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 earliest known infections and deaths clustered around a wildlife market in Wuhan, China.
But 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 seven million people, according to an official count by the World Health Organisation.
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 found SARS-CoV-2.
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 leaked from a Wuhan laboratory, 27km from the market, where researchers study bat viruses.
The idea gained more traction when the US Department of Energy in a report said that a lab leak was likely.
Other US agencies lean toward natural spillover, although also inconclusively. Spillover risk has been increasing in China, including several regions within 400km of the market.
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 spill-overs are most likely.
Between 2002 and 2019, jump zones expanded by 54 per cent, an increase of nearly 150,000 sq km, an area larger than Nepal.
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 infected animal or lab breach.
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 they tested live animals before the government closed the market.
Then, 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, southern China, and became the SARS pandemic of 2003.
At the time, scientists tested animals at a 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.
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 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. Without definitive evidence, speculation will likely continue.
* The writers are from the Reuters news agency