For centuries, people have asked  what makes us different from animals.

There have been a bunch of different  hypotheses for which human trait is the key …. tool use, self-awareness, culture, or  our awareness of death, to name a few. But one of the most accepted hypotheses is  that it’s our language that makes us unique.

Lots of species communicate. But as I’ll explain, that’s different. And for a long time, scientists  have wondered where our special,   complex language skills came from.

Recent research on one of our relatives  may shed some light on that question. Because there’s evidence that  orangutans may use language as well,   and it’s teaching us something about our own. [♪ INTRO] Now one thing that you will have picked up on is that orangutans…. Aren’t our closest relatives.

And it’s true that we’re more closely related to  chimps, bonobos, and gorillas than to orangutans. But regardless of how far away  they are on our family tree,   they’re chatty enough that  researchers took notice. First, a little backstory  on our giant orange cousins.  Orangutans are the largest  tree-dwelling mammals in the world,   and the only living great apes native to Asia.

The two most common species are the  Bornean and Sumatran orangutans,   which are found, unsurprisingly, on  the islands of Borneo and Sumatra. It’s sort of ironic to be talking about  orangutans in the context of language,   because they aren’t particularly social animals. Adult males pretty much want to be left alone   except for interactions with  the females they mate with.

Meanwhile, adult females live  with their kids until they’re   able to care for themselves,  which can take nearly a decade. But once their baby is weaned,  off they go into the jungle,   and mom will live mostly by herself  until the next baby comes along. But while orangutans aren’t  known for throwing parties,   they are known for their sweet karaoke skills.

Adult males have a throat pouch that allows  them to make “the long call”, roars that last   for a minute or more and can be heard up to 2 kilometers away and attract females to mate with. Of course, mating calls aren’t unique  to orangutans, or even to primates. Neither are other kinds of vocalizations, like  specific sounds that mean very specific things.

For example, mongooses have unique alarm calls  to warn about different types of predators. When other mongooses hear the calls,   they respond in ways that are  specific to that particular sound. So a call warning of a bird might cause  them to go to the ground and take cover,   while a call about a terrestrial  predator might result in mongooses   going into sentry mode and scanning  the environment for the threat.

And baboons recognize threat grunts  versus reconciliatory grunts,   and respond differently depending on whether  they’re coming from family or strangers. But there’s a difference between  communication and true language. True language, the kind that makes humans  unique, requires something called recursion.

Recursion is the ability to embed  phrases or clauses or sentences   within each other to create new meanings. It allows us to express a  virtually infinite number of ideas. Here’s an example.

Let’s look at the sentence “I wore the dress that  my mother sewed with fabric that my father wove”. “I wore a dress” is a sentence. “My mother sewed” is a sentence. “My father wove” is a sentence. All of those have meanings. But when you put them together,   “I wore a dress that my mother sewed” means  something different than “I wore a dress”. “My mother sewed with fabric that my father wove”.

Is another complete and different idea,  and stacking all of that together gets you a   completely new meaning than any of the sentences  individually or in any other combination. That ability to stack sentences is recursion,  which we need in order to express complex ideas. And we can see the difference when we  compare this to those mongoose calls.

If they’ve got one sound that means  “bird” and one call that means “urgent”,   they could express bird, urgent, and bird urgent. But that’s pretty much the end of it. So while there are lots of animals that  can help us learn about the evolution of   communication and vocalization, it’s  a lot trickier to find relatives that   can help us figure out where  that recursion part came from.

At least until recently. But before we get to that, all  research needs funding, even ours. So here’s a quick ad break!

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In 2024, a group of researchers from Europe   recorded 66 long calls from  10 male orangutans in Borneo. When they analyzed the rhythms of these calls,  they were able to pick out 5 different elements. One of them was full pulses, and the other four  were subpulses that broke up the full pulses.

The full pulses themselves were regularly paced  across the entire length of the long call. But when they looked at how  those full pulses broke down,   they found that three of the subpulses  occurred at regular intervals within them. So multiple subpulses occurred in a  rhythmic pattern of the full pulses   which occurred in a rhythmic  pattern of the long calls.

That sounds like recursion to me! And in 2025, the same research group looked  at what female orangutans in Sumatra have to   say when they see tigers, which  are their natural predators. Or at least, when they think they see them.

Because this time, the researchers weren’t  just recording calls, they were eliciting them. By having researchers walk around on all  fours draped in sheets that were either   tiger-patterned, white, or had patterns  of other non-native predators on them. You know.

Normal research stuff. “Hey honey, what did you do at work today?” “Well, I dressed up like a tiger  to scare orangutans into screaming. What'd you do?” “I did some spreadsheets…” They recorded everything the  orangutans had to say about this,   and broke them down into three different levels: combinations, which were sets of  sounds less than 0.2 seconds apart,   bouts, which were sets of combinations  less than 2 seconds apart, and series,   which were multiple bouts  between 2 and 20 seconds apart. When they looked at the rhythms of  the alarm calls from the orangutans,   they found that combinations, bouts, and  series happened at regular intervals.

So a rhythmic pattern of combinations  happened in a rhythmic pattern of bouts,   which happened in a rhythmic pattern of series. But there’s even more nuance  in the patterns that suggests   the orangutans are saying a  lot with just a few sounds. The tempo of a series of bouts was a lot  slower when the threat was something that   didn’t look like a scary tiger and looked more  like a researcher wearing a polka-dot blanket.

And while the series tempo would change,  the other parts of the call were all   the same tempo whether the orangutans  bought the researcher’s costumes or not. That suggests that the general alarm  call announces something is there,   the tempo of the bouts seems to indicate  urgency or importance, and the rhythm of   those super short combinations seems to  contain information about the specific threat. Now, this doesn’t mean that orangutans  are speaking in complex sentences,   and they probably can’t use recursive grammar  to express an infinite range of ideas.

But it does provide evidence that complex  recursive grammar could have evolved   slowly from more basic forms of recursion that  appeared before we split off from other apes. So what started off as rhythms embedded in sounds  may have evolved into sounds embedded in words,   which turned into words embedded in phrases,  and then finally phrases into complex sentences. And it’s our fancy, complex sentences that   allow us to bring you videos about the  evolution of fancy, complex sentences.

How’s that for recursion? [♪ OUTRO]