Is A Virus Your Oldest Cousin?

Should you reserve a place at your Christmas table for viruses? A study conducted in 2021 examined the origins of life and concluded that viruses may have existed before cells, or they could have detached from a preexisting cell that eventually evolved into humans and other organisms.

Ah, viruses! These tiny entities that are barely considered alive, yet have the ability to cause chaos in various forms of life. Anything with a metabolism, from bacteria to plants and animals, can fall victim to these merciless organisms. But how can something as small as 20 nm (~0.0000008 of an inch) be related to us? All living beings have their own ways of passing down their genes to offspring independently, without the need to hijack another organism and force it to produce genes for them. However, viruses do not follow this pattern.

How can viruses be considered our relatives? Are we destined to consider these terrifying creatures as part of our family?

A study conducted in 2021 by Hugh Harris and Collin Hill from University Cork College in Ireland presents several intriguing possibilities that suggest viruses could indeed be related to us.

Tree of Life (Photo Credit: Sting – fr:Sting/Wikimedia Commons)

How Can We Establish the Relationship Between Living Organisms?

Molecular biologists typically use a concept known as the molecular clock theory to determine the relationship between different organisms. In simple terms, this theory compares the genetic sequences of two organisms and estimates the degree of similarity.

This is feasible because all organisms share certain genes. For instance, the ribosomal gene, which is present in EVERY organism except viruses, is commonly used to determine genetic closeness or distance between two organisms based on their genetic sequences.

However, viruses mutate at an extremely rapid rate, making it impossible to establish the relationship between different viral species using the molecular clock method. Therefore, this method cannot be applied to viruses.

Thus, it appears that there is no straightforward way to determine our relationship with viruses.

How molecular clocks work based on differences in DNA (Photo Credit: Angelahartsock/Wikimedia Commons)

Nevertheless, there may be alternative methods to determine this relationship. The 2021 paper, which is an extensive review of viruses and their position on the Tree of Life, authored by Hugh Harris and Collin Hill, introduces such hypotheses. While we may not be able to directly measure the relatedness of viruses to other organisms, there are indirect approaches that require creative speculation (based on evidence) regarding the origins of life on Earth.

Could Viruses Be Our Ancient Cousins?

In order to fully comprehend the paper’s explanations regarding the potential origins of viruses, it is important to understand LUCA and FUCA.

LUCA refers to the Last Universal Common Ancestor or the Last Universal Cellular Ancestor. The study emphasizes that this distinction is crucial when considering the inclusion of viruses in the Tree of Life.

In the study, LUCA is used to refer to the last common ancestor of modern cells, excluding viruses. Instead, FUCA is used to denote the First Universal Common Ancestor—the ancestor of LUCA and other lineages that may have become extinct, which could provide clues about the origin of viruses.

An ancient tree of life that predates “The Tree of Life”: The origins of the Last Universal Cellular Ancestor (Photo Credit: Frontiersin.org)

There are several possibilities for the origin of viruses, and the study highlights three main hypotheses that encompass the majority of theories: (1) The viruses-first hypothesis; (2) The reduction hypothesis; and (3) The independent evolution hypothesis.

Viruses: A Lineage Older Than the Existence of Life Itself?

We will begin with the viruses-first hypothesis.

The authors attempt to conceal their excitement, but it subtly emerges in their review. According to this hypothesis, viruses may have existed as “pre-cells,” a type of organism capable of self-replication, but not being part of a cell. Furthermore, these pre-cells were not parasitic; they were self-sustaining non-cell-like entities! Although this is a captivating hypothesis, there is limited evidence to support it.

However, if it were true, it would imply that viruses emerged at the very beginning, shortly after the Earth was formed 4 billion years ago, considering that viruses are not typically considered living entities.

Once Alive, But No Longer Living: Did Viruses Ever Experience Life?

The reduction hypothesis.

The next, more plausible theory suggests that viruses evolved from cellular ancestors, indicating that they were once “alive” (based on our definition of life).

According to this scenario, viruses originated before LUCA (Last Universal Cellular Ancestor), but after FUCA (First Universal Cellular Ancestor).

In this scenario, early viral cells were parasitic organisms that gradually lost their independence by discarding essential self-sustaining genes, ultimately becoming similar to the viruses we are familiar with today.

A similar loss of independence has already occurred! Mitochondria, which are present in all eukaryotic cells (including humans), were initially endosymbiotic bacteria that lost their independence. However, there is currently no way to confirm this hypothesis… yet. And finally…

They Keep Coming Back: How Viruses Could Have Evolved Multiple Times

The independent evolution theory.

Also known as the “escape theory,” this is the most intriguing of all (and also the most likely, according to some scientists).

According to this theory, viruses may have independently evolved at least three times in the three different domains of life: once in Eukaryota, once in Archaea (the ancient “primitive” cells), and once in bacteria. This theory is mind-boggling as it proposes (with strong supporting evidence) that there is a tendency for cell genomes to convert some of their genes and machinery into viruses! These viruses would have originally been part of a normal genome but “escaped” and became parasitic!

Summary

Despite the uncertainty surrounding the classification of viruses and their position on the evolutionary tree, it is likely that they have some connection to us. This connection may depend on our definition of “life” and “family”. Whether viruses have multiple origins or not, if they are indeed part of the tree of life, it suggests that all life on Earth originated from a common ancestor that existed during the early stages of our planet. These small organisms, which could be considered our distant relatives, have the potential to provide insights into the general process of evolution, not only on Earth.