The acquisition of the furin cleavage site has led some to argue that the origins of COVID-19 lie not in natural animal viruses, but in deliberate manipulation in a laboratory. The researchers contacted by Live Science for this story dismissed this as evidence for such an origin, however. The original version of SARS-CoV-2 actually had a wimpy version of the furin cleavage site and was not particularly transmissible compared with what was to come, Wertheim said.

“Anyone who says they’ve never seen a more perfectly adapted human virus, well, they clearly hadn’t met the delta variant,” Wertheim said.

In January 2020, well before the word “variant” exploded into everyone’s consciousness, SARS-CoV-2 acquired a spike protein mutation called D614G that made it perhaps 20% more transmissible. Coronavirus strains with this mutation quickly took over the world. And in the spike protein, evolution has marched on. The alpha variant of coronavirus was 50% more transmissible than the variants with D614G alone, according to Yale Medicine, and the delta variant is around 50% more transmissible than alpha.

The spot on the coronavirus’ genome that encodes for the furin cleavage site is also evidence for a natural origin, Goldstein said. The mutation is a string of 12 nucleotides dropped right in the middle of a codon, or three-nucleotide sequence, that codes for the amino acid serine. By a stroke of evolutionary good luck for the virus, the sequence still works for coding for proteins: All amino acids are coded for by three-nucleotide codons, and because 12 is a multiple of three, the overall rhythm of the sequence remains undisturbed. But the position of the mutation smack dab in the middle of the codon for another amino acid looks far more like an accident of nature than something engineered deliberately.