What happened, during those decades of evolutionary divergence, to bring a still-undiscovered bat coronavirus to the brink of spillover into humans and enable it to become SARS-CoV-2? We don’t yet know. Scientists in China will keep looking for that closer-match virus. The evidence gathered so far is mixed and incomplete, complicated by the fact that coronaviruses are capable of a nifty evolutionary trick: recombination.
That means that when two strains of coronavirus infect the same individual animal, they may swap sections and emerge as a composite, possibly (by sheer chance) encompassing the most aggressive, adaptive sections of the two. SARS-CoV-2 may be such a composite, built by happenstance and natural selection from components known to exist among other viruses in the wild, and emerging from its nonhuman host with a fearsome capacity to grab, enter and replicate within certain human cells.
Bad luck for us. But evolution is not rigged to please Homo sapiens.
SARS-CoV-2 has made a great career move, spilling over from its reservoir host into humans. It already has achieved two of the three Darwinian imperatives: expanding its abundance and extending its geographical range. Only the third imperative remains as a challenge: to perpetuate itself in time.