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Part Human, Part Virus: The Secret Force Behind Evolution

We all know how evolution works: if an organism has a trait that helps it to survive, it will have a much higher chance of successfully reproducing and passing on its genes to offspring. Over generations, the favorable trait will accumulate in the population and in the end, most organisms in the species will have it. But what if that’s not all there is to it? As it turns out, our genes may have changed because of integrated viral DNA. In fact, the human genome contains about 8% viral genes and maybe no more than 2-3% protein-coding genes. In this scenario, we may only be human because of the viruses in our DNA.dna_double_helix_45

Now, I know what you’re probably thinking:

How in the World Could That Possibly be True?

It’s a good question. As Carrie Arnold explains in her article “The Viruses that Made Us Human” (check it out here), when viruses infect our cells, they hijack cellular machinery to copy their genes and translate their proteins at insane rates. A specific kind of virus, called a retrovirus, mixes its DNA with ours to create a sort of hybrid human-virus DNA. It injects cells with its DNA and tricks them into transporting the genes into the nucleus, where our DNA is stored. They are so cunning that they do not only insert their DNA into somatic cells but also in the germ-line cells, which differentiate into egg and sperm cells. If this happens, the hybrid human-virus DNA can be passed down to our children and grandchildren and will be present in every single cell of their bodies.

An electron micrograph of isolated retrovirus serum.
An electron micrograph of isolated retrovirus serum.

So What?

This process can actually be extremely significant. Without it, many different mammals living today (including us) might not have even existed.

The human placenta, which develops around growing embryos, delivers nutrients to the baby and removes waste products of fetal metabolism, while preventing the placental blood to mix with the mother’s. It would not be possible without syncytin, which makes up the boundary between the placenta and the maternal tissue. As it happens, the syncytin gene is nearly identical to the sequence of DNA of a viral protein called env. In other words, it’s quite likely that the human placenta can form because a retrovirus gave it the ability to do so. However, as Arnold continues, it’s not only humans who may form placentas, many other mammals do as well. Their syncytin genes, though, are entirely different from ours. This indicates that different retroviruses entered the gene pools of ancient mammals to give genes for different placentas, ultimately resulting in divergence in the evolution of mammalian species, which we know today.

Of course, this phenomenon can also have serious negative effects. For instance, a population of koalas was found to be exceptionally susceptible to cancer due to a virus within its genome. Avoiding a condition can be quite difficult if an organism is genetically programmed to develop it. It’s also important to note that this is relatively uncommon. Scientists estimate that there have only been about 30 to 40 significant instances of retrovirus interference in the past sixty million years.

What’s More…

In his book, Survival of the Sickest, Dr Sharon Moalem (check out his website here) explains that a special kind of gene, called a “jumping gene”, makes up about 50% of our non-coding DNA. While this DNA constitutes about 97% of what we possess, scientists once thought it served no purpose, actually labeling it “junk DNA”. Dr. Moalem asserts that they couldn’t have been more wrong. Jumping genes, he explains, can actually change an organism’s DNA sequence in the scheme of its own life. He cites the work of Dr Barbara McClintock, who studied corn under extreme conditions. What she found was groundbreaking: instead of the corn best suited to the extreme environments passing its helpful traits to its offspring, the individuals themselves began to change their DNA. Moalem elaborates:

McClintock discovered whole sequences of DNA moving from one place to another, even inserting themselves into active genes. When these genes cut and pasted themselves from one place in the corn’s DNA to another, they actually affected nearby genes- by changing the sequence of DNA, they sometimes turned genes on and sometimes turned them off.

Structurally, these jumping genes are extremely similar to – you guessed it – retroviruses. Thus, it’s quite possible that the simplistic process of natural selection wasn’t enough to get us through periods of intense environmental stress. Many scientists now believe that we have viruses to thank for our humanity and that without their help, we never would have made it this far.

Arnold, Carrie. “The Viruses That Made Us Human.” PBS. PBS, 28 Sept. 2016. Web. 13 Nov. 2016.

Moalem, Sharon, and Jonathan Prince. “Jump into the Gene Pool.” Survival of the Sickest: A Medical Maverick Discovers Why We Need Disease. New York: William Morrow, 2007. 136+. Print.

Edited by: Nelli Morgulchik, Shreya Singireddy, and Arselyne Chery