Since the start of the SARS-CoV-2, or COVID-19, pandemic over a year ago, there have been several different theories as to where the virus originated. Some speculated that it was transmitted from someone eating bat soup in Wuhan, some suggested that it originated from the illegal trade of Malayan pangolins or that it had originated from some animal at a wet market in Wuhan.

Former President Donald Trump even said at a press conference in April 2020 that he had evidence the virus was leaked from the Wuhan Institute of Virology despite reports from U.S. intelligence agencies concluding that there was no evidence the virus was genetically modified or artificially made, according to the Guardian.

The general consensus, though, is that the COVID-19 virus is zoonotic, meaning it came from an animal according to Nature.

Though at first the Huanan seafood market was thought to be the epicenter of the breakout, a study by Huang et al. showed that only 27 of the 41 original cases were directly exposed to the market. This means that the virus did not necessarily originate from animals at the wet market.

There is also speculation that the virus was already circulating before the first known patient contracted it. Peter Ben Embarek, leader of the World Health Organization investigation into the origin of COVID-19, said that because the virus was so well established in Wuhan so quickly, it is likely that the virus had already spread.

The WHO has been working since May 2020 to figure out where the virus came from and how it was transmitted to humans. The study, published March 30, 2021, was inconclusive.

Since several different animals are known to carry different types of severe acute respiratory syndrome, or SARS, animals were looked at first. Bats and pangolins most notably were looked into for a possible link.

“Genome analysis reveals that bats may be the source of SARS-CoV-2. However, the specific route of transmission from natural reservoirs to humans remains unclear,” according to the WHO report. “The whole-genome sequence identity of the novel virus has 96.2% similarity to a bat SARS-related coronavirus.”

This means that although a bat-related coronavirus’s composition is close to COVID-19, it is not the same thing.

The same happened when WHO tested pangolins, revealing that the pangolin virus shared a 90.1% similarity with COVID-19.

Researchers from WHO were sent to the Huanan market in Wuhan to take samples from both the environment — meaning swabs from walls and stalls — and the animals for sale. Though COVID-19 was found in the sewage and drains, it was not found on live or dead animals at the market.

The reason so much focus was placed on the Huanan market was because there were a lot of people who contracted COVID-19 near the beginning of the pandemic whose only exposure to that amount of meat and animals was at the market, according to a study by Huang et al.

Hung Nguyen-Viet, a WHO team member and food safety and environment researcher said that there were many different species available for sale at the market that could have transmitted the virus to humans.

To be sure that there is a connection between SARS-CoV-2 in humans and animals, a direct link needs to be found. So far, 30,000 livestock, wild and domestic animals in China have been tested for previous or current SARS-CoV-2, but only a few cats have tested positive, according to a study by Zhang et al.

Ben Embarek noted that the number of animals tested, though, is not representative of the animal population in China, and many more animals need to be tested to truly have an idea if the virus was transmitted through them.

Researchers have provided many suggestions about what to do next. Many more tests need to be conducted, including extensive tests of livestock animals. Bats and pangolins also need to be tested further, and it was recommended that surveys be conducted to find other viruses in animals related to SARS-CoV-2.

Peter Daszak, a WHO team member, recommended that the wildlife farms that supplied meat to the Huanan market need to be traced back even more to try and find potential transmission routes. This means interviewing and testing workers, vendors, delivery staff and other people involved in the raising, transporting and sale of animals that were sold at markets linked to the virus.

Finding potential transmission routes also includes mapping out species that have been known to carry viruses like SARS. This is difficult because there are limits to detecting animals in larger geographical ranges. Bats and pangolins in Southeast Asia, specifically, need to be researched more.

Though there is not one certain cause of COVID-19, the most likely cause was the wildlife trade, Daszak said, especially with how quickly it spread in Wuhan.