One of the evolutionary benefits gained from a retrovirus is the placenta found in pregnant mammals. Tufts University School of Medicine virologist John M. Coffin published a paper regarding the emergence of placental mammals due to endogenous retroviruses:
“Endogenization of retroviruses has occurred multiple times in the course of vertebrate evolution, with the captured retroviral envelope syncytins [proteins] playing a role in placentation in mammals, including marsupials.”
The placenta is a flattened organ found in the womb that connects the fetus to the uterine wall, and provides nourishment to the fetus. The newly-discovered protein HEMO is a remnant of an ancestral endogenous retrovirus that provided the genetic instructions that tells the body to create a type of viral protein known as a syncytin. These syncytins bond placenta cells together when a woman becomes pregnant – a vital step in fetal development.
The Good, The Bad, and the Virus
The question researchers are now trying to answer is how have these viruses affected our health. Are the DNA remains of these genomic colonizers merely fossils, having lost their function over time, or are the ancient viruses still actively involved in our early development?
“It’s not an either-or — are these things good or bad? It’s a lot more complicated that that,” said Katzourakis in an interview. “We’re barely at the beginning of this research.”
While some of these viruses might protect us from disease, others might cause cancer. Scientists have found viral proteins, produced by endogenous retroviruses, in many types of tumors cells, and a team of French researchers discovered a viral protein that can activate genes linked to cancer.
But Coffin speculates the viral proteins are randomly activating human and viral genes alike. He also points that many ancient viruses have been domesticated by our ancestors, and that our bodies now make essential proteins from endogenous retroviruses that we are dependent on. Others offer protection against other viruses, and we already know the roles of syncytins in placental development. “My speculation is that without syncytins, mammal evolution would have looked very different,” said Coffin. He hopes to discover just how intertwined our history, and our future, is with viruses.
“This research provides important information necessary for understanding how retroviruses and humans have evolved together in relatively recent times.”