In 2003, a team of microbiologists made a huge discovery, one that would force us to reconsider a
lot of what we thought we knew about the evolution - and even the definition - of microbial life.
They were studying a microorganism that had been found in water from a cooling tower in
England a decade earlier. At the time, it was called a bacterium, based on its size.
But the scientists realized that, while this thing was bacteria-sized,
it was built /more like a virus/.
And this made no sense because viruses are
/much/ smaller than bacteria - that’s one of their defining features!
The researchers named it mimivirus, short for ‘microbe-mimicking virus’, and it isn't alone.
In the years since its discovery, many more of these microbial misfits have been found.
They’ve turned up everywhere from the soil,
to the sea bed, and even buried in 30,000-year-old Siberian permafrost.
And none of them neatly fit into the definitions we’ve created for
viruses or cellular life - they seem to have some of the traits of both.
So how did they end up in this weird twilight zone? Do they descend from
viruses that became kinda cell-like, maybe by stealing genes from their more complex hosts?
Or are they the remains of an ancient form of cellular life
that became kinda virus-like, by losing parts over time?
The debate around what they are and where they came from is still going on,
but in the meantime, we’ve been calling them giant viruses, aka giruses for short.
Now, there’s no need to panic about giruses.
They may be bigger than other viruses, but that doesn’t make them badder.
Most of the giant viruses discovered to date, including mimivirus,
have only really been shown to infect single-celled organisms, like amoeba and algae.
And, along with being big, they also have a few other things in
common - clues that let us dig into their potential evolutionary origins.
Like, they all have genomes made up of DNA that are larger and contain more genes than
normal viruses. Mimivirus has around /1000/, while typical viruses have just a handful.
And some of those genes seem…out of place.
Many of them code for proteins that do things that viruses /just don’t do/, like translation.
Now, translation is the molecular process by which proteins are built based on genetic instructions.
It’s the fundamental process at the heart of all living things. Growth, reproduction,
metabolism - they’re all impossible without it.
And because viruses don't carry out translation, they can't do any of those things.
They can only replicate by hijacking the translation machinery of host
cells and tricking them into building more viruses based on the proteins the viral genome codes for.
This is another key trait of viruses and is the main reason why they’ve often been considered too
simple to count as ‘alive’ - a debate we dug into in our episode, “Where Did Viruses Come From?”
So finding that giant viruses carry genes that provide the instructions
for making parts of this translation machinery makes /zero sense/!
Especially because /none of them/ have the central component of the whole translation process:
the ribosome.
It’s a critical molecular machine found in all cellular life, but not in any viruses or
giruses. It’s the site where all those translation proteins come together to carry out the process.
So why do giruses have the blueprints for parts of cellular machinery that they don’t use?
After all, giruses still seem to depend on infecting and hijacking host cells
to reproduce. They’re no different from normal viruses in that respect.
This contradiction has led to competing hypotheses that propose
radically different origin stories for giruses.
One idea that got a lot of attention following the discovery of mimivirus,
is that giruses /started out/ as complex living cells.
In this hypothesis, in the deep past, the ancestors of giant viruses were capable
of translation, self-reproduction, metabolism, and all the other processes we associate with ‘life’.
But, over time, they went through a process known as regressive evolution, losing their cellular
components to the point where they ended up like viruses - just a genome wrapped in protein,
reliant on infecting actual cells to reproduce.
****They went from being a branch of the tree of life to a parasitic vine that wraps around it.
And in terms of what those ancestral cells were, well,
biologists currently only recognize three domains of life.
They are Bacteria, Archaea, and Eukarya, which includes
everything with a more complex cell structure, like all animals, plants, and fungi.
But based on the fact that giruses seem to often have a lot of genes without known
counterparts in any of the three domains, it’s been suggested that they may descend
from an /ancient fourth domain of cellular life that’s now extinct/.
In fact, the discovery of giant viruses
led some researchers to argue that they were a sort of ‘missing link’
that finally gave us an evolutionary connection between the worlds of viruses and cells.
And evolutionary regression from cellular ancestors might be the origin of /all viruses/,
with giant viruses just having kept a few more relics of their cellular history than others.
But, if one explanation is that the out-of-place genes of giruses
are ancient leftovers from their cellular past, the alternative
is that the genes were acquired from their cellular hosts, past and present, instead.
In this hypothesis, giruses started out as regular-sized viruses that, over time,
stole and accumulated DNA from the cells they infected. They didn’t regress from
complex living cells, but expanded from much smaller and simpler viral ancestors.
Now, the exchange of DNA sequences between different groups is known as horizontal gene
transfer. And in recent years, we’ve begun to realize that this process
happens /all the time/, especially in the microbial world.
So the idea that giruses are just skilled genetic pickpockets is totally plausible.
And researchers studying cell-like genes in giruses have reported that some of them
do seem similar to the genes found in different species of living organisms.
This supports the idea that these sequences were
independently and repeatedly stolen and adapted from living cells.
The fact that so many giruses specifically infect amoeba could also be relevant to this hypothesis.
Because! Amoeba regularly ingest smaller organisms, including bacteria, fungi,
and other eukaryotes – so their cells contain a melting pot of ingested DNA sequences
from across the tree of life that giruses could potentially sample from.
The idea that giruses are just talented gene thieves would mean that they’re not a fourth
domain of life - or what’s left of it - but are just genetic mashups of the current three!
And the genes in giruses that don't match any genes in living cells
wouldn’t be evidence of that ‘fourth domain’. Instead,
they’d just be a reflection of the fact that our DNA databases are still incomplete.
Now, while these have been the two main hypotheses about the origins of giruses,
they’re not the only ideas out there – and their actual origin story could be even weirder.
We just don’t know for sure! Researchers still regularly come to opposite conclusions and we
don’t have any evidence that we could call a smoking gun.
But whatever the answer is, it has massive implications for our understanding of life.
The discovery of giruses has already shattered earlier ideas
around the limits of viral complexity and the features that define them.
Beyond virology, the questions raised by their fuzzy, hard-to-classify nature
take us deeper into what it means to be a living cell…
And they point to new possibilities about what both viruses and cells can do and become,
given enough time and strong enough evolutionary pressures.
And there is one thing that I think we can all agree on:
giruses, like mimivirus, have shown us that even when it comes to the supposedly
smallest and simplest biological entities, we still have so much left to learn.
/Giant/ thanks to this month’s Eontologists:
By becoming an Eonite at patreon.com/eons, you can get fun perks like submitting a joke
for us to read, like this one from Michael Leavenworth
A guy walks into a deli and says to the T-Rex behind the counter “Hey! I need a sandwich!”
The T-Rex says “Hang on, can’t you see I’m a little short handed!”
And as always thanks for joining me in the Adam Lowe studio. Subscribe at youtube.com/eons for more journeys in deep time
Viruses are so trippy y'all