Compared to other mammals, the proportion of humans infected with retroviruses is less and we have fewer remnants of viral DNA in our genes, a new research has found.
This could be because of reduced exposure to blood-borne viruses as humans evolved to use tools rather than biting during violent conflict and the hunting of animals, the researchers noted.
"One reason for the reduction in retroviral incorporation into the human genome might be a change in behaviour as humans evolved," the study noted.
"Fewer bloody fights and less exposure to infected meat meant that compared to other animals, our ancestors became less likely to encounter blood, a major route for viral infection," the researchers explained.
"Considering us simply as a primate species, the proportion of human individuals that are infected with retroviruses is much less than among our relatives such as chimpanzees," said Robert Belshaw from Plymouth University in Britain.
Despite natural defence systems, a retrovirus occasionally infects a mammal's egg or sperm, and the virus's genetic code gets incorporated into the animal's own genome.
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This viral 'fossil' then passes down from generation to generation, we all carry remnants of DNA from viruses that infected our ancestors millions of years ago.
For the study, the researchers compared humans with 39 other mammalian species, including chimpanzees, dolphins and giant pandas.
Compared to other animals, far fewer retroviruses were incorporated into the genome for humans and other apes over the last 10 million years, the findings showed.
Even compared to animals very similar to us, humans were unusual in not having acquired any new types of retroviruses into their DNA over the last 30 million years, the study noted.
The study appeared in the journal Retrovirology.