Viruses, Endosymbiosis, and Evolution

Eric Marquette

So, what even are viruses? I mean, seriously, we’ve all heard about them, but they’re kind of tricky to define. Biologists, yeah, they don’t even agree on whether viruses are, you know, actually alive. What we do know is they’re made up of just two main things—RNA or DNA and a protein coat. Super minimal, right? But here's the thing—they can’t, like, reproduce or metabolize on their own. They don’t have cells, no ribosomes, no organelles, nothing like that.

Eric Marquette

Take the influenza virus, for example. It’s an RNA virus, and—get this—it’s classified as what’s called a negative-sense single-stranded RNA virus. Which basically means its genome is kinda like the opposite of the mRNA our cells use to make proteins. On the other hand, viruses like smallpox have a double-stranded DNA genome. Totally different structures, but, you know, both still operate with this same goal: hijack a host, replicate, repeat.

Eric Marquette

Now, here’s where it gets wild. Viral genomes are tiny, but they mutate like crazy. Take influenza again—it’s constantly changing, which is why we’re always updating flu vaccines every year. This rapid mutation makes it nearly impossible to pin down their evolutionary history. Like, you can try, but they’re evolving so fast, it’s, well, really hard to connect the dots over longer time frames. And forget fossil viruses—there just aren’t any. It’s like trying to piece together a puzzle, but the pieces keep, like, shifting around. Makes sense?

Eric Marquette

Oh, and get this—about eight percent of our DNA, eight percent, is made up of stuff called endogenous retroviruses. These are, basically, fragments of ancient viral DNA that got, you know, stuck in our ancestors’ genomes. They’re totally inactive now, but they’re still hanging out, just kind of sitting there. But here’s the thing—they tell us a lot about how humans and viruses have co-evolved over, like, millions of years.

Eric Marquette

So yeah, viruses aren’t really alive, but they’re definitely, you know, part of life. And understanding them helps us see how interconnected everything really is.

Eric Marquette

So, here’s the thing about viruses—they break a lot of rules when it comes to traditional classification. Instead of grouping them by evolutionary relationships like we do with plants and animals, we classify them by their genome types. You’ve got groups like positive-sense RNA viruses, for example, which include coronaviruses—the ones behind SARS and, of course, COVID-19. But here’s the kicker—these groups? They’re probably not monophyletic, meaning they don’t all share one common ancestor. It’s more like, uh, you know, separate groups evolving similar features independently.

Eric Marquette

And speaking of origins, there are these two big hypotheses about where viruses came from. For RNA viruses, the idea is that they might’ve evolved from cellular components—like, pieces of RNA that kind of, you know, escaped and started doing their own thing as parasites. On the other hand, DNA viruses might’ve started as highly reduced parasites, organisms that once had cellular structures but eventually lost them because they just didn’t need them anymore. Either way, viruses almost definitely arose multiple times, independently, which explains their massive diversity. It’s like they’re these, uh, remnants of ancient biology that keep adapting and finding ways to survive.

Eric Marquette

And then there’s the whole RNA versus DNA debate. Take double-stranded RNA viruses—they often wreak havoc on plants, like the rice dwarf virus. But double-stranded DNA viruses? Whole different story. They can infect humans—herpes, for example—or even bacteria, like those bacteriophages we’re experimenting with for phage therapy to treat bacterial infections. It’s pretty wild to think about how two totally different genome types can lead to such varied effects on their hosts.

Eric Marquette

Alright, so let’s talk about one of the coolest, most transformative ideas in biology—endosymbiosis. Yeah, I know, it’s a mouthful, but basically, this theory explains how eukaryotic cells—so, like, the kind of cells that make up you and me—gained mitochondria and chloroplasts. And here’s the crazy part: these essential powerhouses actually started out as ancient prokaryotes. I mean, imagine a smaller cell being engulfed by a larger cell, not, like, to destroy it, but to work together. Over time, they formed this permanent partnership. It’s pretty much teamwork on a microscopic scale.

Eric Marquette

Now, primary endosymbiosis is where it all began. Think of an early eukaryotic cell swallowing a tiny cyanobacterium—a photosynthetic prokaryote—and bam, that cyanobacterium eventually became a chloroplast. This process gave rise to the ancestors of green algae, red algae, and even land plants. Without this, we probably wouldn’t have plants, I mean, at all. And while we’re at it, mitochondria—those little energy factories in cells—followed a similar path when a proteobacterium was engulfed and integrated through endosymbiosis too. So, yeah, your mitochondria? They’ve got bacterial roots.

Eric Marquette

But it doesn’t stop there. Things get even more interesting when you look at something called secondary endosymbiosis. That’s when a eukaryotic cell, already equipped with chloroplasts, gets engulfed by another eukaryote. So now you’ve got, like, a cell-within-a-cell scenario. It happened with green algae, for example, and this process led to groups like euglenids, which have these really unique chloroplasts surrounded by three membranes. And then we’ve got tertiary endosymbiosis, where it goes one step further. A protist engulfs yet another cell that already had chloroplasts, and the result? Dinoflagellates—those fascinating little organisms that light up the ocean in bioluminescent waves. Nature really doesn’t hold back when it comes to creative solutions.

Eric Marquette

And honestly, when you think about it, endosymbiosis is such a perfect example of how cooperation can drive evolution. I mean, these ancient prokaryotes didn’t just survive—they thrived by becoming something bigger than themselves. And the way cells adapted and held onto these partnerships is just... well, it’s kind of inspiring. It shows us how evolution isn’t just about competition, but sometimes, it’s about finding the right partners to team up with.

Eric Marquette

So on that note, that’s all for today. Thanks for joining me to explore these mind-blowing connections in biology. From viruses to endosymbiosis, the story of life is full of surprises—and honestly, we’re just scratching the surface. I’m Eric Marquette, and I can’t wait to dive into more fascinating topics with you next time. Take care, and see you soon.