Viruses have been with us for billions of years, existing in every place where life is found. New strains of viruses emerge, while some die off and disappear. This means that viruses have had a significant impact on the evolution of most organisms. However, these organisms did not leave behind bones or physical artifacts to pick through.
(Photo: Wikimedia Commons/ Kim Hasenkrug, Dave Dorward and Austin Athman, Rocky Mountain Laboratories, NIAID, NIH)
Viral Fossils in Modern DNA
Retroviruses, including modern ones like HIV, replicate by inserting a DNA copy of their RNA genome into the DNA of a host cell. Though the viral genetic material does not generally get passed down from generation to generation, it can still happen if the ancient retroviruses gain the ability to infect germ cells, which pass their DNA down to future generations.
If the retrovirus successfully makes it into germ cells like eggs or sperm, it can become embedded in the genome of a population, an element known as endogenous retrovirus. Up to 8% of the human genome is made of genetic sequences from retroviruses that infected our ancestors a long time ago.
One of the most successful examples is ERV-Fc, a retrovirus believed to have spread globally between 30 million and 15 million years ago.
Viral Family Tree
Genome sequencing has only recently provided experts with an opportunity to trace the past of ancient viruses millions of years back. During an extensive viral evolutionary reconstruction, scientists have generated a "family tree" that shows a virus that jumped from one host species to another.
In 2016, experts analyzed the genomes of 50 modern mammals in search of fossil viral remnants of ERV-Fc. Led by biology professor Welkin Johnson, the research team discovered that this virus was present in the ancestors of at least 28 of them, including the relatives of humans, dogs, chimpanzees, baboons, marmosets, monkeys, squirrels, gray mouse lemurs, aardvark, and panda.
Just as paleontologists can reconstruct a dinosaur skeleton from fossilized bones dug from the Earth, Johnson and colleagues relied on "viral fossils," or the bits of viral genetic sequences that currently persist as harmless codes embedded in the DNA of organisms that once served as hosts to the viruses.
What is more impressive is that ERV-Fc did not infect just one relative of these mammals a single time. Instead, it jumped between species more than 20 times. This is considered an intercontinental pandemic that affected mammals on every continent except in the remote areas of Australia and Antarctica.
The result of the study also places the starting point of ERV-Fc as far back as the beginning of the Oligocene epoch. This historical period is characterized by a global change marked by climatic cooling, which led to the Ice Ages. During this time, vast grasslands emerged, with large mammals as the world's predominant fauna.
Today, ERV-Fc does not seem to impact human health since its genetic material sits dormantly in our genome. However, understanding the nature of ERV-Fc and other endogenous retroviruses can provide information on how we can better address modern-day diseases like HIV.
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