Whether Covid-19 was unleashed by a laboratory mishap or spilled over from animals remains an enduring, fiercely contested mystery. Here are the leading arguments that fuel both sides of this debate, as AFP reflects on the virus’s impact five years after it reshaped the world. The case for lab leak Proponents of the lab-leak hypothesis highlight that the earliest known Covid-19 cases emerged in Wuhan, China — home to the Wuhan Institute of Virology (WIV), a major hub for coronavirus research — located roughly 1,000 miles (1,600 kilometers) from the nearest bat populations carrying similar SARS-like viruses. “Wuhan labs performed research that placed them on a trajectory to obtain SARS viruses having high pandemic potential,” Richard Ebright, a microbiologist and professor at Rutgers University, told AFP. “One year before the outbreak, Wuhan labs proposed research to obtain SARS viruses having even higher pandemic potential and features that match, in detail, the features of SARS CoV-2,” he added. This research proposal included engineering a structure called a “furin cleavage site,” which increases viral growth and transmissibility but is absent in other SARS viruses. Lab-leak advocates also cite concerns over biosafety standards at the Wuhan lab, where personnel reportedly only wore lab coats and gloves. “There is sufficient evidence to conclude beyond reasonable doubt that SARS-CoV-2 entered humans through a research-related incident,” Ebright concluded. The case for natural spillover On the other side, researchers like Angela Rasmussen, a virologist at the Vaccine and Infectious Disease Organization at the University of Saskatchewan in Canada, argue that real-world “hard evidence” consistently points to a wholesale seafood market in Wuhan. “We’ve actually been looking at an evidence base that is hard evidence. It’s evidence that can be measured,” she told AFP, including genomic, geographic and environmental sampling data. She contends that the case for a lab origin, by contrast, is built on “what ifs” and speculation. That would include claims that proposals for research on ways to greatly increase virus transmissibility were publicly rejected but secretly carried out. This perspective is supported by multiple studies, including one published in the prestigious journal Science that analyzed the geographic pattern of Covid-19 cases during December 2019. The study showed cases were tightly clustered around Wuhan’s Huanan Seafood Wholesale Market. Another study, which examined genomic data from the earliest cases, concluded that the virus likely did not circulate widely in humans before November 2019.