Humans Have Far Fewer Integrated Retroviruses Than Other Mammals; Cause May Be Evolutionary Reduction in Blood Encouters That Are Major Routes of Viral Infection; Humans Have Not Incorporated Any New Types of Retroviruses in 30 Million Years

RHumans have fewer remnants of viral DNA in their genes compared to other mammals, a new study has found. This difference could be a result of reduced exposure to blood-borne viruses as humans evolved to use tools, rather than biting during violent conflicts and the hunting of animals. Despite natural defense systems, a retrovirus occasionally infects a mammal's egg or sperm, and the virus's genetic code is incorporated into the animal's own genome. This viral “fossil” is then passes down from generation to generation: we all carry 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. A team of researchers from the University of Oxford and Plymouth University, both in the UK, and the Aaron Diamond AIDS Research Center in the USA, wondered if there was a combination of factors unique to humans that explained why these viral fossils in our genomes remain benign. The scientists counted the number of times that retroviruses appear to have been integrated into an animal's genome, comparing results in humans with results in 39 other mammalian species, including chimpanzees, dolphins, and giant pandas. Reporting their results recently in the journal Retrovirology, the researchers compared the genetic signature of the two edges of the integrate virus. 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.
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