- Okay, so you know when you go to the dentist and they do that thing that they call a routine cleaning but actually feels like some kind of punishment?

The purpose of that cleaning is to scrape away a combination of dental plaque and hardened tarter on your teeth called calculus to keep your mouth healthy and prevent disease.

This might seem like a pretty mundane, slightly painful and kind of gross part of life.

But the stuff that your dentist scratches off your teeth is from the view of deep time, really precious stuff because that calculus is the byproduct of hundreds of species of microbes that are adapted to thrive in the ecosystem that is your mouth.

And left on its own it can last for a very long time.

In fact, your dental calculus, this is fascinating to me.

Your dental calculus is the only part of your body that fossilizes while you're alive, because fossils are just organic material that is mineralized, right?

So that hardened residue on your teeth is actually a fossil record of your own personal mouth microbes.

Now in the last few years, scientists have figured out how to study dental calculus, to trace the evolutionary history of these microbes over tens of millions of years.

And they've found that many have been with us since long before we were human.

They've co-evolved with us through major changes in our evolutionary history and can reveal stories from our deep past like changes in diet, ancient migrations, and even some similarities we had with Neanderthals that remained invisible until now.

And it turns out this surprising wealth of evolutionary information was quite literally under our noses the entire time.

The trillions of microorganisms that live in and on our bodies, including on our skin and in our gut are known as our microbiome.

And it plays a crucial role in our health and development.

It probably always has, we just wouldn't be the same without it.

But the problem is that most of these microbial ecosystems don't even much of a record behind which makes it hard to trace how they shaped us and how we shaped them through time.

But your oral microbiome, the community of microbes that live in your mouth does leave a fossil record.

One that's strong enough to survive and preserve information for millennia, that's your dental calculus.

Calculus starts out as plaque, a thin film that constantly forms on your teeth by the bacteria that live on them.

Over time the plaque mineralizes turning into the much harder dental calculus laid down by generation after generation of microbes, just like geological layers.

And just like the calculus you may have taken in school, your dental calculus is really hard.

And just like the geological or fossil record, this dental fossil record gives us a glimpse into ecosystems that existed in the past because calculus has turned out to be incredibly good at preserving the DNA of your mouth microbes.

And thanks to recent technological advances, even really ancient microbial DNA from this calculus can now be recovered.

Revealing a treasure trove of molecular information, one that could be used to reconstruct the oral microbiomes of people and animals that died long, long ago.

In 2021, a huge study was published that investigated microbial DNA from the dental calculus of over a hundred individuals.

The sample included ancient humans from as far back as 30,000 years ago, Neanderthals from as far back as 100,000 years ago, and museum specimens of chimps gorillas and howler monkeys.

It also included dental calculus from people today who donated it for research after getting their teeth cleaned, which I would like to do.

I'd like to ask Dennis, just like, "No, I'm gonna keep that it's for science."

By reconstructing the microbiome of each group and comparing them with each other, the researchers hit upon a few key discoveries like to their surprise they found that DNA from the same core group of 10 microbes was present in the calculus of all the species they studied.

And this core group wasn't just shared by humans, Neanderthals and the great apes, but also by Howler monkeys from the Americas, a lineage we last shared a common ancestor with around 40 million years ago.

This suggests that these 10 core microbial taxa have been living in our mouths for at least that long.

They were there enjoying the ride as we swung through the ancient rainforests of the (indistinct) Park.

They were there tens of millions of years later as we began to walk upright in Africa.

And they're still there today right now, as you watch this.

Which makes me very, do you have any floss?

And while many these microbes are familiar to microbiologists because of the roles they play in oral health and disease, there are a few we know barely anything about, they haven't even been named yet.

Okay, but why are so many mouth microbes shared across primates that have such different lifestyles and diets, and whose common ancestor lived so long ago?

Well, it seems that the species that make up the core oral microbiome are so dependent on each other that the ecosystem they share has been incredibly stable.

It out that the formation of plaque, and then calculus depends on a succession of microbes working in a certain order.

That means no key part of the ecosystem can change without a lot of other things changing too.

Even over tens of millions of years.

And the fact that this core group has been around for so long and is found in so many different kinds of primates, may mean that they're really important to us and our oral microbiome.

But while parts of our microbiome have been really stable over time, there have been some changes.

When is that point to a big shift in the diet of our ancestors.

When the researchers compared humans with chimps, they found that we have a lot more of a microbe called streptococcus.

Specifically some kinds of streptococcus that seem to have evolved the unusual ability to feed off of starch.

These microbes picked up genes that allow them capture an enzyme called amylase that we produce in our saliva that allows us to digest starch.

By hijacking this enzyme of ours as we started to eat more and more starch, they also became able to digest it.

And this was such an advantage that they became much more abundant in our mouths than in other primates.

And the remarkable thing is, this isn't just a feature of modern humans.

The streptococcus on the Neanderthals teeth also have these genes, meaning that this adaptation probably happened in the mouth microbes of our common ancestor over 600,000 years ago.

Now this is a big deal because for a long time, scientists have suspected that a change in diet during our evolution was probably responsible for some of our more notable features like having really big brains that require lots of energy.

But what exactly that change was and when it happened has been hotly debated.

I mean the oldest fossils of modern humans that we know of so far are about 300,000 years old.

So finding that starch digesting microbes may have been living in the mouths of our ancestors more than a half million years ago has massive implications for understanding our evolution.

Specifically, it suggests that our ancestors started transitioning to a starchier diet, not just before the advent of agriculture which began around 12,000 years ago, but before our species even existed.

And this wasn't the only major event that left a mark in the genetics of our mouth microbes.

When the researchers compared the DNA of the microbes across the different primates, they tried to build a bacterial family tree and found evidence of a more recent change.

Many of the ancient European humans from between 30,000 and 14,000 years ago had a few bacterial strains that were more closely related to the bacteria of Neanderthals than to those of humans from after 14,000 years ago.

Now the microbes that make up your oral microbiome are mostly passed on to you in early childhood from your parents and caregivers through things like nursing and kissing and sharing food which means it's probably a legacy of the interactions and interbreeding that happened between humans and Neanderthals.

And those Neanderthal like microbes stuck around on the teeth of European humans until around 14,000 years ago.

Long after Neanderthals went extinct, and this is right around the time that we know that new human populations migrated into Europe and rapidly replaced the people that were already there, a process called population turnover.

Now we don't know exactly where these new human migrants came from, but we can tell that while they carried the same types of microbes as the people they replaced, their strains lacked the genetic connection to Neanderthals.

So the disappearance of those Neanderthal like microbes, gives us a second line of evidence of that ancient migration.

This all goes to show that our evolutionary journey isn't just our own, our microbes have been with us every step of the way.

And by studying their history, we're essentially looking at ours from a different perspective because we've both been shaped by the same forces in the same events, whether that's dietary transitions, ancient migrations, or interactions with other human relatives.

These events left their mark on us and on our microbes too, which captured them in their own fossil record in our mouths.

And it's only thanks to this gunk between your teeth and the fact that our ancestors probably didn't floss very much, that the deep history of this relationship is finally being revealed.

But this is not an excuse to stop getting your teeth cleaned.

I am more than anyone would love to never go to the dentist again and say I'm doing it so I could leave as much data behind as I can for future scientists.

But trust me, they will have plenty of other stuff to work with, just go to the dentist.

(gentle music)