Research into the origins of the new coronavirus raises questions about how it became so infectious in human beings

New research has deepened, rather than dispelled, the mystery surrounding the origin of the coronavirus responsible for Covid-19. Bats, wildlife markets, possibly pangolins and perhaps laboratories may all have played some role, but the simple story of an animal in a market infected by a bat that then infected several human beings no longer looks credible.

A study published in early May by scientists at the Broad Institute in Cambridge, Mass., and at the University of British Columbia has uncovered an unusual feature of the virus’s recent development: It has evolved too slowly. The genomes of viruses sampled from cases during the SARS epidemic of 2002-2003 showed rapid evolutionary change during the early months of the epidemic, as the virus adapted to its new host, followed by much slower change later. By contrast, samples taken from recent cases of the new coronavirus, SARS-CoV-2, have comparatively few genetic substitutions compared with an early case from December.