Did a Virus Make Us Mammals?

What do you imagine when you think of a bad party guest? It's probably someone who shows up uninvited, annoys everyone, selfishly eats and drinks his fill, and then leaves you exhausted with a mess to clean up. Viruses are a lot like that. They enter our bodies when we don't want them to, take over our cells so they can replicate themselves, and use us as hosts, usually with detrimental effects.

But what if the goals of the virus aren’t necessarily at odds with our own?

A stealthier virus will learn to live more symbiotically with its host. It needs that host, after all, to survive. Think of it as someone who has to couch surf with you for a while, but makes an effort to be a cooperative guest rather than a destructive one. Some viruses have integrated so successfully that they've even played a role in our evolution. In fact, a virus is credited with making us mammals.

How retroviruses affect our DNA

Retroviruses, a subclass of viruses, get their name for the methods they use to replicate within a cell. Once a retrovirus attaches to its would-be host cell it fuses with the cell’s outer membrane so it can slip inside. Retroviruses then use an enzyme called reverse transcriptase to convert their own RNA into DNA. This stealthy tactic makes them more compatible so they can insert themselves into a host cell’s DNA and become part of the host cell’s genome. And once that integration happens, the retrovirus can then use the host cell’s own processes to create new viral components. Those new viral agents can then go on to infect other cells. 

Some scientists estimate that as much as eight percent of our genome may be made up of DNA donated to us by retroviruses.

In the case of the retrovirus known as the human immunodeficiency virus (or HIV), the virus infects CD4 T cells, which means it can have a significant impact on our immune system. And when a host cell dies, this merged viral-and-host genome dies also.

But what happens when a virus infects a sperm or egg cell? That part-host/part-virus genome can get passed on to the host’s descendants. We call these viruses endogenous retroviruses—in other words, viruses that come from within. Some scientists estimate that as much as eight percent of our genome may be made up of DNA donated to us by retroviruses. 

Did a virus make us mammals?

Not all of this viral DNA does anything meaningful for us—it just hitches an evolutionary ride. But sometimes...

Keep reading on Quick and Dirty Tips