Nearly two years into the COVID-19 pandemic, the origin of the virus tormenting the world remains shrouded in mystery. Most scientists believe it jumped from bats to humans, either directly or through another animal. Others theorize it escaped from a Chinese lab. Now, with the global COVID-19 death toll surpassing 5.2 million, a growing chorus of scientists is trying to keep the focus on the animal-to-human theory in the hope that what’s learned can help humankind fend off new viruses and variants. But their search for answers is stymied by tensions between the U.S. and China.