When the Omicron variant took off in southern Africa in November, scientists were taken aback by its genetic makeup. Whereas earlier variants had differed from the original Wuhan version of the coronavirus by a dozen or two mutations, Omicron had 53 — a shockingly large jump in viral evolution.

In a study posted online last week, an international team of scientists further deepened the mystery. They found that 13 of those mutations were rarely, if ever, found in other coronaviruses, suggesting they should have been harmful to Omicron. Instead, when acting in concert, these mutations appear to be key to some of Omicron’s most essential functions.

Now the researchers are trying to figure out how Omicron defied the normal rules of evolution and used these mutations to become such a successful vector of disease.

“There’s a mystery here that someone has to figure out,” said Darren Martin, a virologist at the University of Cape Town who worked on the new study.