It’s been more than three years since the COVID-19 outbreak began in Wuhan, China. A key mystery remains: Where did the virus come from?
Scientists have focused on five clues:
Clue 1: Early cases at Huanan Wholesale Seafood Market
"The first cases that were known were people who had been in or worked in or or visited a market where they sell all sorts of food in the middle of Wuhan," said Betsy McKay, a reporter with the Wall Street Journal.
Because Chinese authorities quickly closed the market and sanitized it, scientists researching how the virus spread didn’t have much to go on.
Clue 2: Genetic material
Scientists collected genetic material from animals at the market known to carry viruses like COVID.
"There was some genetic data collected three years ago that was uploaded to an international genetic database," McKay said. "There's DNA from an animal called a raccoon dog, which is known to be very susceptible to SARS viruses."
SARS viruses include SARS-COV-2, which is the name of the virus that causes COVID-19.
"What the sequences do not show is a raccoon dog infected with the COVID virus. But what they do show is raccoon dogs in the same place where there was a lot of COVID virus collected," McKay said.
Clue 3: The proximity of the initial outbreak to the Wuhan Institute of Virology
"There's a major laboratory in Wuhan that studies coronaviruses. This is a very high level lab that worked with American scientists and other international scientists," McKay said.
"We know that scientists there were genetically engineering coronaviruses under seriously inadequate safety conditions, and we know that viruses escape from labs all the time," said Nicholas Wade, a former science editor at the New York Times.
Clue 4: The structure of the virus itself
Many viruses use their spike proteins to bind and enter human cells. The former head of the CDC says COVID’s spike protein seemed tailor made to infect people.
"It looked like this virus was engineered," said Dr. Robert Redfield, a former director for the CDC.
Clue 5: The close relatives to COVID in nature
Last year scientists discovered decades-old coronaviruses found in cave bats in Laos could enter human cells. The spike protein from one of the bat viruses. And COVID-19’s spike protein — look almost identical.
So, five key clues to consider: the market, genetic material, the lab, COVID’s structure and natural relatives.
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"There is no smoking gun proving a laboratory origin hypothesis, but the growing body of circumstantial evidence suggests a gun that is at very least warm to the touch," said Jaime Metzl, a senior fellow at the Atlantic Council.
Other researchers conclude the virus crossed from animal to human naturally — possibly in the Huanan market.
"There is evidence of a natural animal spillover. And there really isn't much or any [evidence] of a laboratory leak at this point.
Most U.S. intelligence agencies have favored the natural origin, though the FBI and Department of Energy support the lab-leak hypothesis," McKay said. "Those U.S. agencies that lean towards a lab leak or believe it's more likely that the SARS-COV-2 virus came from a lab either expressed low confidence or moderate confidence."
Politically polling suggests two-thirds of Americans believe there was a lab leak. It's a view Republican politicians have largely embraced, while Democratic lawmakers mostly espouse the natural animal to human hypothesis.
"The process, unfortunately, was politicized from the very beginning," McKay said.
That includes the role of China. The international community has continuously criticized China for secrecy, and for impeding investigations into the origin of the virus.
"We continue to call on China to be transparent in sharing data and to conduct the necessary investigations and share the results," said Dr. Tedros Adhanom Ghebreyesus, the director general of the WHO.
China says it’s been "open and transparent" in the search for the virus' origins. Its leaders recently renewed an unfounded theory alleging the virus came from a U.S. military base.
The politics and bad blood surrounding the issue, scientists say, will continue to make solving the mystery all the more difficult.
