Signatures of adaptive evolution during human to mink SARS CoV2 cross-species transmission inform estimates of the COVID19 pandemic timing

One of the unique features of SARS-CoV-2 is that it mainly evolved neutrally or under purifying selection during the early pandemic. This contrasts with the preceding epidemics of the closely related SARS-CoV and MERS-CoV, both of which evolved adaptively. It is possible that the SARS-CoV-2 exhibits a unique or adaptive feature which deviates from other coronaviruses. Alternatively, the virus may have been cryptically circulating in humans for a sufficient time to have acquired adaptive changes for efficient transmission before the onset of the current pandemic. In order to test the above scenarios, we analyzed the SARS-CoV-2 sequences from minks (Neovision vision) and parenteral human strains. In the early phase of the mink epidemic (April to May 2020), nonsynonymous to synonymous mutation ratios per site within the spike protein was 2.93, indicating a selection process favoring adaptive amino acid changes. In addition, mutations within this protein concentrated within its receptor binding domain and receptor binding motif. Positive selection also left a trace on linked neutral variation. An excess of high frequency derived variants produced by genetic hitchhiking was found during middle (June to July 2020) and early late (August to September 2020) phases of the mink epidemic, but quickly diminished in October and November 2020. Strong positive selection found in SARS-CoV-2 from minks implies that the virus may be not unique in super-adapting to a wide range of new hosts. The mink study suggests that SARS-CoV-2 already went through adaptive evolution in humans, and likely been circulating in humans at least six months before the first case found in Wuhan, China. We also discuss circumstances under which the virus can be well-adapted to its host but fail to induce an outbreak.