The Covid-19 pathogen that has swept the world likely evolved after several different coronaviruses merged into one, a new study has found.
High-resolution images studied by the team revealed that the spike on the surface of Sars-CoV-2 — the virus behind the human pandemic — is 97 per cent identical to the spike on RaTG13, its closest known relative.
The RaTG13 coronavirus is found in bats.
But while the spikes as a whole were largely similar, the scientists also discovered a number of significant differences at the location where Sars-CoV-2 bind with a receptor on human cells, called ACE2.
The divergences make Sars-Cov-2 more stable and about 1,000 times better at latching on to ACE2 than RaTG13, their study suggested.
ACE2 has been dubbed the "entry key" to the human body and the mutations on the spike of Sars-CoV-2 make it a perfect fit for the receptor.
Based on their findings, the researchers suggested it was unlikely that a bat virus similar to RaTG13 could infect human cells.
The team said in a statement this supported the theory that Sars-CoV-2 is the result of different coronaviruses coming together and evolving over time, potentially also through several host species.
Antoni Wrobel, co-lead author and postdoctoral training fellow in the Structural Biology of Disease Processes Laboratory at the Francis Crick Institute, said: "The spike is the entry key that allows Sars-CoV-2 into human cells.
"Changes in the virus' genome, which affect the spike's structure, therefore have potential to make the virus either more or less able to enter the host's cell."
"At some point in the evolution of this virus, it seems to have picked up changes, like the differences we found, which made it able to infect humans."
Scientists have previously suggested the coronavirus driving the global pandemic may have emerged after a virus was passed from bats to an intermediary host — potentially pangolins — and then on to humans.
Another theory has suggested the virus jumped directly from bats to humans.
Understanding how Sars-CoV-2 evolved is a key piece of information which could help scientists create a vaccine for the virus.
Meanwhile, the Francis Crick Institute team said they would continue their study, with a view to finding further clues as to its evolutionary path.
Donald Benton, co-lead author and postdoctoral training fellow in the Structural Biology of Disease Processes Laboratory at the Francis Crick Institute, said: “The exact process of how Sars-CoV-2 evolved remains unclear and is something many researchers are trying to piece together. Our work provides a piece of this puzzle, as it suggests that the virus did not come straight from the bat coronaviruses currently known.”
Steve Gamblin, group leader of the Structural Biology of Disease Processes Laboratory at the institute meanwhile added: “The world was caught off guard by Sars-CoV-2. Examining the structure of this virus, and its likely precursor, helps us understand where it came from, and how it interacts with human cells.”