How many types of living things can you name? Often, organisms are grouped into three main categories: bacteria, archaea and eukarya (or eukaryota). Bacteria and archaea are microscopic, single-celled creatures. Eukarya include plants, animals and other organisms that have multiple cells. Together, these three groups make up the Tree of Life.
But wait. What about viruses? Why don’t they get a branch on the tree?
In order to be considered a life form, organisms must meet certain requirements. Viruses don’t meet all of them, so most scientists who classify organisms in this way don’t consider them living things. As they debate whether viruses are really alive, researchers have also put a lot of thought into where viruses might have come from in the first place.
Did you know? In 1901, Walter Reed became the first person to discover a virus that infects humans: yellow fever.
What is “life” and where did it come from?
To be included in the Tree of Life, an organism’s cells must have ribosomes for building proteins, as well as double-stranded DNA. They must also be able to perform oxidation-reduction reactions. This is a type of chemical reaction where electrons get transferred between atoms. Your body depends on oxidation-reduction reactions for many important things, like getting energy from the food you eat.
Viruses don’t meet any of these criteria. And that’s why they don’t get a spot on the Tree of Life.
The three main branches of the Tree of Life are bacteria, archaea and eucarya. The first two groups are made up of single-celled organisms. Eukarya include plants, animals and other organisms that have multiple cells. “Phylogenetic” means that these categories are based on how different life forms evolved (NASA, Wikimedia Commons
Some scientists also think that all extant (still living) and extinct life forms originated from a single-celled organism called the Last Universal Common Ancestor (LUCA). No one really knows much about LUCA, but researchers think it looked like modern-day bacteria.
Did you know? Viruses can infect just about anything, including humans, plants, bacteria, archaea and protozoa.
Where did viruses come from?
Meanwhile, most virologists (scientists who study viruses) don’t think that viruses came from LUCA. So where do they come from? Here are the three of the leading explanations that virologist have put forward:
- 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 symbiotic (cooperative) relationship. At some point, the relationship became parasitic. This happens when one organism starts to rely on the cells of the other organism to stay alive. As one organism became more and more dependent on the other, it started losing some of its genetic material. Eventually, it evolved into a virus.
- The Escape hypothesis (also called the Progressive Hypothesis): Viruses might have come from fragments of genetic material inside early cells. These pieces of genetic material were able to escape their original organism and infect another cell. In this way, they evolved into viruses. Modern-day retroviruses, like HIV, work in a similar way. After they enter a cell, they combine their genetic material with the host’s genetic material.
- The virus-first hypothesis: Viruses might have come from pieces of RNA. RNA is similar to DNA, but it has some structural differences. According to this hypothesis, these pieces of RNA existed before the first cells. They slowly became more complex and eventually gained the ability to self-replicate and infect other cells. In this way, they became viruses.
Did you know? At least two viruses—Sputnik and Mamavirus—appear to infect other viruses!
Which hypothesis is the right one?
So there are clear reasons why viruses aren’t included in the Tree of Life. However, no one knows for sure exactly where viruses came from. The three hypotheses I have presented may be the most popular explanations, but each of them has its limitations. Who knows? A completely different hypothesis may end up being the most convincing.
Scientific ideas are constantly being improved and adjusted. And understanding the true origin of viruses is just one of the questions that scientists will probably continue studying and debating for years to come!
About the Tree of Life:
About viruses and their orgins: