Many conspiracy theorists, who seem to make up about half the posters I'm seeing on Facebook these days, have glommed onto the idea that SARS-CoV-2 (the coronavirus causing the COVID-19 epidemic) was engineered in a lab. Where? Wuhan, China. Why? To cull China's aging population and to sow chaos around the world.

You'll be relieved to know that it originated via purely natural processes. Relieved because if it were that easy to artificially manufacture an infectious virus as a weapon of mass destruction, we'd be seeing a repeat of the present situation every month or so. On March 17, the journal Nature Medicine published the results of a study that seems to rule out any artificial — that is, non-evolutionary — source for the virus. Instead, it's just nature doing what nature does: experiment via mutations.

Background: A virus is a tiny agent, generally about one-hundredth the size of a bacterium, that can infect cells of plants and animals. In humans, most viral infections induce an immune response, either through our natural antibodies or through responses produced by vaccines. (Some viruses are immune to these responses, hence HIV, HPV and viral hepatitis.) A coronavirus has spikes around its spherical body ("corona" = crown). Seven coronaviruses are known to infect humans, three of which are serious: SARS (severe acute respiratory syndrome, China, 2003), MERS (Middle East respiratory syndrome, Saudi Arabia, 2012) and now SARS-CoV-2.

To recap recent history: On Dec. 31, Chinese authorities alerted the World Health Organization about a novel coronavirus. After a spectacularly rocky start in China (healthcare whistleblowers being censured and, in one case, dying from the disease), on Jan. 10, China released the full genetic "sequence" of the SARS-CoV-2 genome to researchers worldwide. The disease has since spread to 170 countries, causing more than 200,000 identified infections and more than 8,000 deaths as of press time.

The cause: The researchers in the study referred to above looked at two telltale features of the SARS-CoV-2 virus, distinctive components of the spikes that give coronaviruses their name: "cleavage sites" — molecular can openers, as it were — to crack open host cells; and "grappling hooks" (RBD, receptor-binding domain) to grab and penetrate the walls of its hosts' cells (specifically, human ACE-2, which regulate blood pressure). The RBD portion of the SARS-CoV-2 spike protein, it turns out, isn't as efficient as on the original SARS virus, implying that it arose naturally, not artificially, since a mad bioengineer would simply have copied SARS. Similarly, you'd expect our mad scientist to create a bomb-proof virus by copying the molecular "backbone" of other disease-causing viruses. Which really does rule out the artificial nature of SARS-CoV-2, because its backbone differs substantially from known coronaviruses. So how did it start? The study identifies two possible scenarios:

"Pre-adaptation": Similar to previous coronavirus outbreaks, the virus — battle-tested and ready to go — jumped straight from animals to humans. For SARS, the animal was a civet (a small tropical animal that's sold for its meat in parts of China), although the virus originated in bats, with civets being the intermediary; for MERS, the virus also came from bats, this time via camels. The study speculates that Malayan pangolins (armadillo-like mammals) may have been the link between bats and humans in the case of SARS-CoV-2.

"Post-adaptation": In this scenario, a non-pathogenic version of the virus jumped from an animal to a human, then mutated into its present form in its human host. The idea here is that the RBD of SARS-CoV-2 is similar to that found in pangolins, while the cleavage sites are similar to strains of bird flu. That is, SARS-CoV-2 appears to have evolved in humans from two different animal hosts.

Why does this matter? Because (quoting from the study), "If SARS-CoV-2 pre-adapted in another animal species, then there is the risk of future re-emergence events," whereas the second scenario is much less likely to recur. More research will decide this question.

Meanwhile, rest assured: Nature didn't need our help with this one; she came up with it all by herself.