“The affinities of all the beings of the same class have sometimes been represented by a great tree. I believe this simile largely speaks the truth.”
- Charles Darwin, English naturalist and geologist
“Viruses are not a missing branch of the tree of life; they are woven into every limb and leaf.”
- Luis P. Villarreal, Professor Emeritus, Molecular Biology and Biochemistry, University of California, Irvine
The inspiration for this week’s Germ Gems post came from a good friend who recently asked this simple, but quite profound, question, “What exactly is a virus?” My answer, which I have expanded considerably beyond the scope of that conversation, follows.
What is a virus? In his article in the December 2020 Scientific American, “Viruses Can Help Us as Well as Harm Us,” science writer David Pride provides a simple definition of viruses: “extremely tiny biological particles made up of strands of RNA or DNA inside a protein coat.” To this, he adds, “they can only replicate with the help of a host cell that they infect.”
Over the past century, a large number of Nobel Prizes were awarded for work involving viruses. Even without SARS-CoV-2, the coronavirus that causes COVID-19, we are living in phenomenally exciting times for virologists. In fact, a majority of my Germ Gems posts over the past two plus years tell stories about virulent viruses that, like SARS-CoV-2, cause our species great havoc, such as HIV, influenza virus, Ebola virus, dengue virus, Nipah virus, and rabies virus. But I’ve also highlighted fascinating and extremely important viruses, called bacteriophages, without which our planet as we know it wouldn’t exist.
Even though viruses are the tiniest microbes (it is estimated that 15,000 SARS-CoV-2 particles can fit on the head of a pin), they are a really big deal. It is estimated that there are more viruses on Earth than stars in the universe, that is, more than a quadrillion quadrillion individual viruses. They’re found everywhere that virologists looked for them. For example, it’s estimated that 380 trillion viruses (belonging to 140,000 species) inhabit the human gut microbiome. That is 10 times the number of bacteria found there, and 100 times the number of cells in your body. And despite their submicroscopic size, the total biomass of viruses on Earth is estimated to equal that of 75 million blue whales, each whale weighing about 300,000 pounds.
Are viruses alive? Despite the large number and combined weight of viruses, most experts in the field of biology don’t consider viruses to be living organisms. Thus, they won’t allow viruses a place anywhere in the official “Tree of Life.”
The reason behind their exclusionary view relates to virologists Marc H.V. Van Regenmortel’s and Brian Mahy’s 2004 nuanced definition of viruses, which classified them as “nonliving infectious entities that can be said, at best, to lead a borrowed kind of life.” (It’s true that viruses must get into cells to survive, but this is the case for some bacteria as well.) The controversy over their fundamental nature (that is, are viruses alive, dead, or somewhere in between?) has festered since the Russian scientist Dimitri Ivanovsky and the Dutch scientist Martinus Beijerack discovered viruses in the late 19th century.
Do viruses belong in the Tree of Life? The idea of a Tree of Life can be traced to Charles Darwin, the English naturalist and father of the theory of evolution, and his 1859 publication of On the Origin of Species. Seven years later, German zoologist Ernst Haeckel drew up a more comprehensive tree representing Earth’s wealth of species.
In 1977, University of Wisconsin microbiologist, Carl Wouse, posited a modern day concept of the Tree of Life containing three major limbs called domains: Bacteria, Archaea, and Eukarya. New molecular biology techniques allowing the probing of every nook and cranny on Earth for DNA or RNA made this radically refurbished Tree possible. Together with geological and paleontological data, the molecular evidence now suggests that life began about 3.8 billion years ago when bacteria first appeared on Earth. To this day, however, nobody knows their origin.
A recent draft of the Tree of Life contains roughly 2.3 million named species of Bacteria, Archaea, and Eukarya (animals, plants, and fungi that harbor a nucleus in their cells). But there’s not a single viral species among them! Just like nobody knows where the first bacteria came from, the origin of viruses is a mystery. Many biologists think that viruses appeared about the same time as bacteria and that they have been co-evolving ever since.
Since the beginning of my career as an infectious disease specialist, I’ve followed the debate on whether viruses belong in the Tree of Life. I believe two recent discoveries ultimately may propel them into the Tree. First, in the journal Nature in 2020 a group of researchers described bacteriophages that are so large and so complex that they blur the line between living and non-living. Second, a group of French scientists showed that giant viruses, called mimiviruses, produce their own energy, that is, they’re not reliant on the cells they infect for an active energy metabolism thus clearing a major hurdle that has kept viruses out of the Tree of Life.
Who really cares whether viruses are living? All the scientific wrangling aside, what most people care about isn’t whether viruses are regarded as inanimate or living creatures, it is their massive destruction of human life that rivets our attention. So far the global death toll from COVID-19 stands at almost 5.5 million. The 1918-1919 influenza pandemic wiped out more than 50 million people globally. And before vaccination eradicated smallpox in 1977, this viral infection had killed more people than all world wars combined.
Nonetheless, only a tiny percentage of viral species are pathogenic. Instead, just like bacteria, a large majority of viruses are either harmless or beneficial to human health. In my June 10, 2020 Germ Gems post, “Viruses That Eat Bacteria: Fighting Fire with Fire,” the case was made for harnessing bacteriophages to treat antibiotic-resistant bacterial infections. According to Forest Rowher, a microbial ecologist at San Diego State University, and his colleagues in their book Life in Our Phage World, bacteriophages destroy up to 40 percent of all bacterial cells in the ocean every day.
Phages also play a role in the oceans’ ability to store excess carbon, and scientists are now considering a way to harness this process to put the brakes on climate change. If viruses can make a significant difference in averting the extinction of our species by climate change shouldn’t that be sufficient to give them a place in the Tree of Life?