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More than 18 months on from the start of the pandemic, the claim Covid-19 may have leaked from a Chinese laboratory is gaining traction. Why are the origins of Covid-19 still a mystery?

After the virus was first detected in China, authorities linked early Covid cases to a seafood market in Wuhan, leading scientists to theorise the virus first passed to humans from animals.

But in recent weeks the controversial claim that the pandemic might have leaked from a Chinese laboratory - once dismissed by many as a fringe conspiracy theory - has been gaining traction.

Now, US President Joe Biden has announced an urgent investigation that will look into the theory as a possible origin of the disease.

So what do we know about the competing theories - and why does the debate matter?

Newsnight’s Mark Urban reports.