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Go to Brilliant.org/SciShow to start their courses in math, science, and computer science for all skill levels. [♪ INTRO]. Viruses are arguably the most notorious parasites on Earth.

And they can be kind of creepy to think about, since they don’t just make you sick, they actually hijack your cells to make more of themselves. We can try to kill them with chemicals or UV light, or design vaccines to thwart them, but even then, they often evolve to evade our efforts. Except, all of that’s kind of wrong.

Or at least really weird to say, because most scientists don’t consider viruses to be living things. How do you kill something that’s not alive? Or, how can a non-living thing evolve?

This isn’t just a philosophical question or a quirk of language. In fact, whether viruses should be considered alive is an incredibly stubborn problem in biology, and when we try to finally pin an answer to it, things get difficult, if not a bit absurd. But maybe that’s okay, because life might just be a bit absurd.

And if we really want to find every living thing on this planet or figure out if any life exists elsewhere, we’ll need to confront that absurdity. Part of the problem is that there’s no single, agreed-upon definition of life. One scientific analysis found more than 100 examples of people trying to come up with a definition!

Potential definitions generally tend to focus on hallmarks of life, like whether the thing in question is able to maintain some sort of metabolism: a stable system of chemical reactions that provides energy. Being able to reproduce on their own is important too, as well as the ability to evolve via natural selection. And a lot of the time, these hallmarks exclude viruses from being placed on the tree of life.

But it turns out that excluding viruses for lacking these features is a bit problematic as well. There are quite a few things we consider “more alive” than viruses even though they also lack some of those essential traits of living things. Like, certain kinds of bacteria can go “inert” or form spores.

They’re not able to grow and divide until they “wake up”... still, we don’t consider them dead. And there are things like Wolbachia, which are bacteria found inside the cells of many insects. They can’t really survive on their own, not for long, anyway.

But they still count as alive, and viruses don’t. You might say, aha! The difference is that Wolbachia or these inert spores still have that all-important cellular machinery.

They’ve still got the genetic code for the machinery that turns genes into proteins, for instance. But… so do some viruses, apparently? Mimiviruses were originally discovered in 1992, and they are huge.

They’re large enough to be seen with a regular microscope! They are, in fact, so large that researchers initially thought they were bacteria. It wasn’t until 2003 that we realized they were actually viruses.

And they’re not only big, they’re weird. Their genomes are complex, with over 900 genes. Previously, it was thought that viruses topped out at around 200.

But more importantly, they seem to have some of the construction machinery needed to build the next generation of viruses. And these parts are functional, they directly participate in the process of making proteins. Don’t get me wrong—they don’t have everything they need to make baby mimiviruses.

They still can’t reproduce all on their own. But neither can Wolbachia. And the presence of these proteins may suggest they could, way back when.

Speaking of things that were once alive and independent, what do we do with mitochondria? They’re specialized structures found in animals, plants, and other eukaryotic cells that look like little bacteria and produce the cell’s energetic currency. They even have their own ribosomes and DNA.

Still, they’re not considered alive, even though they once were. They came into being because some bacteria set up shop permanently inside a cell! Which, come to think of it, isn’t all that different from those Wolbachia.

You might think it’s easy, then! Viruses and mitochondria are alive! But if we’re being generous with the “life” label, should we also extend it to much smaller parasites?

Like, narnaviruses. They’re so small that they don’t even have a capsid, which is the name for a virus's protective coating. They’re basically just a bit of protein and genetic material that floats around until it infects a fungus.

And we can go even simpler. There are things like plasmids or viroids, which are just infectious genes — no proteins at all. There’s even spooky stuff at the genome level.

A certain type of virus called a retrovirus can also integrate it’s genes into its host’s genome, and then emerge again years later. And there are so-called “selfish” segments of DNA that can essentially cut themselves out of the genome, replicate, and then stitch themselves back in at new locations. Should we still talk about them as just a quirk of genetics?

Or are they more like viruses that just happen to live in our genomes? Anyways. The point is that whatever definition of life you use, when you try to actually apply it to the real world, it becomes hard to draw a precise line between alive and not, or chemistry and biology.

Now, there are definitely people who still think viruses shouldn’t be included in the tree of life and that conventional definitions work well enough. But others disagree, especially because delving into the complexity here can help us see viruses with new eyes and maybe even rethink our approach to the science of life. Before we get too deep into some of the alternative concepts here, we should note a few things.

One, we’re going to purposefully give a high-level overview of these ideas, so we’re not going to hit every nuance. If you want to dive deeper, we will link to some key papers in the description. Two, we’re not saying these ideas are universally accepted, just that they’re examples of how scientists are rethinking the definition of life.

