Australia: The Land Where Time Began

A biography of the Australian continent 

Lynn Margulis has said 'We are our viruses'. It appears the evolution of mammals from the pre-mammalian ancestors may have been made possible or even triggered by ERVs. It has been found that mammalian foetuses, as well as the placenta, are seething with ERVs. Though closely resembling the AIDS virus, they are normal in mammal pregnancies. It is believed they became symbiotic following viral infections, becoming symbiotic by adapting to lengthen their survival in the hosts, the early ancestors of mammals. By becoming inserted into the DNA of germline cells, they are passed on to the succeeding generations, bypassing the need to infect another host. Every mammalian genome studied as has been found to contain ERVs. There are at least 1000 different types of ERV in humans. They usually remain dormant until being activated in a foetus and its placenta, though some have been implicated in autoimmune disease.

They have been found to have important, complex roles in pregnancy, protecting the foetus from rejection and the formation of the placenta, allowing it to invade the uterine wall like a cancer, switching off the process after birth. They have a crucial role in protecting the foetus from infection. It is believed that ERVs may also be involved in the production of live young in some amphibians and reptiles. The formation and maintenance of the syncytium,  a microscopically thin membrane that prevents the mothers immune system from rejecting the cells with a DNA component from the father, by preventing them from passing to the placenta. This development of the syncytium is controlled by an ERV protein.

The research on ERVs is in its early stages, but it is known that the interactions between the ERVs and the foetus, placenta and the mother are very complex, with the interactions by chemicals produced by both mother and baby apparently being involved, but these connections are mostly still to be elucidated.

As the number of ERVs found in the human DNA have increased it is becoming apparent that there is also a potential future danger to humans, and other mammals, resulting from the way viruses can swap genetic material with each other, and even with bacteria. It is possible that an infection by a virus or bacterium could activate a previously dormant ERV with possible disastrous consequences, as the human host would lack immunity to it.

Sources & Further reading

Mary E. White, Earth Alive, From Microbes to a Living Planet, Rosenberg Publishing Pty. Ltd., 2003

Links

Endogenous retrovirus