- the study of embryos -- is a fundamental area that is correlated to evolution. Together, they have provided evidence that we all evolved from a common ancestor.

This area of study has showed that embryos that look completely different as adults share many similarities during their development. This is the case of numerous species, such as humans and birds and fish and birds.

Both fish and bird embryos possess gill slits and gill arches. However, only fish possess them as adults. Gill slits develop into gill individual openings and gill arches will eventually support these gills. In contrast, adult birds do not possess gills, they are only present during their development. This provides significant insight into the previously mentioned common ancestor theory.

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