No one needs to be told these days about how big a role viruses can play in our lives. But long before the COVID-19 pandemic – before humans or other multi-cellular organisms even existed – viruses were likely shaping the trajectory of life on Earth.
A team of researchers, including UConn’s Pieter Visscher, have published a paper in the March issue of Trends in Microbiology where they make the case that viruses have been affecting life on our planet for as long as there has been life.
Visscher studies the earliest ecosystems on Earth — mats of microbial communities that formed at least 3.4 billion years ago in primordial tidal pools. Microbial mats like these still exist today, and are used as proxies for early life that help researchers like Visscher and his colleagues reconstruct the past. One controversial question that remains to be answered regards the nature of something that triggers the mats to “lithify,” or transition to limestone formations called stromatolites; no one is sure what causes that to happen.
Visscher and his colleagues argue the switch from microbial mat to stromatolite is mediated by viruses.
Evidence of viruses from the distant past are difficult to come across, because, as Visscher points, out the fragile viruses would be gone in a second, before having the chance to fossilize, unlike the stromatolites they are theorized to play a hand in creating. Therefore, the researchers are relying on other records.
“There is a genetic basis for physiology which has an impact on geochemistry through time, and that geochemistry is found in the rock record,” Visscher says. “This work is about processes and mechanisms, it’s not going back in time and finding viruses in rocks, though that would be great.”
In the paper, the researchers present examples of biological and geochemical signatures that exist in the rock record. They also present examples of mechanisms where viruses can intercede; for example, how viruses can influence photosynthesis by switching genes on or off, or by inserting genetic information into genomes.
“That is something that we believe has been very important in steering photosynthesis in a certain direction,” says Visscher. “Without viruses, some of these processes may never have happened, or at least not in the way they have. We may not be sitting here today otherwise. It is not only the mechanism of the viruses that we are trying to shed light on, but the geological implications of how an ecosystem can move in a certain direction that is successful in the long term, that depends upon viruses to some extent, and that is what we are trying to put forward.”
One way to look at the theory is to visualize an ancient organism as a billiards ball on its evolutionary path and a virus, with abilities to insert or activate genes, as the pool cue that taps the ball into a new and more beneficial direction. Even without fossil evidence, the physiological and genetic mechanisms are the same today as they have been for millennia.
“We are trying to understand these mechanisms and transposing those to geologic time to mechanisms we know were around at that time. The organisms will not be the same, but the principles of their metabolic processes will be. We get so much information from geochemistry and molecular biology and then we get a better grip on deep time,” says Visscher.
Convinced of the connection, the next step the researchers are taking is to prove it.
“The experiments we are doing right now are with viruses in cyanobacteria where we are trying to figure out what role the viruses may have had. We can concentrate viruses and incubate and see what happens and look at shifts in metabolism and physiology. If we are lucky, we can do genomics and high-throughput sequencing on them, and reconstruct what genetic information they contain. The other thing is, we are doing experiments to see if the viruses can be the source of the nucleus to cause the formation of calcium carbonate that makes the stromatolites,” says Visscher.
The researchers are confident, Visscher says: “We know the answer will be yes, but we just have to show that experimentally. I am convinced that we will show that viruses play a role in evolution and in steering it in a certain direction.”
In reflecting the many ways viruses have likely changed the course of history, Visscher points out that though viruses have a bad reputation, especially right now — and for very good reason — they are also indispensable.
“It is not just a puzzle with five pieces, it is a puzzle with maybe a thousand pieces and viruses definitely have a part in many, many pieces. That is why we have the planet with the biodiversity we have today.”