The common ancestors of birds and mammals were very early (stem) reptiles, which almost certainly possessed three-chambered hearts (two atria, one ventricle). Birds and mammals, however, are alike in having four-chambered hearts (two atria, two ventricles). The four-chambered hearts of birds and mammals are best described as
A) structural homologies.
B) vestiges.
C) homoplasies.
D) the result of shared ancestry.
E) molecular homologies.
