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IT LIES at the centre of human experience, and yet how our incredible capacity for complex language arose is a mystery. We are still far from understanding why we are the only living ape with such a skill.

Answering these questions is difficult, not least because speech doesn鈥檛 leave its trace in the fossil record. However, we can look to our ape relatives for clues, as cognitive scientist Gillian Forrester at Birkbeck, University of London, is doing. She has developed puzzle mazes for chimpanzees, gorillas, orangutans and children that shed light on one idea of how language evolved. She tells New Scientist how her findings are challenging our understanding of the brain and painting a clearer picture of how language began.

Alison George: What inspired you to study the evolution of language?

Gillian Forrester: I鈥檝e always been intrigued by the efforts to teach chimpanzees to speak, which were going on while I was growing up in the 1980s. They were a massive failure when it came to chimps learning to combine words into more complex phrases.

This got me intrigued about the common factors between human language and other animal communication systems, and how and why a language system emerged in humans but not for other great apes.

How do we start to answer that question?

We don鈥檛 have our ancient ancestors to look at to see how things changed over evolutionary time because they are all extinct, and cognition doesn鈥檛 fossilise. So all we can do is make suppositions based on their artefacts, such as tools and things they were buried with, to give us an indication of their communication skills.

How can studying the anatomy of great apes help?

We are great apes too. We are tool users and so are they. And we have a particular kind of brain anatomy that we share with other great apes. There is an area of the brain called Broca鈥檚 area, which supports speech production. For a long time, we thought this was unique to humans, but we now know that other apes also have it.

So there鈥檚 a question about precursor behaviour: what were we using this bit of the brain for, initially, that we now use for language? If we understand that, then we might have some clues about what kinds of behaviours supported language emergence over evolutionary time.

What do we know about this brain region?

One of the things that we鈥檝e found in humans is that Broca鈥檚 area, which is active for speech production, is also active during structured motor actions, particularly those involved in tool use and tool making 鈥� things that have to be done in a sequence to reach an end goal.

Think, for example, about making a sandwich. There鈥檚 a hierarchical structure to the task, an ordered sequence of events that have to happen. This 鈥渟yntax鈥� is exactly the same sort of structured sequence that we find in language. To get a sentence out, you have to put the words in the right order. But there is also physical syntax in the way we solve problems with our hands: you have to make the sandwich in the right order to ensure the bread is on the outside.

So there is a link between our hands and our mouths?

Yes, tool making and solving problems with our hands and speech share very similar brain processing. There are highly overlapping areas that generate those behaviours. Neuroscience now highly supports that. And this exists for a reason, because we think that bit of the brain that we use for language didn鈥檛 emerge for language specifically 鈥� it originally emerged for motor action sequences.

This is the idea behind the tool use theory for the evolution of language that was proposed in the 1990s. Maybe tool use was the precursor behaviour that set the brain up for language.

How did language evolve from tool use?

The idea is that our hands were initially our communication system. There is quite a lot of evidence to suggest that we originally 鈥渢alked鈥� not with our mouths, but with our hands. Then, when ancient humans became bipedal about 4 million years ago, our hands were freed and we started doing more sophisticated tool making and tool using.

But if you were using tools and wanted to share that information with other people, there was competition between these two activities, and this created a pressure for our communication to move to another modality: speech. But we don鈥檛 have much hard evidence for this.

How can we test this idea?

We are trying to tease this apart using maze puzzles. These represent components of human language 鈥� a physical syntax. The simplest one is called the flat maze, where you just push a nut through holes. This is like a concrete noun: you always have your finger on the nut, so you know where you are. Next we have a puzzle with a single cog, so you can鈥檛 touch the nut directly but can spin the cog 鈥� and that鈥檚 kind of like an action word, like our verbs.

Then we have boxes that have double cogs, where you can use the bottom cog to spin the top cog to move the nut. This is one layer of abstraction, like how adjectives act upon nouns, for example. They鈥檙e a modifier, so you have to understand the relationship, the mechanisms to move the nut and achieve the goal. Then we鈥檝e got more complex boxes, that are comprised of the components from the smaller boxes, which are like sentences. If you don鈥檛 follow the rules for them, the nut will get stuck in a trap and you won鈥檛 get it as a reward.

So these puzzle boxes represent the basic components of language in physical form?

Yes. We wanted to test if great apes could solve the more complex puzzles, hence the more complex syntax. This would give an indication of whether we inherited this physical syntactical capability.

We鈥檝e worked with gorillas, chimpanzees and orangutans in captivity. We didn鈥檛 know they would be able to solve physical syntax at all, and several people who work with apes told us there鈥檚 no way that the apes were going to be interested or able to solve them. That was absolutely incorrect.

Are you testing it on humans too?

Yes, the second part of the study is to test the puzzles on children between the ages of 2 and 5, who range from single-word users to those who can speak in full sentences. The hypothesis is that the level that they can solve these puzzles 鈥� whether it鈥檚 nouns, phrases or sentences 鈥� should be linked with their current language ability.

What have you found?

All four great ape species (gorilla, orangutan, chimpanzee and children) had at least some individuals who could solve all of the boxes. The fact that some apes can solve the most complex puzzles suggests that we inherited the syntax-processing capability from the last common ancestor we shared with chimpanzees. In children, preliminary analysis suggests a close relationship between the ability to solve the different levels of complexity of the puzzle boxes and language ability, suggesting that the boxes do work as a physical proxy for syntax structure. Together, these findings suggest that our language syntax ability didn鈥檛 emerge for language specifically, and it didn鈥檛 arise within the human lineage after we split from the last common ancestor.

Do you think these boxes could also be used to understand language development in children?

This investigation provides tangible evidence that these specially designed puzzle boxes reflect a physical form of syntax that is similar to the internal syntax that underpins language.

This is important because it could lead to new ways to study child development. Fine motor movements are just so tangled up with our language processing. There is a strong association between a child鈥檚 manual motor ability and their subsequent speech, so, yes, there鈥檚 potential to diagnose conditions earlier.

We are speaking over Zoom, and I can see a beautiful painting of a gorilla on your wall.

That鈥檚 Tibs, one of the gorillas I study. She鈥檚 the cleverest gorilla. She solved the puzzle boxes first. She even managed to make a tool to extract all of the nuts that were hidden in the trap.