Other experts found the study persuasive.

“They showed this mutation is both sufficient and necessary to make the virus worse,” said Hongjun Song, a neuroscientist at the University of Pennsylvania who helped discover how Zika attacks the fetal brain. “I would say this is one of the smoking guns.”

“The scary part, maybe the take-home message, is that it doesn’t take that much — just one mutation — to make something really, really bad,” he added.

The researchers do not claim the S139N mutation is solely responsible for the birth defects among children born to women infected by mosquitoes during pregnancy. Other causes could involve differences in the population in Latin America, including the possibilities that their genetic makeup or exposure to previous mosquito-borne viruses made them more susceptible to harm from Zika.

It is also possible that Zika previously caused microcephaly, but cases simply went unnoticed when the virus reached Asia around the 1960s.

Microcephaly has many causes, many mothers gave birth at home, and newborns with severe brain damage might have died without immediate intensive care. The surge in microcephaly in northeast Brazil in late 2015 was noticed by doctors in hospital neonatal units.

The researchers noted that strains of the virus without the S139N mutation caused some mice to develop mild microcephaly, meaning that the mutation, which occurs on a protein involved in making the virus’s protective coating, is likely only a piece of the puzzle.

But it seems to be an important piece, the scientists said.

“In the beginning, we thought we may need multiple mutations” to create a viral strain that causes severe microcephaly, said Dr. Zhiheng Xu, a principal investigator with the Institute of Genetics and Developmental Biology at the Chinese Academy of Sciences, who led the study with Cheng-Feng Qin, a virologist at the Beijing Institute of Microbiology and Epidemiology.