The origins of the pandemic that has completely disrupted our societies and caused thousands of deaths across the globe have been the subject of a heated debate.

At first, scientists suggested it had come from a wet market in Wuhan, then the U.S. president, Donald Trump, advanced the idea that the virus might have escaped from a Chinese lab – a theory the experts say has no evidence.

So, where did COVID-19 come from? The origin of the virus is so controversial because scientists have not found yet enough evidence to prove one theory, and there are problems with the timeline of the cases.

"We have the first, I think, high-profile study in The Lancet, which in January suggested that a large part of the early cases had contact with the Wuhan seafood market," explains Peter Forster, geneticist at the University of Cambridge in the UK. 

"And so that, I think, gave rise to the idea that you have the seafood market as the origin of the epidemic. However, in The Lancet study, it is interesting that the very first patient fell ill on 1 December and had no known contact with the Wuhan seafood market."

Forster and his colleagues conducted their own study in Cambridge, and found three different strains of the virus, which he calls Type A, B, and C. "So we had samples from 24 December to the end of February and we found these A, B and C types. And what is interesting, A is the oldest type, so we can find that out by comparing it with the bat coronavirus. And what was interesting for me was that the A-type is not common in Wuhan in the early samples."

Out of 23 cases in Wuhan between 24 December 2019 and 17 January 2020 only three were type A, explains Forster. "And what is interesting for us now is to see how the variants, which were low frequency when we published, have now become high-frequency variants. So an important aspect of the research is to take a look at why certain virus types are becoming dominant," he says.

"And the question of why some types are dominant is important. It could be a harmless explanation that it simply is what we call a 'founder effect.' It could be a different clinical background, as a different explanation that the virus is somehow more contagious in certain forms."

Forster thinks more research is needed to be able to identify where exactly the virus originated. "I think we not only have to look at more human samples, we also have to look at more bats and perhaps other animals.

"Because this bat is not very similar to our human virus, it's only 96 percent similar, if the mutation rate is similar to a human mutation rate, and we don't know that, but if it is similar, there will be tens of years of difference," explains Forster.

"So it's probably not the original type. We need to find something that is closer. Perhaps it's a bat. Perhaps it's some other animal."