The conspiracy theory that the novel coronavirus was created in a lab has been circulating since the beginning of the COVID-19 outbreak, although scientists from different countries have refuted it over and again and stressed the natural origin of the virus.

So how exactly do scientists know the virus SARS-CoV-2 originated from an animal, most likely a bat, and was not lab-made?

Polly Hayes, a lecturer in Parasitology and Medical Microbiology at the University of Westminster, explained the question in an article published on British website The Conversation earlier this week, citing multiple studies.

No signs of manipulation in the genetic material of SARS-CoV-2

Hayes pointed out that if the virus had been genetically engineered in a lab, there would be signs of manipulation in the genome data.

This would include evidence of an existing viral sequence as the backbone for the new virus, and obvious, targeted inserted (or deleted) genetic elements.

But the genetic sequence of SARS-CoV-2 has been publicly shared thousands of times by scientists all over the world, and no such evidence has ever been found.

She added that it is very unlikely that any techniques used to genetically engineer the virus would not leave a genetic signature, like specific identifiable pieces of DNA code.

SARS-CoV-2 evolved from a previous wild coronavirus

The genome of SARS-CoV-2 is similar to that of other bat coronaviruses, as well as those of pangolins, all of which have a similar overall genomic architecture.

What's more, differences between the genomes of these coronaviruses show natural patterns typical of coronavirus evolution, which suggests that SARS-CoV-2 evolved from a previous wild coronavirus.

SARS-CoV-2 and a bat virus in Wuhan lab significantly different in evolutionary terms 

Some media reports have said that SARS-CoV-2 may come from another known bat virus (RaTG13) found by researchers at the Wuhan Institute of Virology, since the genomes of the two viruses were 96 percent similar to one another.

But Hayes clarified that in evolutionary terms, this actually makes them significantly different and the two have been shown to share a common ancestor. That means RaGT13 is not an ancestor of SARS-CoV-2. 

SARS-CoV-2 shared similar features with related coronaviruses

One of the key features that makes SARS-CoV-2 different from the other coronaviruses is a particular "spike" protein that binds well with another protein on the outside of human cells called ACE2. This enables the virus to hook into and infect a variety of human cells.

However, other related coronaviruses do have similar features, providing evidence that they have evolved naturally rather than being artificially added in a lab. 

The majority of infectious diseases and all known coronaviruses have animal origins

An estimated 60 percent of known infectious diseases and 75 percent of all new, emerging, or re-emerging diseases in humans have animal origins.

SARS-CoV-2 is the newest of seven coronaviruses found in humans, all of which came from animals, either from bats, mice or domestic animals.

The mixing or "recombination" of distinct coronavirus genomes in nature is one of the mechanisms that bring about novel coronaviruses. There is now further evidence that this process could be involved in the generation of SARS-CoV-2.

SARS-CoV-2 most likely originated from bats

In fact, SARS-CoV-2 most likely evolved from a viral variant that couldn't survive for a long period of time or that persists at low levels in bats.

Coincidentally, it evolved the ability to invade human cells and accidentally found its way into humans, possibly by means of an intermediate animal host, where it then thrived.

Or an initially harmless form of the virus might have jumped directly into humans and then evolved to become harmful as it passed between people.

Bats were also the source of the viruses causing Ebola, rabies, Nipah and Hendra virus infections, Marburg virus disease, and strains of Influenza A virus.