The remnants of the viruses that circulated in the age of the dinosaurs are still hiding in our genes.

About 106 million years ago, viral DNA somehow became integrated into the genome of one of our ancestors. Two million years later, this happened again with the same type of virus. Its ancient remains have now been found in our cells.

“This is very visible in the human genome,” said Aris Katzourakis, an evolutionary biologist at the University of Oxford in the UK.

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The two ‘viral fossils’ may be the oldest ever discovered. In addition, they are also very different from other fossil viruses.

retro

Our genome is full of viral fossils, but almost all of them are retroviruses. These make DNA copies of their own RNA genes. They then implanted that copy in the DNA of the cells they infected.

If this occurs in cells involved in the production of sperm or egg cells, the DNA of this virus can be passed down from generation to generation. Over time, the viral genes mutate and can no longer cause infection. Between 5 and 10 percent of our genome consists of retroviral remains.

Mavericks

The newly discovered virus instead belongs to an ancient group of DNA viruses called maverickMaverick tracks have previously been found in fish, amphibians and reptiles, but so far never in mammals.

Researchers think this virus has been infecting mammals from the very beginning of their evolutionary origin, about 180 million years ago during the Jurassic. This lasted until 105 million years ago, during the Cretaceous Period. Sometime during that last period, this specific virus entered our genome.

After that, the maverick seems to have become extinct in mammals for reasons that are not clear. They can still infect other animals, such as fish. So far, no maverick viruses have been found outside the host.

“There aren’t many non-retroviral viruses in our genome,” Katzourakis said. “This is the only DNA virus in the human genome that we know of, and it is certainly the oldest non-retroviral copy in our genome.”

Even older

There is one fossil retrovirus in the human genome, called ERV-L, which is thought to be older. However, there is some overlap in age estimates. “It’s hard to know for sure which of the two is older because different methods are used to determine their age,” Katzourakis said.

We may have older viral DNA in our genome than this, but it may have been lost or mutated beyond recognition.