through susceptible rodents. Although SARS-CoV-2 has since been engineered (Dinnon et al., 2020) and mouse-adapted by serial passage (Gu et al., 2020; Leist et al., 2020; Sun et al., 2020a), specific mutations in the spike protein, including N501Y, are necessary for such adaptation in mice (Gu et al., 2020; Sun et al., 2020a). Notably, N501Y has arisen convergently in multiple SARS-CoV-2 variants of concern in the human population, presumably being selected to increase ACE2 binding affinity (Khan et al., 2021; Kuzmina et al., 2021; Liu et al., 2021; Starr et al., 2020). If SARS-CoV-2 resulted from attempts to adapt a SARSr-CoV for study in animal models, it would likely have acquired mutations like N501Y for efficient replication in that model, yet there is no evidence to suggest such mutations existed early in the pandemic. Both the low pathogenicity in commonly used laboratory animals and the absence of genomic markers associated with rodent adaptation indicate that SARS-CoV-2 is highly unlikely to have been acquired by laboratory workers in the course of viral pathogenesis or gain-offunction experiments. EVIDENCE FROM GENOMIC STRUCTURE AND ONGOING EVOLUTION OF SARS-CoV-2 Considerable attention has been devoted to claims that SARSCoV-2 was genetically engineered or adapted in cell culture or ‘‘humanized’’ animal models to promote human transmission (Zhan et al., 2020). Yet, since its emergence, SARS-CoV-2 has experienced repeated sweeps of mutations that have increased viral fitness (Deng et al., 2021; Otto et al., 2021; Simmonds, 2020). The first clear adaptive mutation, the D614G substitution in the spike protein, occurred early in the pandemic (Korber et al., 2020; Volz et al., 2021). Recurring mutations in the receptor binding domain of the spike protein, including N501Y, K417N/T, L452R, and E484K/Q—constituent mutations of the variants of concern—similarly enhance viral infectivity (Cai et al., 2021; Khan et al., 2021; Kuzmina et al., 2021) and ACE2 binding (Liu et al., 2021; Starr et al., 2020; Zhu et al., 2021a), refuting claims that the SARS-CoV-2 spike protein was optimized for binding to human ACE2 upon its emergence (Piplani et al., 2021). Further, some pangolin-derived coronaviruses have receptor binding domains that are near-identical to SARS-CoV-2 at the amino acid level (Andersen et al., 2020; Xiao et al., 2020) and bind to human ACE2 even more strongly than SARS-CoV-2, showing that there is capacity for further human adaptation (Dicken et al., 2021). SARS-CoV-2 is also notable for being a host generalist virus (Conceicao et al., 2020), capable of efficient transmission in multiple mammalian species (including mink, tigers, cats, gorillas, dogs, raccoon dogs, and ferrets), and large outbreaks have been documented in mink with spill-back to humans (Oude Munnink et al., 2021) and to other animals (van Aart et al., 2021). Combined, these findings show that no specific human ‘‘pre’’ adaptation was required for the emergence or early spread of SARS-CoV-2, and the claim that the virus was already highly adapted to the human host (Zhan et al., 2020), or somehow optimized for binding to human ACE2, is without validity. The genesis of the polybasic (furin) cleavage site in the spike protein of SARS-CoV-2 has been subject to recurrent speculation. Although the furin cleavage site is absent from the closest known relatives of SARS-CoV-2 (Andersen et al., 2020), this is unsurprising because the lineage leading to this virus is poorly sampled and the closest bat viruses have divergent spike proteins due to recombination (Boni et al., 2020; Lytras et al., 2021; Zhou et al., 2021). Furin cleavage sites are commonplace in other coronavirus spike proteins, including some feline alphacoronaviruses, MERS-CoV, most but not all strains of mouse hepatitis virus, as well as in endemic human betacoronaviruses such as HCoV-OC43 and HCoV-HKU1 (Gombold et al., 1993; ll OPEN ACCESS Cell 184, September 16, 2021 4851 Review de Haan et al., 2008; Kirchdoerfer et al., 2016). A near identical nucleotide sequence is found in the spike gene of the bat coronavirus HKU9-1 (Gallaher, 2020), and both SARS-CoV-2 and HKU9-1 contain short palindromic sequences immediately upstream of this sequence that are indicative of natural recombination break-points via template switching (Gallaher, 2020). Hence, simple evolutionary mechanisms can readily explain the evolution of an out-of-frame insertion of a furin cleavage site in SARS-CoV-2 (Figure 2). The SARS-CoV-2 furin cleavage site (containing the amino acid motif RRAR) does not match its canonical form (R-X-R/KR), is suboptimal compared to those of HCoV-HKU1 and HCoV-OC43, lacks either a P1 or P2 arginine (depending on the alignment), and was caused by an out-of-frame insertion (Figure 2). The RRAR and RRSR S1/S2 cleavage sites in feline coronaviruses (FCoV) and cell-culture adapted HCoV-OC43, respectively, are not cleaved by furin (de Haan et al., 2008). There is no logical reason why an engineered virus would utilize such a suboptimal furin cleavage site, which would entail such an unusual and needlessly complex feat of genetic engineering. The only previous studies of artificial insertion of a furin cleavage site at the S1/S2 boundary in the SARS-CoV spike protein utilized an optimal ‘‘RRSRR’’ sequence in pseudotype systems (Belouzard et al., 2009;