Covid-19 cases detected before December 20 in 2019 were from the western side of the Wuhan market where mammal species were also sold.
Meanwhile, another study that also backs the claim on China's Wuhan market to be the epicentre of Covid-19 disease, used genetic information to track the timing of the coronavirus outbreak.
It suggests that a total of two variants were introduced into humans in November or early December 2019. This study in the journal Science, which is titled "The molecular epidemiology of multiple zoonotic origins of SARS-CoV-2", took a molecular approach and seems to determine when the virus crossed from animals to human beings.
According to this study, the first animal to human transmission might happened around November 18, 2019 which later, came to lineage B that was found only in people who had a direct connection to Huanan seafood market.