The mosquito-borne Zika virus has significantly evolved since it was first discovered in 1947, and researchers said Friday these genetic changes could shed light on why it has the power to cause birth defects.

The research in the journal Cell Host and Microbe was led by scientists at the University of California, Los Angeles and the Chinese Academy of Medical Sciences and Peking Union Medical College in Beijing.

Researchers looked for individual differences between more than 40 strains of Zika virus -- 30 of which came from people, 10 from mosquitoes, and one from a monkey.

They found big differences between the Asian and African lineages of the virus, and "significant changes in both amino acid and nucleotide sequences during the past half-century," said the study.

Zika strains found in humans are more genetically similar to a strain identified in Malaysia in 1966 than that found in Nigeria in 1968, "suggesting the strains in the recent human outbreak evolved from the Asian lineage," it said.

All of the human strains identified in the 2015-2016 outbreak appear most closely related to the virus identified in 2013 in French Polynesia.

The team also found that a certain protein, called the pre-membrane precursor or prM, was quite different between the Asian human and the African mosquito subtypes.

"We believe these changes may, at least partially, explain why the virus has demonstrated the capacity to spread exponentially in the human population in the Americas," said senior study author Genhong Cheng, a professor at the University of California, Los Angeles's department of microbiology, immunology and molecular genetics.

"These changes could enable the virus to replicate more efficiently, invade new tissues that provide protective niches for viral propagation, or evade the immune system, leading to viral persistence."

More research is needed to uncover the precise reasons for Zika's ability to cause birth defects in some children.

Never before has a mosquito-borne virus shown the potential to cause a condition known as microcephaly in babies born to mothers infected while pregnant, leaving the infants with small heads and deformed brains.

Thousands of babies in Brazil, the epicenter of the latest outbreak, are suspected of being born with microcephaly since last year.

Experts say the Zika strain causing the 2015-2016 epidemic has not yet been isolated from a mosquito.

A vaccine against the virus, which can also be sexually transmitted, is years away, US health authorities say.