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.
There are histones in the nucleus that DNA coils around. The part of the DNA that is coiled around the histones is not accessible, and cannot be expressed in that cell. DNA that is not wrapped around a histone can be transcribed.