Chris Newman, a wildlife biologist at the University of Oxford and a co-author of one of the studies, said that his Chinese colleagues saw a number of wild mammals for sale at the Huanan market in late 2019. Any of them might have been responsible for the pandemic, Dr. Holmes said.

“You can’t prove raccoon dogs yet, but they’re certainly a suspect,” he said.

Some critics have questioned how sure Dr. Holmes and his colleagues can be that a Huanan animal was to blame. Although many of the earliest Covid cases were linked to the market, it’s possible that other cases of pneumonia have not yet been recognized as early Covid cases.

“We still know far too little about the earliest cases — and there are likely additional cases we don’t know about — to draw final conclusions,” said Filippa Lentzos, an expert on biosecurity at King’s College London. “I remain open to both natural spillover and research-related origins.”

Another problem: If infected animals indeed started the pandemic, they’ll never be found. In January 2020, when researchers from the Chinese C.D.C. arrived at the market to investigate, all the animals were gone.

But Dr. Holmes argues that there’s more than enough evidence that animal markets could spark another pandemic. Last month, he and Chinese colleagues published a study of 18 animal species often sold at markets, obtaining them either in the wild or on breeding farms.

“They were absolutely full of virus,” Dr. Holmes said.

Over 100 vertebrate-infecting viruses came to light, including a number of potential human pathogens. And some of these viruses had recently jumped the species barrier — bird flu infecting badgers, dog coronaviruses infecting raccoon dogs. Some of the animals were sick with human viruses, too.

The simplest way to reduce the odds of future pandemics, Dr. Holmes has argued, is to carry out studies like this one at the interface between humans and wildlife. His own experience discovering new viruses has convinced him that it doesn’t make sense to try to catalog every potential threat in wildlife.

“You could never possibly sample every virus out there and then work out which one of those can infect humans,” Dr. Holmes said. “I don’t think that’s viable.”