Around 106 million years ago, the DNA of a virus somehow got integrated into the genome of one of our mammal ancestors. Two million years later, something similar happened again with the same kind of virus. Now, the ancient remnants of that virus has been found inside our cells.
鈥淚t鈥檚 kind of hiding in plain sight in the human genome,鈥� says听听at the University of Oxford.
These two viral 鈥渇ossils鈥� are some of the oldest ever discovered, and possibly even the oldest. They are also rather unusual.
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Our genomes are strewn with 鈥渇ossil鈥� viruses, but almost all are retroviruses, which actively insert DNA copies of their RNA genes into the genomes of the cells they infect.
If this happens in cells that give rise to sperm or eggs, this virus-derived DNA can be passed down the generations. Over time, the viral genes mutate and eventually can no longer give rise to infectious viruses. Between 5 and 10 per cent of our genome consists of retroviral remnants.
The newly discovered virus instead belongs to an ancient group of DNA viruses called Mavericks. Fossil Mavericks have been found in various animals, including fish, amphibians and reptiles, but until now had never been found in mammals.
The researchers think these viruses plagued mammals from the time these animals first evolved around 180 million years ago during the Jurassic Period until at least 105 million years ago during the Cretaceous Period, when the insertions took place.
After that, Mavericks appear to have died out in mammals for reasons that aren鈥檛 clear. They might still infect other animals, such as fish, but as yet no free-living Maverick viruses have ever been found.
鈥淭here aren鈥檛 that many non-retroviral viruses in our genome,鈥� says Katzourakis. 鈥淭his is the only DNA virus in the human genome that we know of, and it鈥檚 certainly the oldest non-retroviral insertion in our genomes.鈥�
There is one fossil retrovirus in the human genome, called ERV-L, that is thought to be older, but there is some overlap in the age estimates. 鈥淚t is difficult to know for certain whether the ERV-L retrovirus or this Maverick is indeed older as slightly different methodologies were used to ascertain their age,鈥� says Katzourakis.
There would have been viral insertions even earlier than these ones, too, but their fossil remnants may have been lost or mutated beyond recognition. Occasionally, though, integrated viral genes are co-opted by evolution and become useful to their hosts.
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