Friendly viruses aided human evolution

 

Humans and viruses enjoy a symbiotic relationship, one that has had a crucial effect on our evolution and may hold the key to future medical advances, writer Frank Ryan tells JOHN HOLDEN

THE BRITISH zoologist and Nobel laureate, Sir Peter Medawar, once described the virus as “a piece of bad news wrapped in a protein”. This characterisation wouldn’t be far off for most people. Smallpox, HIV/Aids, human swine flu and other pandemics have left us feeling less than comfortable with the smallest of all organisms. But one scientist is now arguing that humans are in fact much closer to viruses than first thought.

“In the past people regarded viruses as selfish genetic elements, and very often they are,” explains Frank Ryan, author of Virolution. “I’m not trying to dismiss the prevailing ideas in evolution or medicine. Viruses are selfish entities, this is true, but even acting as brutally as they do, there’s a secondary evolutionary dynamic going on,” he argues.

“People who view viruses only as chemical assemblages miss the vitally important point that they have arrived on the scene through a vast, and exceedingly complex, trajectory of evolution, much as we have ourselves.”

If you think that viruses have played no part in human evolution think again. Up to 46 per cent of our DNA was actually provided some time in the past by invading viruses, Ryan says. In comparison, just 1.5 per cent of our genomes arise from the genes we inherit from our primate and animal ancestors.

Ryan argues that Human Endogenous Retroviruses, or Hervs, are a viral integration process integral to the genome of all mammals. It came about by an extensive series of retroviral epidemics that infected our ancestors over tens of millions of years. This means that the virus is something more than just a pest.

One example familiar to most would be that of the British red squirrel and imported American grey squirrel. Initially biologists believed the grey squirrel was winning the survival battle because it was larger, more aggressive and began feeding earlier in the year than the native counterpart, but what Ryan suggests is that the grey squirrel carries a squirrel pox virus that causes no disease symptoms in its symbiotic partner, but is lethal in the red squirrel; what he refers to as “aggressive symbiosis”.

The same is the case with HIV/Aids in humans. “HIV 1 is an acute retrovirus like the viruses in our genome,” says Ryan. “Even at its worst, we know that it is changing and selecting specific genotypes that it feels most comfortable with, while killing off others. So you can see natural selection at the level of interaction even in the Aids pandemic.

“There’s even an evolutionary dynamic to the current swine flu pandemic. This virus has arisen through the genomic union of three different viruses which amalgamated their pre-evolved genomes following a symbiotic evolutionary pattern. Evolutionary phenomena can be very hard to stop.”

Ryan suggests that certain viruses play vital roles in human activity. “There are eight full-length viruses in the human placenta playing different roles, such as a human virus that codes for syncytin 1, which fuses cells together to create a protective layer between mother and baby,” says Ryan. “Proof of the symbiotic inclusion of such viruses as part of our DNA is the fact they have been conserved throughout evolution.”

Understanding both the positive and negative roles played by the virus in human activity will, Ryan argues, further our understanding of how best to treat all manner of human conditions.


Frank Ryan will be publishing his third paper on viral symbiosis in the Journal of the Royal Society of Medicinelater this month. Virolutionis published by Harper Collins