During their early stages of development, the embryos of reptiles, birds, and mammals look very similar. this suggests that reptiles, birds, and mammals:

This suggests that these animals have a common ancestor.

Explanation:

Common ancestor is a species that, through the process of descent with modification, gives rise to new species. The evolutionary relationships of organisms can be represented by phylogenetic trees (or phylogenies), which are branched diagrams in the shape of a tree that show ancestral relationships between species.

Organisms that share a common ancestor are more related to each other. For this reason, when we observe that when reptile embryos, birds and mammals look very similar, they probably share a common ancestor.