Researchers in Australia and the United States combined their efforts to figure out where COVID-19 may have originated and published their results in a paper in Nature Medicine. This question is perplexing scientists everywhere, and several ideas have been proposed. The internet at large has also produced a few interesting guesses as to how we got into this situation, but do they hold water?
The prevailing hypothesis among scientists is that the virus came from bats, and by some unknown process, became adapted to infect humans. According to the authors, this unknown process may have been either 1) mutations while the virus was inside an animal host followed by a jump into humans or 2) mutations of the virus inside the human body. A third hypothesis is that the virus acquired these mutations in a research lab and accidentally leaked into the public.
To investigate the likelihood of each of these claims, the researchers collected all the coronavirus literature they could find to learn about the virus structure and what other scientists have done to trace its family tree. Here is what we know, and what we don’t.
The virus that causes COVID-19, called “SARS-CoV-2”, is structurally different from other coronaviruses in two ways. The first is called the “receptor binding domain,” which is basically the part of the virus that allows it to attach to a human cell. The second is a part of the binding domain called the “spike protein” which allows it to enter the cell. Once the virus gets inside the cell, it is able to copy itself and carry out the infection that your immune system eventually responds to, making you feel sick. The virus’s genetic information gives instructions for making these proteins, so knowing about its genome can tell us a lot.
There’s a place on the human cell called the ACE2 receptor, located on cells in places like the lungs and intestines, where SARS-CoV-2 attaches. Interestingly, computer models indicated that SARS-CoV-2 does not bind as well to the location on the human cell surface that as the 2003 SARS human coronavirus. While worried citizens wonder if SARS-CoV-2 was engineered in a lab for destructive purposes, their fears may be relieved. The researchers say if a computer predicted the virus to be worse at binding, it is not likely that it would have been engineered accordingly.
What about the spike protein? It is also different from the 2003 SARS virus, and different from the other bat coronaviruses. A weird feature of the spike protein in SARS-CoV-2 (called a “polybasic cleavage site”) has only been found in more distantly-related coronaviruses. The function of this feature is not really known, but it might make the virus more pathogenic. This mutation can either be added in a lab or acquired naturally by moving through a dense population of hosts – like a flock of birds or a crowd of people – for a long period of time.
However, before you jump to conclusions about foul play, the researchers also pointed out that there is no genetic evidence that SARS-CoV-2 has markers of any coronavirus gene editing kits on the market. They conclude that the trait was most likely naturally acquired.
There have been SARS infections in lab workers in the past, where the random mutations allowing it to infect humans were acquired during the process of growing the virus. However, they point out that the SARS-CoV-2 binding site has been found in other coronaviruses, so it was likely acquired by a process called recombination – where viruses encounter one another in nature and essentially swap genes.
The authors conclude that it is unlikely that this virus was manufactured, but more evidence is needed to shift the balance toward either an animal origin or a lab origin. In particular, they say that finding a coronavirus mutant in a mammal host that has the same spike protein as SARS-CoV-2 would be evidence in favor of an animal origin. Since COVID-19 is such a new disease, more studies are most definitely on the way.