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#Question id: 9617


The common ancestors of birds and mammals were very early (stem) reptiles, which almost certainly possessed 3-chambered hearts (2 atria, 1 ventricle). Birds and mammals, however, are alike in having 4-chambered hearts (2 atria, 2 ventricles). The 4-chambered hearts of birds and mammals are best described as

#Unit 9. Diversity of Life Forms
  1. structural homologies.
  2. vestiges.
  3. homoplasies.
  4. the result of shared ancestry.