It's the enduring mystery of the COVID-19 pandemic: Where did the virus come from?
Scientists know that SARS-CoV-2, the pathogen that causes the illness, is part of a family of viruses found in some horseshoe bats, a type common to the tropics and subtropics outside the Americas. They also mostly agree that many of the earliest known infections and deaths clustered around a wildlife market in Wuhan, China.
But in the three years since COVID-19 emerged, researchers have been unable to determine how it infected its first victim, triggering a pandemic that has since killed almost 7 million people, according to an official count by the World Health Organization, and many more if the tally included deaths caused by unreported infections.
Debate intensified recently after a Chinese research team uploaded DNA evidence, swabbed from the market during the outbreak, to an international gene sequence database. The previously undisclosed data indicated the presence of wild animals in the same section of the market where the team had found SARS-CoV-2. The animals, known to be susceptible to bat viruses, included raccoon dogs, bamboo rats and porcupines.
Although not conclusive, some scientists said the data add evidence to the theory that the virus jumped from animals to humans through what is known as "zoonotic spillover," a source of many infectious diseases in humans.
Others suspect the pathogen somehow leaked from a Wuhan laboratory, 27 km from the market, where researchers study bat viruses. The idea gained more traction earlier this year when the U.S. Department of Energy, with "low confidence," in a report said that a lab leak was likely. Other U.S. agencies that have studied the matter lean toward natural spillover, although also inconclusively.
The specific basis for the assessments hasn't been publicly disclosed.
Though it remains a mystery how the virus got to Wuhan, spillover risk has been increasing dramatically in China, including several regions within 400 km of the market. Chinese court records are replete with cases of poachers taking wildlife from risky areas, and scientists from the Wuhan lab have collected bat samples there.
A Reuters data analysis found that human encroachment on bat habitats in recent decades has turned parts of China into an epidemiological minefield. These areas, termed "jump zones" by the news agency, combine factors including tree loss, precipitation and bat species to create conditions where spillovers are most likely.
Between 2002 and 2019, the data show, jump zones in the country expanded by 54%, an increase of nearly 150,000 sq km, an area larger than Nepal. One concentration of jump zones includes a region of mountains and lakes about 175 km southeast of the Wuhan market. That region, around China's giant Poyang Lake, has been heavily degraded by the construction of dams, mining and pig farming.
Still, scientists have struggled to find conclusive proof about the geographic or biological origins of COVID-19. In part, the mystery has continued because Beijing hasn't allowed an independent investigation into either hypothesis – infected animal or lab breach.
China's government has said that it has supported and taken part in research to determine COVID-19's origin. It has accused the United States of politicizing the matter, especially because of efforts by American intelligence agencies to investigate.
"Putting the intelligence community in charge for a matter of science is a clear sign that the issue has been politicized," Mao Ning, a spokesperson for China's foreign ministry, said at a March press conference.
Early in the outbreak, Chinese scientists found the COVID-19 pathogen on stall surfaces and floor drains of the Wuhan market where wild animals had been sold. But there's no evidence so far they tested live animals before the government closed the market.
Without more evidence, theories about a lab leak began to circulate.
Both possibilities have precedents in China.
In late 2002, the SARS-CoV-1 virus emerged in Guangdong province, in southern China, and became the SARS pandemic of 2003. At the time, scientists tested animals at a local market and found the virus in palm civets as well as evidence of infection in a raccoon dog and a ferret badger. Transmission from animal to human is widely considered by scientists as the source of that outbreak.
After the SARS pandemic ended, two graduate students contracted SARS while working at the National Institute of Virology in Beijing, where scientists were studying the pathogen. That outbreak, while contained, spread to nine people and killed one. It now serves as a reference for those who suspect a lab leak with COVID-19.
In 2010, Zhong Nanshan, the physician who led China's responses to SARS – and COVID-19 years later – told a Chinese newspaper that "the ecological balance between man and nature is overexploited." He noted the discovery at the time in Wuhan and Hong Kong of SARS-like viruses in horseshoe bats. His comments were cited in a research paper.
"If we resolutely take measures, I guess SARS will not return," he said. "If we do not strengthen management, it will definitely return."
Zhong didn't respond to Reuters' request for comment.
Since SARS-CoV-2 emerged, scientists have discovered close matches to the virus in samples collected from bats in Yunnan province. Researchers also found viruses closely related to the COVID-19 pathogen in bats across the border in Laos.
But none has been close enough to SARS-CoV-2 to be a direct ancestor. Locating an exact match could be like finding the proverbial needle in a haystack.
Without definitive evidence, speculation will likely continue.
Western governments, and much of the world's scientific community, have urged China to be more open and more collaborative with findings by its researchers.
"We continue to call on China to be transparent in sharing data, and to conduct the necessary investigations and share the results," said Tedros Adhanom Ghebreyesus, director general of the World Health Organization, in a March briefing. "Understanding how the pandemic began remains both a moral and scientific imperative."