Apologies for this lengthy addendum, which is a summary of Nicholas Wade's excellent article on the origins of the virus. It is a long and technical piece which not everyone will have the time (or interest) to get through. But it is, in my view, essential to understanding why the lab origin hypothesis is probable, so here I provide the Cliff's Notes version.

He makes four key arguments for the lab leak hypothesis, some of which are a little abstruse to the non-scientific reader, but here they are:

1) The place of origin. If the virus had an animal origin there would be evidence of the virus in infected animals or humans outside of Wuhan, specifically in the region where these types of coronaviruses are known to come from. In a year of looking, no such evidence has materialized.

2) No evidence of gradual zoonotic mutation. In every other case of a virus making a jump from an animal host to a human host, it has gone through a process of evolutionary development which incrementally makes it more adapted to humans. This was the case for SARS and for MERS. In the case of COVID the virus emerged fully suited to human infection with no evidence of less successful antecedents.

3) The furin cleavage site. (Here we get technical.) The spike protein on the virus is composed of two separate sub-units, known as S1 and S2. Cleaving the spike protein into these two subunits is the key mechanism that allows the virus entry into a human cell. However it takes a specific enzyme on the surface of a cell to do this cleaving. There are many types of enzymes that can potentially cleave a protein, depending on the particular amino acids that comprise the protein. However only in humans is there the furin enzyme, which does its protein cutting thing in the presence of a specific amino acid sequence on the protein to be cleaved. COVID is the only coronavirus yet discovered that has this ‘furin cleavage site’ in its spike protein (between the S1 and S2 sub-units, naturally).

4) Human specific codons. (Here we get much more technical, so I will try to compress this a bit.) Proteins are simply chains of amino acids. Each amino acid is built from three units (nucleotides) of DNA, which are called codons. It turns out that there is more than one combination of the four DNA nucleotides (A, C, G, T) that produces the same amino acid. For example, the codons CGT and CGC both produce the amino acid arginine. But the key fact is that humans have a definite preference for a specific codon producing a given amino acid. It just so happens that in the furin cleavage site the codons that produce arginine are specific to humans and are very rare in the other parts of the coronavirus genome.

These are the technical arguments upon which Wade’s lab origin hypothesis rests. I find them compelling, particularly the last two points. The full paper can be found here: thebulletin.org/2021/05/the-origin-of-covid-did-people-or-nature-open-pandoras-box-at-wuhan/ and I recommend it highly.