We're The Only Ones With Chins - And We Don't Know Why

You share a trait with every single human who's ever lived –

but no other animal on Earth has it.

It's not your big brain,

or your opposable thumbs... it's actually this little shelf on your face

that we call a chin.

Yes, really.

And here's the thing: we're not totally sure why it exists.

Ever since Homo sapiens was named in the 1700s,

the question of how to define our species has been, well…a question.

See, when Carl Linnaeus added humans to his ‘System of Nature,’ he didn’t include

a description of distinct traits,

like he did for other species.

Next to Homo sapiens,

he simply wrote a Latin phrase that translates to 'know thyself.’

And it turns out, that’s…a lot harder than you might think.

Ever since, scientists have struggled to identify traits that are uniquely ours.

Many ideas have been proposed, ranging from anatomical to cultural.

Humans as 'the' hunters, 'the' tool makers, 'the' large-brained species, for example,

have all been debunked.

But what hasn’t been debunked?

Chins.

So what is a chin even for and why do we have them?

Something so unique to us must have a purpose…right?

Well, maybe not.

It could just be an evolutionary accident.

But even if that’s the case,

the chin is still useful – just maybe not in the ways you’d expect.

And we brought in paleoanthropologist Ella Al-Shamahi,

host of the new NOVA and BBC series ‘Human’ for a closer look.

Around the time that Linnaeus named Homo sapiens,

scientists had already noticed that all of our skulls have a bony, projecting chin.

Paleoanthropologists call this the 'mental eminence'

or ‘protuberance’ and it has a specific anatomical configuration that's present from the time

we’re fetuses.

That configuration is often called the “inverted T,”

because it looks like an inverted T.

So there's a central ridge that runs down the middle of the jaw,

meeting a horizontal section that juts out along the lower edge,

that’s framed by depressions on either side.

It's kind of funny that we need to give a technical definition for chins,

like...we all know a chin when we see one, right?

But we're not just talking about a random bony bump.

This is, instead, a specific piece of facial architecture.

And it’s not something that any other animal has.

Compare our projection to a chimpanzee's chin, for example.

Theirs—like most other animals—basically just slopes backwards in what's called a 'receding symphysis.’

When scientists began uncovering fossil human relatives, the pattern became clear.

Neandertals, discovered in the 19th century, had receding chins like chimpanzees.

You can see the difference when you compare it to Homo sapiens.

And as additional hominin discoveries –

so those fossils belonging to species more closely related to us than chimps –

began to trickle in,

they only added more support to the growing consensus

that we alone share this feature… Like our 3 to 4 million-year-old ancestor,

Australopithecus afarensis – they lack a projection.

Even our more closely-related ancestor, Homo erectus, is missing this feature as well.

So it became increasingly clear

that this might be a sort of signature of our species.

Scientists began to wonder: what is it for, exactly, these projecting chins?

And once they dove into that question, they found a mystery.

I’ll let Kallie take it from here.

There have been some wild hypotheses over the years trying to explain why we

have chins.

Some scientists have suggested they evolved

as a clamping device to hold cloth

while working,

while others have maintained that they are merely decorative features shaped by sexual selection.

But the most outlandish?

That chins evolved to take a punch.

Researchers used data from actual experiments

that had Olympic boxers punch a dummy to better understand head injuries to explore

whether this bony reinforcement might help in fights.

And they argued that facial features like robust bones

and muscles strategically placed around the jaw could better withstand impacts…

But ideas like the punching hypothesis often amount to little more than 'just so'

stories—seemingly logical explanations that crumble under scrutiny.

The more credible ideas

that have emerged over the years are centered around other unique features

that we often think of

as important for humans to be,

well, human.

The first major hypothesis appeared in a 1913 article called 'The Story of the

Chin.'

In it, a physician suggested the chin resulted from our complex language abilities.

Since human speech requires many different tongue movements and precise muscle coordination,

he reasoned that the chin might anchor those muscles, aiding speech production.

The problem is, the chin isn’t actually the attachment point for those muscles.

Instead, the main muscle he highlighted attaches to the inside of the jawbone.

So most researchers didn’t buy it.

Meanwhile, more hominin fossils were being discovered – like Peking Man in China,

now classified as Homo erectus.

These, too, lacked chins,

reinforcing that this feature belonged uniquely to Homo sapiens…

Which meant that it could potentially be used

as a diagnostic feature – helping us recognize our own species from our close

relatives in the fossil record.

But even as the chin's uniqueness became more clear, its evolutionary purpose remained elusive.

In the 1950s, scientists turned to biomechanics—studying how muscles, tendons,

and ligaments work together in the lower jaw.

And they came up with a new idea:

the ‘airway impingement hypothesis,’ which basically says that chins arose in response to breathing.

Specifically, that it was a response to constraints in the respiratory system resulting from

our upright posture.

But not everyone was convinced.

Other researchers stuck with the biomechanical approach,

but focused on the other main thing our jaw does: chewing.

This seemed more promising, because we know Homo sapiens chewing is pretty unique.

