In the early 2000s, bats infected with coronavirus transmitted the virus to raccoon dogs and other wild mammals in southwestern China. Many of these animals were sold in markets where the coronavirus resurfaced. Consequently, the SARS pandemic spread to 33 countries, resulting in 774 fatalities. Months later, researchers identified a mammalian coronavirus known as Palmcoten, found in markets central to the outbreak.

In a study published Wednesday, a team of researchers drew comparisons between the evolutionary paths of SARS and COVID-19, 17 years apart. They examined the genomes of the two coronaviruses responsible for the pandemics, alongside 248 related coronaviruses in bats and other mammals.

Jonathan Pechal, an evolutionary virologist at the University of Edinburgh and author of the study, noted that the histories of the two coronaviruses mirrored each other. “In my opinion, they are very similar,” he stated.

In both instances, Dr. Pekal and his team assert that coronavirus transmission originated from bats in southwestern China to wild mammals. Soon after, wildlife traders transported infected animals hundreds of miles to urban markets, leading to widespread human outbreaks.

“When wildlife is sold in urban centers, pandemics often follow,” stated Michael Warby, an evolutionary biologist at the University of Arizona and co-author of the research.

This research appears at a politically charged moment. Last month, the White House launched a web page titled “Laborek: The True Origins of COVID-19,” asserting the pandemic stemmed from a lab accident in Wuhan, rather than market interactions.

In a budget proposal issued on Friday, the White House indicated that it was “confirming” the lab leak theory, which justified an $18 billion cut to the National Institutes of Health.

The Chinese government responded with a flat denial of claims that COVID was caused by a lab leak in Wuhan, suggesting instead the virus may have originated from a laboratory in the U.S.

“A thorough and detailed investigation into the origin of the virus should be conducted in the United States,” the statement read.

Sergei Pond, a virologist at Temple University, expressed skepticism regarding the resolution of COVID’s origins. He voiced concerns that political rhetoric from both governments could hinder scientific inquiries into the virus’s origins.

“If it weren’t tragic, you’d have to laugh at how things have unfolded,” Dr. Pond commented.

In the initial weeks of the early 2020 pandemic, the virus responsible, SARS-CoV-2, emerged alongside notions of biological weapons created by the Chinese military. A cohort of scientists analyzing available data at that time dismissed this claim, indicating that while they could not dismiss accidental lab leaks, they leaned towards the natural origins of COVID.

Over the ensuing months, Dr. Warby, who was not part of that group, grew dissatisfied with the lack of compelling evidence favoring one theory over another. He co-signed an open letter with 17 other scientists advocating for further investigation to ascertain the most likely explanation.

“We felt there was much yet to be learned, so let’s not dismiss the lab leak theory,” Dr. Warby asserted. “Let’s investigate.”

As Dr. Warby and other researchers scrutinized COVID’s origins, the American intelligence agencies conducted their assessments. Their conclusions were varied. The FBI and CIA support the lab escape theory from the Wuhan Institute, albeit with little certainty. The Department of Energy expresses low confidence in the possibility of a virus escape from another lab in Wuhan, while other institutions favor a natural origin.

Scientists have struggled to evaluate the evidence behind these conclusions as the relevant institutions have not published supporting data or analyses. However, Dr. Warby and his colleagues have released several papers in scientific journals. Along this journey, Dr. Warby became convinced that the COVID pandemic originated at the Huanan Seafood Market in Wuhan.

“Scientifically, that’s evident,” Dr. Warby remarked, referencing both HIV and the Spanish flu, two diseases with origins that have been thoroughly studied.

In their recent study, Dr. Warby, Dr. Pekal, and their colleagues examined 250 genomes of coronaviruses, leveraging genetic similarities and differences to establish relationships. They successfully reconstructed the evolutionary timeline of the coronaviruses responsible for both SARS and COVID-19, known as SARS-CoV and SARS-CoV-2.

The ancestors of both coronaviruses have been circulating in bats across China and its neighboring regions for hundreds of thousands of years, specifically inhabiting southwestern China and northern Laos for the last 50 years.

