500 million years ago, back in the Cambrian Period, a pioneering little mollusk floated up off the ocean floor.

It had developed a way to use its defensive shell for a whole new purpose - buoyancy.

It turned out that by filling its shell with gas, this mollusk could literally reach new heights, gaining a key advantage over its relatives on the seafloor.

Scientists believe that this was the very first cephalopod - a group that now includes squids, octopuses, cuttlefish, and nautiluses.

While we might think of the nautilus with its shell as an oddity today, the fact is that the ancestors of modern, squishy cephalopods like the octopus and the squid all had shells.

And early cephalopods are actually defined by their shells -- or, more specifically, by how their shells adapted to suit their needs.

Some cephalopods truncated their shell.

Others acquired a different shape.

Some of them internalized their shell like a backbone.

And, in some cases, they got rid of the thing altogether.

In ancient times, the shell was cephalopods' greatest asset.

But it also proved to be their biggest weakness.

Mollusks were some of the first truly complex animals, probably appearing in the late Ediacaran Period.

Although there is some evidence of hard mineralized shells from this time, shells as we know them today become much more common after the Cambrian Explosion, which -- not coincidentally -- is when the first evidence of predators appears.

So, early shells worked like shields, protecting the soft body, or mantle, of the animal from predators lurking above.

But by the late Cambrian, one mollusc known as Plectronoceras had acquired a couple of adaptations that marked the beginning of a brand new form of transportation -- and a whole new kind of mollusk.

For one thing, its shell was divided into sealed-off chambers by thin walls called septa.

As the animal grew, it added new chambers to its shell.

This in itself wasn't new, but it ended up being instrumental to another adaptation that was.

As Plectronoceras added septa to its shell, it left behind a small, tube-like part of its mantle in each chamber.

This little tube of tissue is known as a siphuncle, and as unassuming as it seemed, it helped Plectronoceras perform a trick the world had never seen before.

By making the blood that flowed through the siphuncle super salty, Plectronoceras was able to absorb all of the water from the chambers in its shell.

As water diffused out of the shell and into the salty blood, gas seeped in, and what was once a suit of armor became a personal floatation device.

The very first true cephalopods had arrived, and they looked like tiny, adorable, upside-down ice cream cones.

This development of a gas-filled, chambered shell, also known as a phragmocone, was a triumphant, history-making adaptation.

By the time the Cambrian had segued into the Ordovician, cephalopods had entered a golden age.

There were few predators to threaten them, and a rise in ocean oxygen levels caused life to flourish, diversify, and occupy new habitats, providing an abundance of food.

This is known as the Great Ordovician Biodiversification Event.

And that's when they got very big.

The Ordovician endoceratids cephalopods were the biggest animals of their time, reaching an impressive 6 meters in length.

And as the Ordovician progressed, cephalopods began to leave the shallows to explore the open ocean.

So they had to find ways to become more fast and agile.

Some species developed shells that coiled, forming a more compact and maneuverable form, like the modern nautilus.

By the Silurian Period, a genus called Sphooceras tried a different tactic: Instead of coiling its shell, it broke off the end of it.

Sphooceras periodically wrapped part of its soft mantle around the outside of its shell, and then secreted enzymes that helped break off the chambers at the end.

This made the end blunter, shorter, and sturdier.

Which in turn made the shell less vulnerable to breaking and easier to maneuver.

Sphooceras might be the very first cephalopod that kept its shell inside its mantle for any length of time - and this was an experiment that was about to be taken to a whole new level.

That's because a new evolutionary pressure was waiting for cephalopods in the Devonian Period: fast, jawed fish.

While fish with jaws first appeared in the Silurian, they proliferated in the Devonian.

And that kicked off an evolutionary arms race between fish and cephalopods.

Up until this point, all cephalopods had been members of the slow and steady group known as nautiloids, from the pioneering little Plectronoceras to the imposing Cameroceras.

This ancient lineage still survives today in the form of the modern nautilus.

But in the Devonian, a new branch of the cephalopod family tree appeared: ammonites.

And they coped with the rise of fish with a live-fast, die-young strategy.

Unlike nautiloids, which grew slowly and invested a lot of energy into making a few offspring, ammonites grew quickly and had many offspring.

They ended up being so successful, diverse, and numerous that their shells are now used as index fossils to define Periods in the Mesozoic.

And ammonites developed a huge variety of shell sizes and shapes, growing shells that looked like hooks or knots or even paper clips.

Then, around the beginning of the Carboniferous, a new lineage appeared with an even more radical strategy to deal with the fish problem.

They were the first coleoids Like Sphooceras millions of years before them, coleoids wrapped their soft mantles around their hard shells.

But unlike Sphooceras, they kept it there permanently.

Hematites, for example, was one of the earliest coleoids, and it had a cone-shaped shell inside its soft body.

Then, over millions of years, the shells began to shrink, and what remained was built with lighter-weight material.

After all, internal shells no longer offered protection, so there was no reason to keep lugging around all that extra weight.

So they lost the gas-filled chambers that had kept them afloat, and developed new ways to stay buoyant, and new, faster forms of jet propulsion to get around.

In time, the internal shell was streamlined down to a long, chitinous structure, kind of like a backbone, called a gladius.

All squid alive today still have some kind of gladius, while octopuses have a pair of similar structures called stylets.

Armed with these adaptations, coleoids began to take advantage of a new niche: the deep sea.

While the old gas-filled phragmocone couldn't withstand the pressure of the deep ocean, the gladius had no such problem.

And their ability to live in the deep turned out to be what saved coleoids from extinction.

At the end of the Cretaceous Period, a fatal blow struck the ammonites and most of the nautiloids: the Cretaceous-Paleogene extinction event -- the same event that killed the non-avian dinosaurs.

Acid rain changed the pH of the oceans, compromising the integrity of the shells these animals needed to survive.

This hit baby ammonites, which relied on their thin, fragile shells to passively float near the ocean surface, especially hard.

At the same time, there was likely a massive die-off of ammonites' main food source, plankton.

The nautiloids were probably saved by their slow and steady lifestyles, and six species in two genera have survived to the present day.

But the coleoids were able to take refuge in the deep sea, and were no longer dependent on their shells.

So with the ammonites gone, when conditions improved, the coleoids rose up and took their place.

Today, coleoids have colonized every marine ecosystem on the planet, and they play a vital role in ocean food webs.

Instead of relying on a suit of protective armor, they now use intelligence, camouflage, and agility to outsmart predators and prey alike.

Their journey from small, passive molluscs to sleek, voracious predators took hundreds of millions of years of trial and error -- from developing shells to survive, to finally learning to thrive without them.

And the squid still swims around with its gladius intact, and the octopus with its stylets - - reminders of the history they share with the shelled creatures of the past.