The different molecular mechanisms used by the two S1-CTDs to recognize ACE2 suggest a convergent evolutionary relationship between the two S1-CTDs (i.e., the two S1-CTDs evolved independently to recognize the same virus-binding hot spot on ACE2), althogh a divergent evolutional relationship cannot be completely ruled out (i.e., the two S1-CTDs both evolved from a common ancestral protein that bound ACE2).