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While our early human ancestors and other great apes were all tool users, it is only humans who eventually began walking upright and developed spoken language.

This evolutionary divergence has long fascinated . As director of the Comparative Cognition Group at Sussex, she investigates the behaviour and brain organisation of human and non-human great apes and how cognitive abilities evolve and develop over time and across species.

Although spoken language is often seen as a marker of human superiority, Professor Forrester emphasises that this ability is rooted more in physiology than intelligence.

“The way our skull sits on our spine gives us space at the back of the throat, allowing us to make open vowel sounds,” she explains. “Apes have different morphology and cannot produce those sounds in the same way.”

However, her research reveals that apes use their hands and bodies to communicate in ways that mirror the structural foundations of language.

“Whether you sign as a deaf speaker or speak verbally, language is a set of motor actions that must be sequenced correctly to make sense” she says. “Tool use also involves a hierarchical sequence of motor actions.”

Neural imaging supports this connection, showing that the same brain regions are activated during tool use, problem-solving, and both verbal and gestural communication.

“Archaeological evidence suggests that we didn’t start out speaking,” Professor Forrester notes. “We were gesturers - using our hands, bodies, rhythm, dance, postures, and signals to convey meaning. Over time, these may have evolved into more symbolic gestures.”

She believes that all our behaviours and brain functions have evolutionary precursors, developed gradually over millions of years. Through close observation of the behaviours of our primate cousins, we can begin to understand this evolution.

Since 2004 she has been studying families of western lowland gorillas living at in Kent, which is a breeding sanctuary for endangered species. Over the years some individuals have even been successfully released into a nature reserve in Gabon in 2015.

In addition to observing their communication interactions, she has begun to develop scientifically-based measures of great ape wellbeing. Gorillas are supersensitive, which means that when they are stressed their immune systems plummet. This makes them more susceptible to respiratory disease than humans, which can be fatal for them.

Her research often begins with studying humans. She then applies the same principles and techniques to great apes.

“We know how humans respond to stress – in particular, our heart rate and respiration increase” she says. “But we now also know that blood flow to our face changes when we are stressed.”

In an experiment involving apes and now humans, she and her team used thermal imaging equipment to show that, when under stress – such as carrying out mental arithmetic or public speaking – blood flow is increased to the eyes and decreases to the nose.

“The theory is that blood increase to the eyes is for vigilance, to keep you safe,” she says. “Vision is the primary sensory channel for primates,” she says. “Even if people say they feel calm, the blood flow changes show what they are really feeling.

“This is a window into your arousal system without you using language to complicate the matter. What’s lovely about it is that it translates between human and non-human primates.”

This technology is now being used to monitor stress in other primates, offering new ways to develop interventions based on human mental health therapies.

“Primate brain structures are similar to ours, and they experience psychological conditions, such as anxiety, depression, trauma, and obsessive behaviours. These are common across apes in captivity, but without language, detecting and measuring them is a challenge.”

Thanks to thermal imaging, researchers can now visibly measure stress in animals, making this a translational breakthrough - applying human-based technology across species.

“Thermal imagers were originally designed to detect heat leaks in buildings or for airport surveillance,” says Professor Forrester. “It took creative insight to see how they could be used to study blood flow under the skin. This is where science and creativity merge.”

As the academic lead for public engagement in the School of Psychology, Professor Forrester is passionate about sharing her work with wider audiences.

“I’ve taken part in ‘Soapbox Science’ events, given interviews on TV and radio, and hosted talks at festivals - including . That’s the fun part of my job, but it’s also fundamental for communicating research to as many people as possible.”