Our genomes are peppered with ancient relics of viruses that infected our ancestors millions of years ago. These remnants, which make up as much as 8% of our genome, have also been found other animals, but it turns out that humans possess fewer than other mammals. Interestingly, according to new research, this could be because ancient humans evolved to gather food and fight using tools rather than their teeth, which would have reduced our exposure to blood-borne viruses. The findings have been published in the journal Retrovirology.
Humans are infected with viruses all the time. Sometimes they kill us, but more often than not our immune systems fight them off, leaving us with nothing but antibodies as a reminder of the infection. However, sometimes they can become a permanent addition to our bodies as some viruses are capable of inserting their DNA permanently into ours. If these viruses manage to infect our sperm or egg cells, they can be passed on from generation to generation. Over time, these viruses acquire mutations and degrade, and their activity dampens. But because these viruses can sometimes still copy themselves and then insert these genes at other locations, our genomes are littered with these so-called viral fossils, or endogenous viral elements, which trace back to millions of years ago.
The vast majority of these fossils originate from the integration of a type of virus called a retrovirus, and so are named endogenous retroviruses (ERVs). While many studies have demonstrated a link between ERVs and certain diseases in animals, they seem to remain benign in humans and have not been definitively associated with any diseases, although some research has hinted at possible connections.
In order to find out more about the differences between ERVs of humans and other mammals, scientists from the University of Oxford, Plymouth University and the Aaron Diamond AIDS Research Center compared our genomes with those of 39 other mammalian species, including apes, dolphins and Old World Monkeys- a group of primates which all descend from a common ancestor that lived around 40 million years ago.
More specifically, they were looking at rate at which these ERVs acquired mutations over time, which can be used to determine how long the virus has been in that particular genome. Interestingly, compared with other animals, they found that humans and other apes experienced significantly fewer retroviral incorporations over the last 10 million years. Furthermore, the rate at which the viruses copied themselves in Old World Monkeys remained relatively unchanged over the past 30 million years, but decreased in humans and other apes. However, humans were distinct in that they seem to have not acquired any new types of retroviruses over the past 30 million years.
According to the researchers, this could be because of the way that our ancestors’ behavior changed as they evolved over time. While other primate species were using their teeth to fight or kill prey, running the risk of exposing themselves to retroviruses in the blood, ancient humans began to use tools for the same purpose. This would mean that our ancestors were less likely to become infected with viruses capable of incorporating themselves into the genome.
[Via University of Oxford, Retrovirology and National Geographic]