As Human Vision Improved, Sense Of Smell Decreased
January 15, 2018 at 4:25 p.m.
By Max [email protected]
Dogs have a scent sensitivity many thousands of times greater than humans. In fact, this ability is one of the more curious facts in nature.
Humans, however, and other primates have relatively good senses of smell. This is true even though human evolution has been characterized with a gradual increase in vision and a reduction in the ability to smell. The change is likely due to a progressive diminution of the nose over many eons as the eyes gradually moved to the middle of the face to enhance the depth of vision.
There is little doubt that over many years this facial change has resulted in a progressive reduction in the proportion of functional olfactory receptor genes. Mice, for example, have approximately 1,300 olfactory receptor genes, of which some 1,100 are functional, whereas, humans have only some 350 functional genes (smell receptors) of approximately 1,000. (Scientists still cannot explain how the 350 smell receptors are able to account for the human ability to detect thousands of different odors.) Humans, however, perform as well as, or better than other mammals.
Many humans actually make a living with their noses; think of oenologists (wine experts), perfumers and food tasters. The subject matter may appeal to those of us with a nose for news about the nose.
Smell is the most direct of all senses. Only substances volatile enough to spray microscopic particles into the air have an odor. (This ability allows us to enjoy the taste of food.) The nose, where the ability to smell begins, is composed of two nasal cavities separated by a middle wall, the nasal septum. Initial detection of odors takes place at the posterior of the nose, in the small region known as the olfactory epithelium. Odor molecules flow back into the nasal cavity behind the bridge of the nose, where they are absorbed by the mucosa-containing receptor cells bearing microscopic hairs (cilia).
Specialized proteins, known as receptor proteins, extend from the cilia. There are approximately 5 million of these cells, which then send impulses to the brain’s olfactory bulb or smell center via fibers known as axons. The olfactory epithelium also contains neuronal stem cells, which generate olfactory neurons though out the life span.
Each of these neurons expresses only one odorant receptor. The receptor is activated by a specific odor, which in turn activates a specific region in the brain. Unlike most neurons, which die and are never replaced, the olfactory sensory neurons are continually regenerated.
Sniffer Dogs
Scientists have known since the 1950s that dogs and other keen-scented mammals such as rats and rabbits have a specialized anatomical structure in their nasal cavities. Called the olfactory process, it is a large maze of highly convoluted airways that humans and other primates lack. In dogs, the recess lies right behind the eyes and takes up almost half of the interior of the nose.
Using a computer model of the canine nose, scientists have discovered that when a dog sniffs, each nostril pulls in a separate odor sample. The dog can tell which nostril is pulling in the scent so it knows which direction to go when tracking. The dog’s nose has a unique nasal airflow pattern, which helps transport odor molecules quickly via a single airway to the olfactory recess.
There are several differences between the human and dog olfactory systems, which explains the dissimilarity in olfactory acuity or the sensitivity of the sense of smell.
The first is the size of the nasal cavity and the amount of air that can be inhaled. German shepherds, for example, can breathe in five times more air than a human. Dogs have more olfactory sensory cells than humans; estimates of 5 million in a human compared to 125 million in a dachshund and 300 million in the bloodhound.
The dog’s olfactory bulb is about 40 times larger than in the human. These differences result in a human being able to smell a mixture of odors compared to the dog that can smell a range of distinct and different scents.
The dog also has something else a human doesn’t: the vomeronasal gland. The gland is actually a pair of long, fluid-filled sacs that open into either the mouth or the nasal cavity and located in the area above the roof the mouth. It appears to be an accessory olfactory organ that allows dogs to identify scent and possibly pheromones.
In a recent book about dogs written by Alexandra Horowitz, she summarized the difference between dogs and humans thusly: "What dogs see and know comes through the nose, and the information the dog has about the world based on smell is unthinkably rich. It is rich in a way we humans once knew about, once acted on, but have lost, and have since neglected."
Max Sherman is a medical writer and pharmacist retired from the medical device industry. He has taught college courses on regulatory and compliance issues at Ivy Tech, Grace College and Butler University. Sherman has an unquenchable thirst for knowledge on all levels. Eclectic Science, the title of his column, will touch on famed doctors and scientists, human senses, aging, various diseases, and little-known facts about many species, including their contributions to scientific research. He can be reached by email at [email protected].
