There are three possible explanations for why the evolution of SARS-CoV-2 looks so different from that of other viruses, and they are not mutually exclusive. First of all, we really haven’t looked that hard at other respiratory viruses. More than 7.5 million genomes of SARS-CoV-2 have been sequenced; just a few hundred or a few dozen for each of the four seasonal coronaviruses have been. When scientists try to reconstruct the relationship among these sequenced viruses in evolutionary trees, “the trees are so sparse,” says Sarah Cobey, a biologist at the University of Chicago. A whole suite of other viruses also cause common colds: rhinoviruses, adenoviruses, parainfluenza, respiratory syncytial virus, metapneumovirus, and so on. These, too, are poorly sampled. More than
100 types of rhinoviruses alone infect humans, but we don’t have a great understanding of how that diversity came to be or evolved over time.