The RBM  of S1-CTD is located on the very tip of the trimeric spikes and evolves at the fastest pace. On the other hand,  S1-NTD is likely located underneath S1-CTD, is less exposed to the host immune system, and evolves at a slower pace than S1-CTD. Therefore, between the two S1 domains, the more conserved S1-NTDs may function as the more reliable RBDs that recognize sugar receptors, allowing coronaviruses to search for additional and high-affinity protein receptors using their fast-evolving S1-CTDs. Such dual-RBD structures in coronavirus spikes may give coronaviruses an evolutionary advantage in finding new receptors and expanding their host ranges.