Cast your mind back 40 million years and think about your ancestors. You’re probably picturing creatures that looked like a bit like today’s monkeys, but they’re only part of your family tree. To see your other ancestors, you’d have to whip out an imaginary microscope. Meet your great-great-great-etc-grandviruses.
The human genome is littered with the remains of viruses that, in ages past, integrated their genes into the DNA of our ancestors. They became a permanent fixture, passed down from parent to child. Today, these “endogenous retroviruses“, or ERVs, make up around 8% of our genome. They’re a living fossil record of prehistoric plagues.
Until recently, scientists thought that the only viruses to have left such a legacy were the retroviruses, a group that includes modern members such as HIV and hepatitis B. But they are no longer alone. Masayuki Horie and Tomoyuki Honda from Osaka University have found that another viral dynasty, the bornaviruses, have repeatedly inveigled their way into the genomes of mammals. They’re found in humans, the great apes, elephants, rodents and many more. Those that lurk amid our genes have been our partners in evolution for at least 40 million years.
Almost all of these hidden sequences match the N gene of the most famous bornavirus – BDV or Borna disease virus. As a result, Horie and Honda christened their newfound sequences as EBLNs (or “endogenous Borna-like N” elements). We carry four such sequences but many others exist. A scan of over 234 genomes revealed that EBLNs are found in all manner of mammals, including chimps, gorillas, orang-utans, macaques, lemurs, bushbabies, African elephants, hyraxes, ground squirrels, mice, rats, guinea pigs, bats and opossums.
Most of these EBLNs are the result of independent invasions, at different points in the history of mammal evolution. Squirrels have only carried their EBLNs for less than 10 million years but our own genetic passengers have been riding and co-evolving with us for over 40 million years.