And lastly, while defining life based on what exists on one planet is bound to miss things, it’s not a useless exercise. Different definitions can lead to different questions and different ways of understanding the world. And that’s why some scientists have tried to re-think what a virus is and even proposed entirely new definitions for life.

One interesting idea is that the little seed-of-illness version of a virus, what’s called the virion, isn’t really a virus; no more than a sperm is a person. Instead, it’s only part of the virus life cycle. And viruses can be considered living, cellular organisms once they’ve hijacked the internal machinery of cells.

Inside a cell, a virus can reproduce, metabolize, and evolve. In this phase, it’s what one researcher has dubbed the “virocell”. Strangely, this can happen in coexistence with the old cell, we know of viruses that don’t destroy their host, just kind of hang out.

And since both organisms could use the same cellular machinery to build things and reproduce, this means one cell could belong to two organisms at the same time. The distinction, then, between viruses and regular cells would be whether they have genes that code for ribosomes or for capsid coats. This doesn’t totally give us clarity on what is and isn’t alive, though.

Like, those weird replicating proteins aren’t included. For that, one of the main scientists behind this idea proposed that any individual thing that participates in a living process, from plasmids to elephants, should count as alive, a purposefully broad definition. Others think that the very idea of putting the label of “life” on things is somewhat silly.

What you’re really asking is where can you draw the line, but everything we know of suggests there are no lines, just fuzziness. So, in a 2016 paper, two researchers proposed something a bit radical. They suggested that all biological entities, from humans down to viruses and even extending to those weird edge cases like plasmids, are built of “replicons,” or units of reproduction.

Any of these that are at least partly autonomous get dubbed “replicators”. And to count as that, there must be some trigger, some signal that can cause the replicator to reproduce independent of its environment. So a gene embedded in a huge chromosome might not count, because it can’t replicate independently of the rest of the chromosome.

But a virus would. We can still draw some distinctions, like whether a replicator can produce its own energy and resources, as a cell does, or whether it parasitizes others, like a virus. Some borderline situations may still exist within those.

But, notably, this does away with the question of whether viruses are alive, because it groups everything we’ve discussed so far under one header, the replicators. Also, because replicators can cooperate with each other, they can build into yet larger replicators. Genes build cells, which build organisms.

We can go from jumping gene and mitochondria to elephant without breaking the system. And it allows us to talk about the selfish end of things, like those weird self-replicating genes, without needing to draw too many lines. A self-copying gene is just a replicator in the larger “elephant” replicator group.

In the end, when you consider these newer ideas and traditional viewpoints, viruses might be dead, or they might be alive, or they might be a process or event more than a thing, or they might be part of a spectrum of living things. We’ve certainly got a lot of options. As for why this matters, well, it’s important to note that viruses are not just an unfortunate quirk of biology.

They play important roles in ecosystems. For example, viruses are a key part of the ocean’s nutrient cycle because they prey on and break apart bacteria and phytoplankton. They’re also big players in evolution.

By moving genes from one species to another, or by causing mutations, they’ve shaped the genomes of all living things. We may have viruses to thank for our placentas, for example. It’s even been suggested that the nucleus in our cells may have started as a virus that invaded some ancestral, nucleus-free cell way back when.

But, more broadly, this matters because different definitions can affect the way we think and ask questions. The way we frame things can influence the kinds of questions we ask. For instance, if you’re buying a house as an investment, you might have some questions about property taxes.

On the other hand, if you’re buying it as a home for you and your future family, you might ask about the school district or the neighborhood culture. Similarly, exploring different definitions of viruses could help us ask better questions about early life on Earth and what role viruses were playing back then. If we automatically accept the idea that viruses are a process cells go through, then, by that reasoning, cells must have already existed.

On the other hand, if viruses are just replicators, maybe some proto-viruses did exist before cells. In which case, what were they doing? And was it important?

Thinking about definitions could also help us search for life elsewhere in the universe. For example, a few papers have pointed to NASA’s 1976 Viking mission to Mars. During that mission, in between all the geology and chemistry that needed to be done, scientists had enough room for just one experiment to look for life.

After all, you can only fit so many instruments on a probe. The one that got selected looked for metabolic activity in the soil, and got inconclusive results. It made sense at the time; metabolism is one of the traditional hallmarks of life.

But would a different definition have resulted in a different test getting put on the probe? And if so, you just have to wonder: what kind of results might that test have gotten? I love questions like that, that really make you think about how the world works.

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