See, our faces and jaws have shrunk significantly over our evolutionary history,

retracting in response to the unique shape of our brain case – a shift

that has fundamentally reshaped our facial architecture in the process.

And this smaller face and jaw raised the question of:

what new stresses might this have created?

Since bone remodels in response to stress – changing density

and shape based on the loads placed on it – maybe the chin is

simply a response to chewing pressures.

Maybe, these researchers argued, it acted as a buttress,

strengthening our smaller jaws for the demands of processing food.

For a while, this seemed like the likely answer.

But no one had definitively proven it.

And around this time, in 1967,

scientists uncovered the fossil called Omo 1 from Ethiopia.

Considered to be one of the oldest Homo sapiens ever found,

at almost 230,000 years old, Omo 1 is pretty fragmentary.

But it does preserve the lower jaw,

which shows that this human had a prominent chin.

This underlined the fact that the human chin is pretty old,

and added confirmation that it’s likely been around since the beginning of our species

– which evolved potentially about 300,000 years ago.

After almost a century of searching for a purpose,

some scientists began asking a radical question:

What if the chin doesn’t have a function at all?

What if it's just... there?

As weird as that sounds,

evolutionary biologists have a term for these nonadaptive traits – they call them 'spandrels.'

They’re just byproducts of other evolutionary processes, never directly shaped by natural selection.

But proving that something is just an artifact is pretty difficult.

After all, how do you prove something doesn’t have a purpose?

So, the science of the chin stayed in limbo.

It wasn’t until the early 1990s

that researchers moved away from theoretical approaches

and really started putting the chewing hypothesis to the test.

One scientist compared the geometry of the human jaw to other primates

and found that the proportions of our jaws are unique in how short

and broad they are.

He then built upon experimental work

that had examined the precise stress patterns

that different primate jaws experience

when eating food and applied

that work to this human shape.

This showed that the short, broad shape of Homo sapiens’ jaw does, in fact,

create a different set of stresses.

And the structure of our bony chin seems to exactly match the needs raised

by this distinctive shape

and the stresses it causes,

he argued.

For example, the lower jaw experiences stresses caused by twisting of its left

and right halves,

called vertical bending.

And the outer layer of bone is exceptionally thick right where vertical bending creates

the most stress – down at the base of the symphysis,

where the two halves come together.

Comparing this to other hominins,

the researcher pointed out

that the timing of the chin’s appearance lines up precisely with these changes in

jaw proportions.

So, he argued that the reason the chin doesn’t appear until Homo sapiens is

because earlier species – from Australopithecus to early Homo – retained those more ape-like

jaw proportions.

Which meant that they didn’t need to solve the problem of counteracting this stress

– because the problem didn’t exist yet.

So this suggested the chin truly was an adaptive response,

one that was responding to changed mechanical demands on the jaw.

And while this study has won over some scientists,

others are more skeptical and continue to maintain that the chin is just...there.

In large part because more recent biomechanical modeling reveals

that jaws with chins actually aren’t any better at handling those particular types of

stresses than those without chins.

So there’s still no consensus on the chin’s evolutionary origins.

And in recent years,

some scientists have argued that all chin hypotheses – functional or not – face some challenges,

either on a theoretical level or an empirical one.

And while more discoveries of hominin fossils – both others without chins

and Homo sapiens with chins – are adding data points to the timeline,

they still aren’t telling us why this characteristic exists.

But, what we can say is that,

even if the chin doesn’t actually do anything evolutionarily,

it does allow us to identify ourselves in the fossil record – to ‘know

ourselves’… So something that’s functionally useless can still be useful.

And as humbling as it is to not know why we have this feature,

at least it’s given us something

that can be listed under Linnaeus’s distinctive traits of Homo sapiens…at least,

for now.

Big thanks to Ella for joining us on this episode!

Ella is the host of an amazing new series from NOVA called Human

that tells the epic story of Homo sapiens

as they outlast other human species

and spread across the globe.

Head over to the link in the description to check it out,

and tell them Eons sent you!

Did our most famous fossil ancestor, Lucy, who did not have a chin,

die by falling out of a tall tree?

The answer is part of a decades-long debate over how, exactly,

our ancestors transitioned from life in the trees to life on the ground.

Find out more in our episode, “How Did Our Most Famous Ancestor Really Die?”

Chins up!

to this month’s Eontologists!

Addie, Annie & Eric Higgins, Carl Woelfel, Jackie Scott-Ralston, Jake Hart, John Davison Ng,

JuanM, Melanie Lam Carnevale, Nico Robin, Raphael Haase, Tony Dai, and STEVE.

Become an Eonite at patreon.com/eons

and you can get fun perks like access to the Patreon-only activity feed

and behind the scenes content!

And as always thank you for joining me in the Annie

and Eric Higgins Studio!

Subscribe at youtube.com/eons for more Pleistocene people.

So funny - now I'm, like, always, like - Not so much UFC, but boxing - like,

whoever has the bigger chin, that's who I'm gonna guess is gonna win.

Yeah, I wonder if I've, I've found a hack to boxing.

There's too many ways to get knocked out in UFC,

so I don't think the chin would help, but in boxing, though?

Boxing maybe.