The evidence that supports the theory that fish and mammals evolved from the same ancestor is:
Their embryos share features in the early stages of development.
Both fish and mammals, being vertebrates, exhibit a number of shared embryonic features in the early stages of development, such as the presence of structures like pharyngeal arches (which can develop into gills in fish and structures in the jaw and ear in mammals) and similar overall body plans. This embryological evidence points to a common ancestor from which both groups evolved.