The authors of the new work studied how the new type of coronavirus spread among not only people, but also animals. Immediately after the outbreak in China, it made another interspecies transition and began infecting minks on farms in Denmark, Poland, Canada and several other countries.
The authors concluded that the rate of evolution of the virus increased dramatically after it entered the mink population. As a result, SARS-CoV-2 acquired several unique mutations that allowed it to better adapt to infecting another mammalian species.
Based on this information, the authors tried to estimate how quickly the ancestor of SARS-CoV-2 could go from spreading among bats or other wild animals to humans.
To get the exact time span, the authors analyzed how quickly mutations accumulated in the genome of those coronavirus samples that were isolated from mink organisms and working on farms. The researchers calculated the time that had to elapse between the appearance of the first strains of SARS-CoV-2 capable of infecting humans and the appearance of the virus in Wuhan.
They went on to compare how early variations of the causative agent COVID-19 and its putative ancestor, the RaTG13 bat virus, differed. As a result, they concluded that the virus appeared in China six months before the first massive outbreak.
SARS-CoV-2 has already experienced adaptive natural selection in the human population. This suggests that the ancestor of the pathogen COVID-19 circulated among people for at least six months before the first infections were recorded in Wuhan.