A new study showed on Friday that the virus that causes COVID-19 could have started spreading in China as early as October 2019, two months before the first case was identified in the central city of Wuhan,.

According to a paper published in the PLOS Pathogens journal, researchers from Britain’s University of Kent used methods from conservation science to estimate that SARS-CoV-2 first appeared from early October to mid-November 2019.

The most expected date for the virus’s emergence was Nov. 17, 2019, and it had probably already spread globally by January 2020, they estimated.

China’s first official COVID-19 case was in December 2019 and was linked to Wuhan’s Huanan seafood market.

However, some early cases had no known connection with Huanan, implying that SARS-CoV-2 was already circulating before it reached the market.

A joint study published by China and the World Health Organization at the end of March acknowledged there could have been sporadic human infections before the Wuhan outbreak.

In a paper released in preprint form this week, Jesse Bloom of the Fred Hutchinson Cancer Research Center in Seattle recovered deleted sequencing data from early COVID-19 cases in China.

The data showed that samples taken from the Huanan market were “not representative” of SARS-CoV-2 as a whole, and were a variant of a progenitor sequence circulating earlier, which spread to other parts of China.