You share a trait with every single human who's聽 ever lived – but no other animal on Earth has it.

It's not your big brain, or your聽 opposable thumbs... it's actually聽聽 this little shelf on your face聽 that we call a chin.

Yes, really.

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

Ever since Homo sapiens was named in the 1700s,聽聽 the question of how to define our聽 species has been, well…a question.

See, when Carl Linnaeus added humans聽 to his ‘System of Nature,’ he didn’t   include a description of distinct聽 traits, like he did for other species.

Next to Homo sapiens, he simply wrote聽 a Latin phrase that translates to 'know聽聽 thyself.’ And it turns out, that’s…a  lot harder than you might think.

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

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

Humans as 'the' hunters, 'the' tool makers,聽聽 'the' large-brained species, for聽 example, have all been debunked.

But what hasn’t been debunked?

Chins.

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

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

Well, maybe not.

It could just聽 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聽 Ella Al-Shamahi, host of the new NOVA聽聽 and BBC series ‘Human’ for a closer look.

Around the time that Linnaeus named Homo sapiens,聽聽 scientists had already noticed that all of聽 our skulls have a bony, projecting chin.

Paleoanthropologists call this the 'mental聽 eminence' or ‘protuberance’ and it has a   specific anatomical configuration that's聽 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聽 the middle of the jaw, meeting a horizontal聽聽 section that juts out along the lower edge,聽 that’s framed by depressions on either side.

It's kind of funny that we need to聽 give a technical definition for chins,聽聽 like...we all know a chin when we see one, right?

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

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

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

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

Theirs—like most other   animals—basically just slopes backwards  in what's called a 'receding symphysis.’ When scientists began uncovering fossil human聽 relatives, the pattern became clear.

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

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

And as additional hominin discoveries聽 – so those fossils belonging to species   more closely related to us than聽 chimps – began to trickle in,   they only added more support to the growing聽 consensus that we alone share this feature… Like our 3 to 4 million-year-old ancestor,聽聽 Australopithecus afarensis聽 – they lack a projection.

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

So it became increasingly clear that聽 this might be a sort of signature of聽聽 our species.

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

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

I’ll let Kallie take it from here.

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

Some scientists have suggested they evolved as聽 a clamping device to hold cloth while working,聽聽 while others have maintained that they are merely聽 decorative features shaped by sexual selection.

But the most outlandish?

That聽 chins evolved to take a punch.

Researchers used data from actual experiments聽 that had Olympic boxers punch a dummy to better聽聽 understand head injuries to explore whether聽 this bony reinforcement might help in fights.

And they argued that facial聽 features like robust bones聽聽 and muscles strategically placed around聽 the jaw could better withstand impacts… But ideas like the punching hypothesis聽 often amount to little more than聽聽 'just so' stories—seemingly logical  explanations that crumble under scrutiny.

The more credible ideas that have emerged聽 over the years are centered around other聽聽 unique features that we often think of as聽 important for humans to be, well, human.

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

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

Since human speech requires many different聽 tongue movements and precise muscle coordination,聽聽 he reasoned that the chin might anchor聽 those muscles, aiding speech production.

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

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

So most researchers didn’t buy it.

Meanwhile, more hominin fossils were being聽 discovered – like Peking Man in China,   now classified as Homo erectus.

These,聽聽 too, lacked chins, reinforcing that this聽 feature belonged uniquely to Homo sapiens… Which meant that it could potentially be聽 used as a diagnostic feature – helping   us recognize our own species from our聽 close relatives in the fossil record.

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

In the 1950s, scientists turned to聽 biomechanics—studying how muscles,   tendons, and ligaments work聽 together in the lower jaw.

And they came up with a new idea:聽 the ‘airway impingement hypothesis,’   which basically says that chins聽 arose in response to breathing.

Specifically, that it was a response to聽聽 constraints in the respiratory system聽 resulting from our upright posture.

But not everyone was convinced.

Other聽 researchers stuck with the biomechanical聽聽 approach, but focused on the other聽 main thing our jaw does: chewing.

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

See, our faces and jaws have shrunk聽 significantly over our evolutionary history,聽聽 retracting in response to the unique聽 shape of our brain case – a shift   that has fundamentally reshaped our聽 facial architecture in the process.

And this smaller face and jaw raised the question聽 of: what new stresses might this have created?

Since bone remodels in response to stress聽 – changing density and shape based on the   loads placed on it – maybe the chin is  simply a response to chewing pressures.

Maybe, these researchers聽 argued, it acted as a buttress,聽聽 strengthening our smaller jaws for聽 the demands of processing food.

For a while, this seemed like the likely聽 answer.

But no one had definitively proven it.

And around this time, in 1967, scientists聽 uncovered the fossil called Omo 1 from聽聽 Ethiopia.

Considered to be one of聽 the oldest Homo sapiens ever found,聽聽 at almost 230,000 years old,聽 Omo 1 is pretty fragmentary.

But it does preserve the lower jaw, which聽 shows that this human had a prominent chin.

This underlined the fact that聽 the human chin is pretty old,聽聽 and added confirmation that it’s  likely been around since the聽聽 beginning of our species – which evolved  potentially about 300,000 years ago.

After almost a century of searching for a purpose,聽 some scientists began asking a radical question:聽聽 What if the chin doesn’t have a function  at all?

What if it's just... there?

As weird as that sounds, evolutionary biologists聽聽 have a term for these nonadaptive聽 traits – they call them 'spandrels.'

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

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

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

So, the science聽 of the chin stayed in limbo.

It wasn’t until the early 1990s that  researchers moved away from theoretical聽聽 approaches and really started putting聽 the chewing hypothesis to the test.

One scientist compared the geometry聽 of the human jaw to other primates聽聽 and found that the proportions of our jaws聽 are unique in how short and broad they are.

He then built upon experimental work that聽 had examined the precise stress patterns that聽聽 different primate jaws experience when eating聽 food and applied that work to this human shape.

This showed that the short, broad聽 shape of Homo sapiens’ jaw does,   in fact, create a different set of stresses.

And the structure of our bony聽 chin seems to exactly match the聽聽 needs raised by this distinctive shape聽 and the stresses it causes, he argued.

For example, the lower jaw聽 experiences stresses caused聽聽 by twisting of its left and right聽 halves, called vertical bending.

And the outer layer of bone is exceptionally聽 thick right where vertical bending creates聽聽 the most stress – down at the base of the  symphysis, where the two halves come together.

聽 Comparing this to other hominins, the聽 researcher pointed out that the timing of聽聽 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聽聽 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聽 truly was an adaptive response,聽聽 one that was responding to changed聽 mechanical demands on the jaw.

And while this study has won over some scientists,聽聽 others are more skeptical and continue to聽 maintain that the chin is just...there.

In large part because more recent biomechanical聽 modeling reveals that jaws with chins actually聽聽 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聽 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聽 fossils – both others without chins   and Homo sapiens with chins – are  adding data points to the timeline,聽聽 they still aren’t telling us  why this characteristic exists.

But, what we can say is that, even if the chin聽 doesn’t actually do anything evolutionarily,   it does allow us to identify ourselves in聽 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聽 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.