For a year now, as the world tried to figure out how to stop the COVID pandemic, many people have been consumed by a different question: How did it start? In an interview with CNN that aired on March 28, a prominent scientist speculated, without evidence, that the origin was when the SARS-CoV-2 virus escaped from a laboratory in Wuhan, China, where the outbreak was first noticed. Virologist Robert Redfield, a former director of the U.S. Centers for Disease Control, said “That’s my own view. It’s only opinion.”

Two days later, advocates of a different origin gave their view: there was a wildlife spillover, with a virus that started in bats in China. A joint report from the World Health Organization and the Chinese government speculated, again without direct evidence, that a bat virus went through other animals and ended up infecting humans.

Nobody has found a coronavirus in a Wuhan lab that experiments made more transmissible, became identical to SARS-CoV-2, and then infected a worker. Likewise, nobody has found a coronavirus in the wild that mutated to become similar to SARS-CoV-2 as it passed through other animals, and then infected humans. Both ideas are largely evidence-free at this point. They are both possible.

But they are not, however, equally probable. They differ in the number of events that could create each scenario. Redfield’s lab leak idea relies on one event, or perhaps a small handful: a mistake in the lab. The wildlife spillover idea has millions of chances to occur.

Redfield’s speculation is that any virus that comes from animals and became so efficient at infecting humans had to have lab help to do so in one quick jump. That single quick leap is a big assumption.

In fact, Redfield himself, in the same CNN interview, said he thinks the virus was circulating for months before we noticed it. That is not a quick jump. It is an extended time period that fits idea no. 2, the wildlife spillover.

That idea holds there are billions of bats in China, and millions of encounters every week among bats and other wild animals and, in some cases, humans. The virus has many chances to jump. In its original form, it is inefficient at replicating in people. But it has millions of chances to get better even before it infects the first human. Bats go out foraging and have numerous encounters with other animals, such as pangolins, badgers, pigs and many others, and an opportunistic virus can infect these species. Coronaviruses mix among bat colonies, giving them chances to re-sort their genes. They even mix among single bats: a bat has been observed harboring several different coronaviruses.

These viruses have time. They don’t take one jump but spend months moving from host to host, mutating as they go. Once in people, virus versions that get mutations that improve their ability to infect human cells get chances to replicate more often. They soon become good enough at infecting those cells that humans become noticeably sick, and we finally notice a new disease. This happens in the same timeframe that Redfield says the virus was circulating.

We can actually see this occurring with the novel coronavirus right now. It is quickly gaining mutations, called E484K and 501Y, that make it more infectious, and doing so in independent lineages across the globe, according to research by evolutionary microbiologist Vaughn Cooper of the University of Pittsburgh Medical Center. This is happening naturally, because millions of infections around the globe have provided millions of opportunities for mutations, virologist Adam Lauring of the University of Michigan told Scientific American. It is not happening because of a lab leak.

So which scenario do you think is more likely? Redfield’s lab leak, relying on one speculative episode? Or the notion of a wildlife spillover, with a million or so chances to occur?

If you had to bet on a particular card turning up in your poker hand, would you put your money on the card that only has one chance? Or the card that has a million chances to show up? Both scenarios are possible. One is a lot more probable.

This is a major reason why you’re hearing most scientists betting on wildlife spillover, as noted in a letter to the Lancet as well as the WHO report. (Several other researchers have told Undark magazine that the lab leak notion has not been given a fair hearing.) This question of origins is not an idle debate, either. It matters a lot, because knowing how a virus-driven pandemic begins focuses our attention on preventing similar situations. There are many more disease-causing viruses out there. It matters in another important way too. Fact-free speculation that sloppy Chinese scientists released a virus, which was common in the Trump administration, has fueled a tremendous wave of anti-Asian racism in the U.S., contributing to hundreds of acts of violence and terrorized communities.

Again, there is not much direct evidence on either side of this origins debate. The numbers, however, make Redfield’s notion a much longer shot and bigger gamble.

This is an opinion and analysis article.

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