Viral fossils, embedded in our genetic codes, reveal an evolutionary history of ever more fastidious slaughter. Bloody encounters—whether from hunting, fighting, or the butchering of carcasses—spread retroviruses that bespatter the genomes of those who survive the violence. But lately these retroviral flecks, or rather their stains, have been darkening genomes less and less, at least among humans.

Despite natural defense 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. 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.

Humans, however, have fewer remnants of viral DNA in their genes compared to other mammals, report scientists from the University of Oxford, Plymouth University, and the Aaron Diamond AIDS Research Center. These scientists attribute the decrease to reduced exposure to blood-borne viruses.

Basically, humans became less blood-soaked as they evolved to use tools. Humans could kill more elegantly. No longer did they need to bite and claw.

This conclusion emerged from a study that culminated in a paper published February 2 in the journal Retrovirology. The paper, entitled, “The decline of human endogenous retroviruses: extinction and survival,” described how scientists counted the number of times that retroviruses appear to have been integrated into animals’ genomes. The scientists compared the counts for humans to those for 39 other mammalian species, including chimpanzees, dolphins, and giant pandas.

In addition, the scientists examined the genetic signatures of viral edges. These edges are identical when the virus first invades the genome, but as they acquire random mutations over time, they slowly begin to diverge. By tracking this divergence, the research team could measure how long the retrovirus had spent in an animal's genome.

“By dating the integration of endogenous retrovirus (ERV) loci in 40 mammal species, we show that the human genome and that of other hominoids (great apes and gibbons) have experienced an approximately four-fold decline in the ERV integration rate over the last 10 million years,” the authors wrote in the Retrovirology article. “Humans are unusual compared to related species—Old World monkeys, great apes, and gibbons—in (a) having not acquired any new ERV lineages during the last 30 million years and (b) the possession of an old ERV lineage that has continued to replicate up until at least the last few hundred thousand years.”

After accounting for possible confounding factors—differences in body weight (which correlate negatively with ERV integration rate) and the disappearance of some ERV lineages—the scientists speculated that “the decline in ERV integration in the human genome has been exacerbated by a relatively low burden of horizontally transmitted retroviruses and subsequent reduced risk of endogenization.”

In other words, the reduction in retroviral incorporation into the human genome might be a consequence of fewer bloody fights and less exposure to infected meat among humans compared to other animals. Our ancestors simply became less likely to encounter blood, a major route for viral infection.

“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, Ph.D., an associated professor of genomics at Plymouth University.

However, lead researcher Gkikas Magiorkinis, M.D., Ph.D., from Oxford University's Department of Zoology pointed out 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,” continued Dr. Magiorkinis, “and this has severe consequences for viral infections.”

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