Curtis Suttle | Without viruses we would not be alive
Viruses have helped to sculpture life and the world we live on
By 2019 Wall Scholar Curtis Suttle, a Professor and Fellow of the Royal Society of Canada at the University of British Columbia, where he shares an office with billions of viruses.
The novel coronavirus, SARS-CoV-2, is just the latest in what seems to be a never-ending war against the spread of viral diseases; Measles, Influenza, HIV, Ebola, Zika, SARS. It is estimated that the Spanish flu killed 50 to 100 million people worldwide, and that smallpox killed more than 20 million people in North America alone, so it is no wonder that public health authorities are on edge about the emerging viral disease caused by the novel coronavirus. Indeed, the word “virus” is rarely mentioned in good news stories in the media. Thus, it is surprising that we wouldn’t exist and could not survive without them.
Viruses are tiny. If an average virus was the size of a pinhead, and an average height American was scaled accordingly, the person would be 150 km tall. Viruses are unlike other forms of life, and in fact, are often not considered to be alive, at all. They have no metabolism on their own, and no capacity to reproduce unless they infect a suitable host – viruses typically infect a specific host, analogous to a key opening a lock. It is this high host specificity that endows viruses with their critical role in maintaining life.
Viruses are everywhere. We are swathed by the hundreds of millions of viruses that fall from the atmosphere on each square foot of Earth, every day. In a tablespoon of ocean or lake water there are more viruses than people in North America, and most of us will swallow a billion viruses every time we swim in a lake or the ocean. If the viruses in the ocean were stretched end-to-end they would span 10 million light years, or farther than the nearest 60 galaxies. We are inundated by viruses that don’t make us sick.
But, clearly, viruses can also infect us; so, how is it that we would not exist without them? Our long and complex history with viruses dates back millions of years, and is written in the 8% of our DNA that is viral in origin. Every time a virus infection occurs, there is a very small probability that a bit of its genetic material will be left behind. Even though such events are exceedingly rare, over millions of years and billions of infections, genetic material has been left behind many times. In fact, our genome contains about 100,000 remnants of genes originating from viruses.
These contributions by viruses to our genetic makeup have been crucial to our evolution. One example is the placenta; if it were not for a gene donated by a virus there would be no placenta, and mammals would not exist. Another, is a gene that encodes a protein that moves information between cells in the nervous system, and which appears to be critical to long-term memory. Without the intervention of viruses, we would not exist as a species.
We could also not survive if viruses did not carry out ecosystem functions on which we depend. For example, about 95 percent of the living material in the oceans, by weight, is microbial, and these microbes produce about half of the oxygen on Earth. Viruses are estimated to kill about 20% of these organisms, each day. This continuous mortality liberates the nutrients that sustain oxygen-breathing organisms.
Viruses are also crucial for maintaining biodiversity. Not only does much of the Earth’s genetic diversity reside in viruses, they also help maintain biodiversity by infecting the organisms that would otherwise outcompete all others. This process, termed “kill the winner”, occurs because viruses spread rapidly among hosts when their density is high; thus reducing their population, and freeing up resources so other species can grow. Indeed, outbreaks of influenza and other viral infections occur every fall and winter as children head back to school and we crowd together indoors.
Viruses have been sculpting life on our planet for a very long time. They existed billions of years before our first multicellular ancestors emerged. Yet, through evolutionary time, humans and all life have found a way to co-exist with the viruses that infect them.
Paradoxically, even though viruses continue to make us ill, humans would not exist without viruses, and our ongoing survival depends on them. The central role of viruses in our evolution and survival is something that needs to be acknowledged in the classroom. Viruses get a lot of bad press, but their integral role in our survival is something that scientists and creationists should be able to agree on, and is worth pondering the next time a rhinovirus gives you a runny nose.
Curtis Suttle is a Distinguished University Scholar at UBC, and a professor in the departments of Earth, Ocean and Atmospheric Sciences, Microbiology & Immunology, Botany, and the Institute for the Oceans and Fisheries.