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