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Dogs have a scent sensitivity many thousands of times greater than humans. In fact, this ability is one of the more curious facts in nature.
Humans, however, and other primates have relatively good senses of smell. This is true even though human evolution has been characterized with a gradual increase in vision and a reduction in the ability to smell. The change is likely due to a progressive diminution of the nose over many eons as the eyes gradually moved to the middle of the face to enhance the depth of vision.
There is little doubt that over many years this facial change has resulted in a progressive reduction in the proportion of functional olfactory receptor genes. Mice, for example, have approximately 1,300 olfactory receptor genes, of which some 1,100 are functional, whereas, humans have only some 350 functional genes (smell receptors) of approximately 1,000. (Scientists still cannot explain how the 350 smell receptors are able to account for the human ability to detect thousands of different odors.) Humans, however, perform as well as, or better than other mammals.
Many humans actually make a living with their noses; think of oenologists (wine experts), perfumers and food tasters. The subject matter may appeal to those of us with a nose for news about the nose.
Smell is the most direct of all senses. Only substances volatile enough to spray microscopic particles into the air have an odor. (This ability allows us to enjoy the taste of food.) The nose, where the ability to smell begins, is composed of two nasal cavities separated by a middle wall, the nasal septum. Initial detection of odors takes place at the posterior of the nose, in the small region known as the olfactory epithelium. Odor molecules flow back into the nasal cavity behind the bridge of the nose, where they are absorbed by the mucosa-containing receptor cells bearing microscopic hairs (cilia).
Specialized proteins, known as receptor proteins, extend from the cilia. There are approximately 5 million of these cells, which then send impulses to the brain’s olfactory bulb or smell center via fibers known as axons. The olfactory epithelium also contains neuronal stem cells, which generate olfactory neurons though out the life span.
Each of these neurons expresses only one odorant receptor. The receptor is activated by a specific odor, which in turn activates a specific region in the brain. Unlike most neurons, which die and are never replaced, the olfactory sensory neurons are continually regenerated.
Sniffer Dogs
Scientists have known since the 1950s that dogs and other keen-scented mammals such as rats and rabbits have a specialized anatomical structure in their nasal cavities. Called the olfactory process, it is a large maze of highly convoluted airways that humans and other primates lack. In dogs, the recess lies right behind the eyes and takes up almost half of the interior of the nose.
Using a computer model of the canine nose, scientists have discovered that when a dog sniffs, each nostril pulls in a separate odor sample. The dog can tell which nostril is pulling in the scent so it knows which direction to go when tracking. The dog’s nose has a unique nasal airflow pattern, which helps transport odor molecules quickly via a single airway to the olfactory recess.
There are several differences between the human and dog olfactory systems, which explains the dissimilarity in olfactory acuity or the sensitivity of the sense of smell.
The first is the size of the nasal cavity and the amount of air that can be inhaled. German shepherds, for example, can breathe in five times more air than a human. Dogs have more olfactory sensory cells than humans; estimates of 5 million in a human compared to 125 million in a dachshund and 300 million in the bloodhound.
The dog’s olfactory bulb is about 40 times larger than in the human. These differences result in a human being able to smell a mixture of odors compared to the dog that can smell a range of distinct and different scents.
The dog also has something else a human doesn’t: the vomeronasal gland. The gland is actually a pair of long, fluid-filled sacs that open into either the mouth or the nasal cavity and located in the area above the roof the mouth. It appears to be an accessory olfactory organ that allows dogs to identify scent and possibly pheromones.
In a recent book about dogs written by Alexandra Horowitz, she summarized the difference between dogs and humans thusly: "What dogs see and know comes through the nose, and the information the dog has about the world based on smell is unthinkably rich. It is rich in a way we humans once knew about, once acted on, but have lost, and have since neglected."
Max Sherman is a medical writer and pharmacist retired from the medical device industry. He has taught college courses on regulatory and compliance issues at Ivy Tech, Grace College and Butler University. Sherman has an unquenchable thirst for knowledge on all levels. Eclectic Science, the title of his column, will touch on famed doctors and scientists, human senses, aging, various diseases, and little-known facts about many species, including their contributions to scientific research. He can be reached by email at [email protected].
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