The common ancestors of birds and mammals were very early (stem) amniotes, 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 ________.

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The four-chambered hearts of birds and mammals are an example of homoplasy, meaning they evolved independently in each group and are not due to shared ancestry or structural or molecular similarity. Vestigial structures are remnants of once-functional features that have lost their original function over time, while structural and molecular homologies would refer to similarities between different species due to shared ancestry or functional constraints.

The Common Ancestors of Birds and Mammals Were Very Early