Sentient (REVIEW): Deciphering the wonders and science behind our senses.

Being sentient—is being aware of the world around us and conscious of our own place within it—is a marvel that science has yet to explain and that we take for granted because it seamlessly defines our every waking moment. 

The 5 senses we grew up being taught—sight, smell, hearing, touch, and taste—conceived by Aristotle have become a nearly universally accepted belief system.  And yet it is a myth. Many neuroscientists say up to thirty-three distinct senses are served by dedicated receptors in our bodies.

In SENTIENT: How Animals Illuminate the Wonder of Our Human Senses (Atria Books; Hardcover; On Sale: February 22, 2022; $28.00; ISBN 9781982156558), Jackie Higgins explores the breadth of our capabilities as sentient beings through the lens of the animal kingdom. Her book looks to animals to better understand the ways we sense and make sense of the world. Scientific research shows that there is more to unite, than divide us and that all creatures are built on the same foundations—a message of particular importance right now.  Through their eyes, ears, skin, tongues, noses and more, we can uncover what it means to be human and rediscover the extraordinary in the ordinary waking moment.

Jackie Higgins took a moment to discuss Sentient with SCINQ.

Jackie Higgins. (CREDIT: Atria Books)

Can we start with some background?

I come from a storytelling background. After getting a degree in zoology, I made wildlife documentaries and then science documentaries. I’ve made films that are broadcast in the States on channels like National Geographic and Discovery, as well as in the UK on the BBC for their series Horizon, many of which were co-productions with PBS. 

In your book, you use animal biology as a vehicle to contextualize and introduce similar aspects of human biology. Did it take you a long time to come up with the narrative structure of your book?

I use zoology to get a bit of distance or perspective on ourselves. A book that I loved when I was growing up was The Naked Ape by Desmond Morris. He looked at primates to better understand human behavior. I use a similar thesis in this book, but instead of looking just at primates, I cast the net far and wide to look at the whole animal kingdom. 

The subject of Sentient is how we sense the world. However, our senses circumscribe every waking moment; they inform every moment of every experience.  So by looking at, for example, our sense of color through the eyes of a mantis shrimp, one gets a distance and a different perspective on how we see color. With this remove, we can begin to understand ourselves more clearly.

I use a Leonardo da Vinci quote at the beginning of the book: “The average human looks without seeing, listens without hearing, touches without feeling, eats without tasting, and inhales without awareness of odor and fragrance.” Reframing our senses through those of other animals helps us to more fully appreciate them.

It seems like the scientific community took our senses for granted for a long time. They took for granted ideas that had been established in the past, which often was quite faulty. Why do you think it took so long to reexamine our senses?

I don’t think it took so long.  The idea that we only have five senses – sight, smell, hearing, touch and taste – was started by Aristotle over two millennia ago.  To his mind, pain was an emotion.  However, the Persian philosopher Avicenna, who lived some 1000 years ago, proposed that pain was a sense in its own right, a sense independent from touch.  Then in 1906, in The Integrative Action of the Nervous System, Charles Sherrington described senses beyond Aristotle’s classic five; he coined the word ‘nociception’ for the sense of pain, as well as ‘proprioception’ for the sense of body.  I write chapters on both in Sentient.

So scientists started questioning what Aristotle had said about the senses a very long time ago.  What’s interesting is that despite this, the myth that we have fives senses has persisted.  I think the answer to that is because the idea is just so appealing in its simplicity.

Your question also hints at the vast amount of scientific research going on at the moment, revealing how immeasurably more exciting our senses are than perhaps we have assumed. 

While researching and writing Sentient, one of my favorite senses became touch. At the very beginning, I hadn’t really thought much about it. If you ask people, what’s the sense you would least like to lose, most would say your eyesight; very few would say touch.  Yet current research is revealing so many surprising aspects to this sense, not least that it might encompass different senses.

In the end, I decided to split touch into two senses: how we feel the topography of the world – the lie of the land, the smoothness of a ball bearing or the corrugated roughness of a walnut – and how we feel when touched.

Only now is this second sense of emotional touch being unraveled and understood.  For example, scientist have relatively recently found a sensor in our skin that fires to a low pressure, moving slowly over our skin, at body temperature.  It has been called the ‘caress sensor’.

Godwit. (CREDIT: Caroline Church)

One of the things I find interesting is the way you compare animals and humans and how there are a lot of overlapping features shared by the two. Can you discuss that?

There is a huge amount of overlap or conservancy.  The fact that the taste buds on my tongue are the same as those on the body of the Catfish is proof that all of life on this planet is part of the same evolutionary family tree. We are all part of one immense and sprawling family. 

Conservancy is a recurring theme within the book.  Talking again about touch, for example, I use two animals to describe its two aspects. The Star-nosed Mole is an extraordinary creature that essentially finds its way through burrows with this little star on its nose.  The sensory Merkel cells in that star are much like those found in our fingertips: it feels the world in much the same way as us. 

