Evolution: Why humans are affected less by retroviral infections than other animals?

The proportion of humans infected with retroviruses is less when compared with other animals as we have fewer remnants of viral DNA in our genes, said a new research.

Attributing this phenomenon to 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 said, “One reason for the reduction in retroviral incorporation into the human genome might be a change in behaviour as humans evolved.”

“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.

They counted the number of times that retroviruses appear to have been integrated into an animal’s genome in humans, comparing humans with 39 other mammalian species, including chimpanzees, dolphins and giant pandas.

Despite natural defence systems and immunity, a retrovirus occasionally infects a mammal’s egg or sperm, and the virus’s genetic code gets incorporated into the animal’s own genome.

This viral ‘fossil’ then passes down from generation to generation, carrying remnants of DNA from viruses that infected our ancestors millions of years ago. These ‘endogenous retroviruses’ (ERVs) appear not to cause us any harm, even though they are known to result in diseases such as cancer in other animals.

The researchers compared humans with 39 other mammalian species, including chimpanzees, dolphins and giant pandas and found that far fewer retroviruses were incorporated into the genome for humans and other apes over the last 10 million years.

Even when compared to animals like chimpanzees or other apes 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 said in its highlights.

However, Dr Magiorkinis from Oxford University’s Department of Zoology cautions: “We have shown in the past that Hepatitis C, a virus transmitted mainly through blood, was spread massively after World War II. There is no doubt that the past trend of reduced blood contacts has been reversed in the last century, and this has severe consequences for viral infections.”

The study has been published in the journal Retrovirology.


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