Eons

66 million years ago, after an asteroid slammed into the Earth and wiped out the non-avian dinosaurs, the world became a dark wasteland.

Plants couldn't photosynthesize, food chains collapsed, and about 75% of species went extinct.

But among the survivors were two distantly-related groups of animals that, on the surface, seem to have nothing in common: tiny mammals and a group of lizard-like reptiles.

They did share one important trait, though: the ability to chew their food.

Really well.

So why did these two completely different groups of super-chewers independently evolve the same superpower?

Turns out, this strange ability might have helped them survive when the world almost ended.

INTRO While mammals like us chew our food without a second thought, this process is not normal compared to most other animals.

Much of the animal kingdom saves the majority of the digestion process for after their food has already been swallowed.

For example, frogs, snakes, and birds swallow their food either whole or in sizable chunks, leaving their stomachs and intestines to do most of the work.

Birds and crocodilians even have an extra organ called the gizzard that uses swallowed stones to grind down food much like our teeth do.

...Which may seem bizarre, but chewing is ev en weirder.

Now, it might seem basic to us since it's so automatic, but it's actually really complicated.

We mammals don't just chop up our food; we grind it down like a mortar and pestle.

Assuming we do chew and aren't like, filter-feeding whales or anteaters.

When mammals chew, our jaws don't just move up and down, they go side-to-side, as well.

These movements are powered by specialized, robust muscles that allow the jaw to move in a variety of directions.

And the shape of our teeth is also essential to the unique way that mammals process food.

Almost all mammals have some arrangement of raised points on their teeth, called cusps, that allow them to crush and grind.

The exact arrangement varies by diet - humans, for example, have four primary cusps on our molars, arranged in a square, perfect for our omnivorous lifestyle.

Non-mammal teeth tend to be much simpler.

Many animals are haplodont, meaning they have conical teeth with a single cusp, and without any extra ridges.

We see this type of dentition in crocodilians and snakes, as well as many species of fish and amphibians.

And the ancestors of mammals started out with these simple teeth, too.

Take Archaeothyris from the Late Carboniferous Period, 306 million years ago, for example.

This little carnivore had basic, single-cusped teeth.

It didn't need anything too complicated to tear off chunks of meat from its prey.

Over time, that single cusp split into three cusps, lined up in a row.

This is known as triconodont dentition and it appears to first pop up in mammal relatives from the late Triassic Period, like Morganucodon.

Triconodont teeth allowed Morganucodon to slice through meat, rather than just rip and tear.

But the real game-changer came about 145 million years ago, in the Cretaceous Period with one of the great milestones in mammal evolution: the tribosphenic molar.

The earliest example of this kind of tooth shows up in a little mammal called Tribactonodon, who sported molars with three cusps arranged in a new pattern: a triangle.

With these tribosphenic molars, mammals could do more than just tear and slice.

They could crush and grind, too, turning their food into an easily-digestible pulp.

Almost all mammal teeth today are thought to have been derived from some form of these molars, so I guess you could say Tribactonodon, was really on the cusp of something great!

The combination of complex jaw movement with shearing crests and crushing basins on our teeth helped turn many mammals into masters of mastication.

But if most of the animal kingdom has gotten along just fine without chewing, why do we do it?

For decades, scientists thought they knew why mammals evolved such high-powered chewing.

See, mammals are warm-blooded, or endothermic -- we burn energy constantly.

The idea was that it takes a lot of energy to maintain a high body temperature, a speedy metabolism, and to stay active for long periods.

So all that crushing and grinding would help extract maximum calories from every bite to fuel that need for energy.

Chewing breaks food down into smaller pieces, which greatly increases its surface area.

This makes the chemical breakdown of the material easier and digestion more efficient.

It made sense... Until they looked closely at the tuatara.

Tuataras are reptiles that live in New Zealand.

You might remember them from our episode "When Lizards Took Over the World."

They look a lot like lizards, but they're not.

They're the last survivors of an ancient group called Rhynchocephalians that once spread across the globe.

If you look inside a tuatara's mouth, you'll find something strange: they have two extra rows of teeth along the roof of their mouth.

When they close their mouth, the lower jaw slides forward between those rows, shearing food through the teeth like a living hand saw.

And for a long time, scientists couldn't agree about just how complex their chewing was.

But new computer modeling has put this debate to rest, showing that tuatara jaws can actually move in very dynamic ways, making their chewing quite impressive.

It turns out, these little guys are the only other animal in the world that is considered to have as complex a chewing system as mammals'.

But here's the thing: tuataras are notoriously not warm-blooded.

And, not to throw shade, but they're kinda lazy too.

Tuataras are generally ambush predators, meaning they sit around and wait for food to wander by and basically deliver itself to them.

So if complex chewing evolved to fuel the high-energy mammalian lifestyle, why does this lazy reptile have it too?

The answer might lie in when these abilities evolved.

While the tuatara is the only living Rhynchocephalian, this group was much more diverse and prolific during the Mesozoic Era.

And fossil teeth of some extinct rhynchocephalians from the Late Jurassic Period show the same wear patterns that we see in modern tuataras.

This is around the same time that we see early mammals flexing their jaw muscles and right on the verge of developing tribosphenic molars.

And if we fast-forward to the end of the Cretaceous, 66 million years ago, mammals and rhynchocephalians were two of the few groups to survive the K-Pg extinction event.

But how could chewing prevent extinction?

Well, that asteroid kicked massive quantities of soot up into the atmosphere, leading to global cooling and lots of plant death due to their inability to photosynthesize.

Many groups were going extinct, making all kinds of food resources unreliable.

Now, many animals are dietary specialists, meaning they are really good at acquiring one specific type of food.

But in this wasteland, being a dietary specialist was a death sentence.

If you only ate leaves or one type of prey, you were probably done.

During a time when food resources are scarce and unpredictable, it's pretty dangerous to be a picky eater.

But mammals and rhynchocephalians had an advantage.

Their complex chewing systems meant they could process lots of different types of food.

Worms?

Sure.

Snails?

Why not?

Beetles?

Absolutely.

Lizards?

Eh, I'll try it.

Eggs?

In a pinch, might as well.

Now, this isn't to say that all of these animals didn't have preferences.

This isn't about what they would generally eat, but instead what they can eat when nothing else is available.

When resources are plentiful, it doesn't hurt to go for your favorites.

But when resources are scarce, wasting energy hunting for a specific meal is a dangerous gamble.

We can see this flexibility in modern tuataras.

They're primarily insectivores, but when times get tough, they'll eat seabird eggs and chicks, lizards - even other tuataras.

And there are also many living mammals that are generally herbivores, but will eat meat opportunistically... Like, sika deer - studies show they supplement their diet with bones and dead birds.

So, food preferences are fine most of the time, but in times of crisis - like a giant asteroid impact - the ability to eat whatever is available can come in pretty handy.

Which kinda means it took a lack of food for our ancient relatives to start eating more food - or, at least, a wider variety of food.

66 million years later, we're still here.

So is the tuatara.

And at least part of that probably comes down to our willingness to chew...and to not be so choosy about what we're chewing on.