The other chapter was told through another nose: this one belonging to the Vampire Bat. David Julius, who won the Nobel Prize in Physiology or Medicine this year, has done fascinating research into TRP proteins.  They allow us to gauge temperature as well as  spicy chilies, pain, and sunburn.  However, Julius also found them in nose of the Vampire Bat.  Here, they enable the animal to detect the temperature of blood vessels pulsing just beneath their victim’s tough hide. 

Moles and bats are mammals, like us.  The Catfish I referred to earlier is more distantly related.  So the conservancy in taste buds between it and us proves that, along with all other vertebrates, we inherited our taste system from a fish-like ancestor: a reminder than we originated in water many millions of years ago.

The first chapter is about an invertebrate: a mantis shrimp with an exceptional color sense. It has many more color sensors in its eyes than we have in ours. We have three different types of color light sensor, typically; it has 12. But what’s extraordinary is that both our and its color sensors are primed with different types of opsins. Again, you can trace back through this opsin family tree to the mother opsin: the one that gave rise to all others seen throughout the animal kingdom which scientists date back some 700 million years. That’s not long after the common ancestor of all animals took its final breath.

This deep conservancy found throughout the animal kingdom is a reminder of our deep connections to one another. All creatures on earth are distant cousins, admittedly some more distant than others.

Great Grey Owl. (CREDIT: Caroline Church)

Sentient is filled with all these fascinating anecdotes and experiments Are there any that stand out? 

When researching the Spookfish chapter, I met Ron Douglas, a professor who studies how fish see in the ocean depths using the same rod sensors that we use to see in the dark. He alerted me to another photo sensor in the eye beyond the two that we are taught about at school – a sensor that it neither a rod, nor a cone – and he introduced me to the man who discovered it. 

Russell Foster was the first to realize that there must be another light sensor in the eye.  It responds to the change of day to night and keeps our bodies on track with its rhythm.  It gives us a sense of, what’s called, circadian time. However, when he first suggested its existence, ophthalmologists and other vision scientists were so convinced he was wrong that they called him a fool and hounded him out of scientific conferences.  He would prove them wrong.  His extraordinary journey and his even more extraordinary discovery are told in my chapter on the Trashline Orbweaving Spider.

At the beginning of Sentient, you pose the age old question: if a tree falls in the forest and no one’s here to see it, does it really fall?

Yes, it falls, but does it make a sound? Or is the apple on its branch red? And the answer to both those questions is ‘no’. Even if the tree has fallen, color and sound do not exist without a spectator or a listener to see or to hear them. The great thing about perception is that it happens within us.  Color starts in the eye of the beholder, sound starts in our ear, and so on, but perhaps the most important sensory organ is our brain.

Duck-billed Platypus. (CREDIT: Caroline Church)

One of the most common arguments people have against scientific explanations of feelings, wants, and desires is that things like love are too complex to be attributable just to chemicals. But you hint at the fact that it just may be a chemical thing. It presents all sorts of philosophical problems. Did you understand at the beginning that that’s what you’re getting into?

For me, science takes on a new life when it brushes up against philosophy. I find the coalface between those two disciplines endlessly fascinating and knew that a book on perception would naturally mine these thought-provoking issues.

I should flag that I am a student of Richard Dawkins.  I believe that many of our behaviors, no matter how complex, will eventually find scientific explanation.  This includes the intricate web of human feeling and emotion, from grief to joy and even to falling in love. Some might call this approach reductionist, but I think this is a mean term for what is actually a rather majestic endeavor.  There is much awe and wonder to be found as we peel back our understanding of what it means to be human. 

In Sentient, I use a wild Silkmoth to explore our sense of desire. The male can smell a female’s airborne pheromone wafting on the air from five kilometers away. Mind-boggling research has shown that they can detect concentrations so dilute that they are in the parts per quadrillion, even outcompeting Bloodhounds that deal in parts per trillion.

Once a molecule of that pheromone has hit their antenna, it acts like a chemical command. And the male immediately changes direction and starts to zigzag upwind until it eventually finds the female. This is how the males find females and how baby moths come to be.  The question that concerns many people is whether we are as susceptible as male moths to such airborne intoxication.  Sentient investigates the controversies surrounding the putative human pheromones, while revealing the strange ways, proven by science, in which we rely on scent to find our partners in love.

What is the one thing you’d like your readers to take away from Sentient?

I want the book to reinvigorate people’s curiosity. I want people to be curious about themselves and the world around them. I believe that if we take that time to look, we find that the world is an infinitely intriguing place and that we are marvels. 

Oliver Sacks appears quite a few times throughout the book. I’ve long been a fan of his. I borrow a quote taken from his last op-ed, written when he knew he was dying: “I can’t pretend I’m without fear. But my predominant feeling is one of gratitude. Above all, I’ve been a sentient being, a thinking animal on this beautiful planet. And that, in itself, has been an enormous privilege and an adventure.” 

My hope is that people will come away from Sentient  not simply with more curiosity about their world about them and their place within it, but also with gratitude and with the realization that life is an enormous privilege and an adventure.

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