When coronaviruses infect bats, they sometimes cohabitate with another coronavirus. This can lead to the accidental creation of a hybrid virus carrying genetic material from both original coronaviruses, a process referred to as recombination.

“These are not ancient events,” noted David Rasmussen, a virologist at North Carolina State University involved in the new research. “These occurrences happen frequently; these viruses are truly mosaic in nature.”

In 2001, shortly before the SARS pandemic emerged, researchers found that SARS-CoV had undergone significant genetic mixing among bats. This led to the virus’s potential evolution into a human pathogen. However, given that Guangzhou is hundreds of miles from the ancestral area of SARS-CoV, it’s improbable that the virus gradually reached the city through bats.

Researchers generally concur that ancestors of SARS-CoV infected wild mammals, which were subsequently sold in markets around Guangzhou. Shortly after the onset of the SARS pandemic, the presence of SARS-CoV was confirmed in palm civets and other wild mammals traded in the market.

A similar trend was observed with SARS-CoV-2, the virus responsible for COVID-19. The final recombination event in bats occurred between 2012 and 2014, just five to seven years prior to the COVID outbreak, hundreds of miles away from northeastern Wuhan.

Moreover, this signifies a considerable distance from the area where the virus’s ancestors circulated, paralleling the journey that SARS-CoV undertook via the wildlife trade.

Proponents of the lab leak theory emphasize the significant distance between Wuhan and the habitats of the closest relatives of SARS-CoV-2. They argue that if bats traveled to the area around Wuhan without infecting local mammals, scientists must have been collecting coronaviruses from bats in southwestern China and experimenting on them in the lab.

American scientists have critiqued the Wuhan Institute of Virology for lax safety measures in their coronavirus experiments. Nevertheless, no evidence has confirmed that the ancestors of SARS-CoV-2 were present at the institute prior to the pandemic. Recent research by Dr. Warby and his colleagues illustrates that bat coronaviruses can traverse considerable distances via wildlife trade without any scientific intervention.

The researchers argue that these findings align with a study published in 2022, identifying the Huanan Wet Market in Wuhan as the site of the initial COVID outbreak. Wild mammals were sold at this venue, and early cases of COVID were documented there. Moreover, Chinese researchers collected various strains of SARS-CoV-2, exhibiting different mutations at this location. Dr. Warby and his team posited that the virus likely spilled over from wild mammals at the market on two separate occasions.

Dr. Pond stated that while the new study supports the wildlife spillover theory, he does not believe the issue has been definitively settled. He highlighted two statisticians’ critiques from last year, regarding the modeling behind the 2022 study. Dr. Warby and his colleagues have provided a rebuttal to those criticisms. “That debate is still ongoing,” Dr. Pond remarked.

Mark Eloit, former director of the Pasteur Institute in Paris, emphasized the importance of the new research in clarifying the origins of SARS-CoV-2.

However, he also noted that the coronavirus exhibits significant differences from closely related bat viruses. Following its divergence from those viruses, it would have undergone mutations or recombination to effectively spread among humans.

“I contend that the potential for recombination events—either incidental or deliberate—remains as plausible as the hypothesis of zoonotic transfer via intermediate hosts at the market,” Dr. Eloit argued.

Dr. Eloit and other scientists concurred that discovering intermediate hosts of SARS-CoV-2 among wild mammals would significantly bolster the argument for natural spillover. However, Chinese officials examined various animals at the onset of the pandemic but found no traces of the virus.

Before scientists could conduct studies, wildlife vendors at the Huanan Market removed animals from the stalls, and when China ceased wildlife trade, farmers culled their animals.

“There are large gaps in our knowledge, and we can’t overlook that,” Dr. Pond remarked.

Stephen Goldstein, a geneticist at the University of Utah, remarked that while he was not involved in the new study, the findings serve as a cautionary reminder of future pandemic risks. Wild mammals traded in markets within regions where SARS and COVID-19 emerged can wreak havoc in urban centers hundreds of miles away. “These viral fragments are present in numerous places,” Dr. Goldstein concluded.