Where Did Viruses Come From?

How many types of living things can you name? 

Organisms are usually grouped into three main categories. These are archaea, bacteria and eukarya or eukaryota. Bacteria, archaea and some eukarya are microscopic, single-celled creatures. Plants, animals and other organisms that have multiple cells form the majority of eukarya. These three groups make up the Tree of Life.

But wait. What about viruses? Why don’t they get a branch on the tree?

Organisms must meet certain requirements in order to be called a life form. Viruses don’t meet all of them. So most scientists who classify organisms this way don’t recognize them as living things. 

Researchers may argue about whether or not viruses are really alive. But they have also thought a lot about where viruses came from in the first place.

Did you know?

Walter Reed became the first person to discover a virus that infects humans. He discovered the virus that causes yellow fever in 1901.

What is “life” and where did it come from?

To be included in the Tree of Life, an organism’s cells must have ribosomes. Ribosomes help cells to assemble proteins. Cells must also have double-stranded DNA. 

Living organisms also need to be able to perform oxidation-reduction or Redox reactions. These are chemical reactions where electrons are transferred between atoms. Your body uses oxidation-reduction reactions for many important things, like getting energy from the food you eat.

Viruses don’t meet any of these criteria. That’s why they don’t get a spot on the Tree of Life.

Some scientists think that all life forms came from a single-celled organism called the Last Universal Common Ancestor or LUCA. This includes extant life forms, or life forms that are still living, and extinct life forms, or life forms that no longer exist. No one really knows much about the LUCA. But researchers think it looked a lot like bacteria looks today. 

Did you know?

Viruses can infect just about anything. This includes humans, plants, bacteria, archaea and protozoa.

Where did viruses come from?

Virologists are scientists who study viruses. They don’t think that viruses came from a LUCA. So where did they come from? 

Here are the three main explanations that virologists have suggested.

1. The Reduction Hypothesis, also called the Regressive Hypothesis.

Viruses might have come from more complex organisms. This could have happened when two single-celled organisms formed a mutualistic or cooperative relationship. 

At some point, this relationship would have become parasitic. This happens when one organism starts to rely on the other organism to stay alive. As the smaller organism became more and more dependent on the larger organism, it lost its protein-building structures. However, it could still replicate itself. This new organism would be able to infect new organisms. It is now a virus.

2. The Escape hypothesis, also called the Progressive Hypothesis.

Viruses might have come from broken pieces of genetic material inside early cells. These pieces were able to escape their original organism and infect another cell. In this way, they evolved into viruses. Modern-day retroviruses, like the Human Immunodeficiency Virus (HIV), work in much the same way. After they enter a cell, they combine their genetic material with the host’s genetic material.

3. The virus-first hypothesis.

Viruses might have come from pieces of RNA. RNA is similar to DNA, but it has some structural differences.

• RNA is a single-stranded molecule, not double like DNA.
• The sugar-phosphate “backbone” of RNA contains ribose instead of deoxyribose.
• RNA has self-complementary sequences. This allows parts of it to pair and fold with itself, like a protein.

In this explanation, these pieces of RNA existed before the first cells. They slowly became more complex. Eventually they gained the ability to self-replicate and infect other cells. In this way, they became viruses.

Did you know? 

At least two viruses appear to infect other viruses! These are Sputnik and Mavirus.

Which hypothesis is the right one?

There are clear reasons why viruses aren’t included in the Tree of Life. But no one knows for sure where viruses came from. The three hypotheses above are the most popular explanations. Each one has its limitations though. 

Who knows? A completely different hypothesis might end up being the most convincing.

Scientific ideas are constantly being improved and adjusted. The true origin of viruses is just one thing scientists will keep studying and debating for years to come!