Between 535 and 520 million years  ago, a new kind of biological litter   began collecting in the ancient  oceans of the Cambrian period.

Before that, life had been pretty  soft and squishy, but the Cambrian   saw the spread of the first groups with  a major innovation: hardened body parts… Like early arthropods with armored exoskeletons.

These were made of a tough material  that’s rich in carbon called chitin,   and their modern relatives - including insects,  crustaceans, and arachnids - still have it today.

This adaptation helped these  armored critters expand in   huge numbers throughout the world’s oceans.

As they did, the oceans became seeded with  tiny floating fragments of their exoskeletons,   which they shed as they grew larger  or left behind when they died.

And it turns out, without these broken bodies of  these bug-like creatures flooding the ancient   sea with bits of chitin, our world  might be totally unrecognizable today… Because these tiny exoskeleton fragments may  have allowed some of the most important microbes   in the planet’s history to set sail out into  the open ocean and change the world forever.

The Cambrian is famous for being a pretty  formative period in the history of life on Earth.

It saw the rise and spread of the  first of the major animal groups that   were, well, recognizable as animals - with  body plans that resemble modern groups today.

This radiation of big, complex  animal life came with a lot   of knock-on effects that shaped  the planet in all sorts of ways.

And bits of arthropod chitin becoming common in   the oceans might have been one  of those planet-shaping effects… Because it may have started a chain reaction  of microscopic events with global consequences.

And until recently, we had no idea about it.

See, animals weren’t the only group  undergoing changes half a billion   years ago that set the stage for later ecosystems.

Another game-changing evolutionary innovation  was brewing in the microbial world that,   at first glance, appeared totally unrelated… Tiny, single-celled photosynthesizing  organisms called cyanobacteria started   spreading like a floating  forest in the open oceans.

Today, their descendants are  called picocyanobacteria,   and they’re the most important microbes  you’ve probably never heard of.

They’re the smallest and most  abundant photosynthesizers on Earth… And they drift out in the open oceans  and draw down enormous amounts of   carbon dioxide from the atmosphere,  locking the carbon up in their cells.

By some estimates, picocyanobacteria alone   are responsible for around 25% of  all photosynthesis in the ocean.

This makes them a major part of  the base of the marine food chain,   where they play an outsized role in  determining how much energy ecosystems   have available to start off with, and so how  rich and complex those ecosystems can get.

Life on Earth just wouldn't  be the same without them.

But back before the Cambrian,  their ancestors lived a more   sheltered lifestyle as bacterial  mats on the coastal seafloor.

So how did these cyanobacteria go from  seafloor mat-makers to open ocean drifters?

Enter the Chitin Raft Hypothesis.

This idea was proposed in 2023 by a team of  researchers, and basically says that, around 500   million years ago, these ancient microbes rafted  out to sea on bits of dead bug-like critters.

Hear me out.

The first clue to the chitin raft hypothesis  was something the researchers stumbled onto   by accident while studying modern  picocyanobacteria in the lab.

While almost all of them are full-time  photosynthesizers, some strains can supplement   their diet of sunshine with organic carbon  from the environment when light levels are low.

This trait is called mixotrophy.

And while studying these mixotrophic strains,  the researchers found that several of them   carried genes for breaking down one type  of organic carbon in particular - chitin!

When they introduced chitin particles  to the microbes carrying those genes,   they found that these low-light strains were  not only feeding on chitin when they could,   they were attaching themselves to it, too!

Could this be how picocyanobacteria originally  made the transition to life in the open oceans?

By surfing on edible rafts  made of arthropod exoskeletons?

This was the researchers’ first lightbulb moment.

Because, after all, the open ocean  is not an easy habitat to adapt to.

It’s a harsh, salty, unsheltered  environment that’s both low in   nutrients and exposed to  high levels of UV radiation… Definitely a much less comfy  and stable habitat than the   mats on the coastal seafloor where the  ancestors of these microbes had lived.

So, latching onto carbon-rich chitin  particles from arthropod exoskeletons   and using them as a back-up source of  food and shelter may have been what   set them on the path to taking over the oceans.

The hypothesis was plausible, but to  test if the timing actually matched up,   the researchers then performed  a phylogenetic analysis.

They calculated when different bacterial groups   diverged from each other based on  the mutation rate of their DNA.

Then, they looked at how genes related to chitin  use are distributed across the family tree.

And they found that the first  chitin-use genes appeared deep   in picocyanobacteria evolution,  about 500 million years ago,   when their ancestors were making that  initial transition to the open oceans.

This is also right around the time when  arthropods were expanding in the oceans,   too, flooding the place with bits of chitin.

To the researchers, this seemed  unlikely to be a coincidence.

Instead, they argued that the ancestral microbes   that lived as mats on the coastal seafloor  adapted to make use of this new resource.

They acquired genes that allowed them  to stick to and feed on chitin and   used particles of it as rafts to  voyage out into the open oceans.

It was this rafting that acted as a transitional  stage in their huge ecological shift, allowing   them to establish an evolutionary foothold  in their new, much harsher environment.

After millions of years of rafting on bits of  dead bugs, many lineages gained the adaptations   they needed to thrive in that environment,  eventually becoming completely planktonic.

They jumped ship, leaving the chitin rafts  behind to be full-time photosynthetic drifters.

And while many picocyanobacteria then lost the  genes related to chitin-use, not all of them did.

Some floating lineages kept  those ancestral abilities,   probably because they were  useful in certain situations… Including the mixotrophic ones that often live in  especially low-light regions of the water column… Which is where using chitin is still a  solid plan B when photosynthesis is hard.

And without the lineages holding  onto those ancient adaptations,   we might never have stumbled on to  the Chitin Raft Hypothesis at all.

While it’s still a hypothesis that requires  more research, if it’s actually true, then   these ancient microbes riding out to sea on the  broken bodies of bugs was a very important event… One that we’re still experiencing the  consequences of half a billion years later.

When those floating picocyanobacteria  spread throughout the oceans,   the planet’s primary productivity  - the amount of organic matter at   the base of the food chain available for  ecosystems to use - got a pretty big boost… A boost that helped create the rich and  complex biosphere that we all know and love.

When we tell stories of epic journeys to  strange new environments here on Eons,   animals or other forms of complex  life tend to be the main characters.

But the story of picocyanobacteria is a reminder  that the history of microbes is also full of