IN OCTOBER 2014, virologist Edward Holmes took a tour of the Huanan Seafood Wholesale Market in Wuhan, a once relatively overlooked city of about 11 million people in the central Chinese province of Hubei. The market would have presented a bewildering environment for the uninitiated: rows of stalls selling unfamiliar creatures for food, both dead and alive; cages holding hog badgers and Siberian weasels, Malayan porcupines and masked palm civets. In the southwest corner of the market, Holmes found a stall selling raccoon dogs, stacked in a cage on top of another housing a species of bird he didn't recognize. He paused to take a photo.

Eight years on, that photo is a key piece of evidence in the painstaking effort to trace the coronavirus pandemic back to its origins. Of course, it's been suspected since the early days of the pandemic—since before it was even a pandemic—that the Wuhan wet market played a role, but it's been difficult to prove it definitively. In the meantime, other origin theories have flowered centered on the Wuhan Institute of Virology, a biological research lab which, it's argued, accidentally or deliberately unleashed the virus on the city and the world.

The overwhelming scientific consensus is that Covid originated in a similar way to related diseases such as SARS, which jumped from bats to humans via an intermediate animal. Figuring out exactly what happened with Covid-19 could prove immensely valuable both in terms of finally disproving the lab leak theory and by providing a source of information on how to stop the next pandemic. "This is not about placing blame," says Kristian Andersen, a professor of immunology and microbiology at the Scripps Research Institute in California. "This is about understanding in as much detail as we can the origins of the Covid-19 pandemic."

For the last two years, an international team of scientists including Andersen and Holmes has been trying to pinpoint the epicenter of the pandemic, using methods ranging from genetic analysis to social media scraping. Their research, which attracted widespread coverage in preprint before being published in its final form last week, reads as much like a detective report as an academic study….