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The Fox, the Shrew, and You: How Brains Evolved
A leading neuroscientist describes the long evolutionary process that led to the human brain
Our human brain is both unique and similar to that of other species. The only way we can trace its evolution is by comparing it to the brains of animals alive today. In this book, leading neuroscientist Rogier Mars offers an engaging account of the evolution of the brain by exploring the brains and cognitive capacities of animals from the humble sea squirt to the socially minded fox and the tiny shrew.
By examining the challenges that different animals and their ancestors faced, Mars shows that we can understand what drove the evolution of their brains. Early vertebrates became predators of the sea; mammals evolved a complex neocortex to deal with foraging for high-energy food; and social primates adapted to contend with a fast-changing environment in which groups of individuals team up to get food. Over the course of a long evolutionary road, the ancestors of present-day animals and their descendants continually adapted to challenges, modifying their brains again and again. For us humans, this process gradually led to a brain that is capable of so much, from inventing language to traveling into space.
Mars leads readers across eras and species, showing us how we resemble our animal cousins, how we differ from them, and how animals in one branch of the evolutionary tree did the hard evolutionary work of becoming human.
Our human brain is both unique and similar to that of other species. The only way we can trace its evolution is by comparing it to the brains of animals alive today. In this book, leading neuroscientist Rogier Mars offers an engaging account of the evolution of the brain by exploring the brains and cognitive capacities of animals from the humble sea squirt to the socially minded fox and the tiny shrew.
By examining the challenges that different animals and their ancestors faced, Mars shows that we can understand what drove the evolution of their brains. Early vertebrates became predators of the sea; mammals evolved a complex neocortex to deal with foraging for high-energy food; and social primates adapted to contend with a fast-changing environment in which groups of individuals team up to get food. Over the course of a long evolutionary road, the ancestors of present-day animals and their descendants continually adapted to challenges, modifying their brains again and again. For us humans, this process gradually led to a brain that is capable of so much, from inventing language to traveling into space.
Mars leads readers across eras and species, showing us how we resemble our animal cousins, how we differ from them, and how animals in one branch of the evolutionary tree did the hard evolutionary work of becoming human.
- GenresNonfictionScience
248 pages, Hardcover
Published March 10, 2026
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Displaying 1 - 5 of 5 reviews
July 18, 2026
A slim book that explores how brains have evolved over more than 500 million years. Ultimately, it explores different brains on the quest to show that the human brain is not a unique anomaly but the result of countless small adaptations shared with other animals. Rogier B. Mars draws on comparative neuroscience to explain how studying species such as fish, shrews, foxes, and primates helps us understand the origins of memory, decision-making, cooperation, and intelligence.
Brain evolved in response to the practical challenges of survival, as a foraging device to find food, avoid predators, navigate environments, and live alongside others. Rather than placing humans above the rest of the animal kingdom, Mars highlights the continuity between species and shows how many of our most distinctive cognitive abilities have deep evolutionary roots.
I enjoyed this insightful introduction to brain evolution that combines neuroscience, evolutionary biology, and animal behaviour to reveal what makes us human.
Cool Facts:
“Not all animals have neurons. Neurons probably evolved at least twice, once in the lineage leading to comb jellies and once in the lineage leading up to Cnidaria (anemones and jellyfish) and Bilateria (which include all vertebrates). Exactly when and why they appeared is hard to determine. The most likely scenario is that early animals benefited from electrically connected cells that could communicate the presence of a sensory stimu-Tus outside the body to other cells. The first neurons, including chemical transmission at synapses, are thought to have appeared at a similar time as simple muscles did, helping to create a stimulus-detection-to-movement-execution pathway in the body. When animals started to engage in proper predator-prey arms races, muscles that allowed faster contractions appeared.
Neurons guided the coordinated contraction and relaxation of these muscles. Once synaptic transmission was established, neurons could start forming connections with one another, allowing much more complex information processing.”
“Brains seem to have evolved independently in arthropods (insects, spiders), mollusks (ranging from oysters and slugs to octopuses), worms, and chordates (the group including the sea squirt and vertebrates such as the lamprey. How brains evolved is different in each of those different groups; evelution does not always follow exactly the same path.”
“In the reptile brain, toward the top and the middle part of the brain is a large space normally filled with cerebrospinal fluid that provides mechanical and immunological protection for the brain. Even more to the middle is a layer of brain cells that is termed, because of its position, the medial cortex. If we look at how this part develops and which genes are expressed here, we can find a very similar structure in the mammalian pallium, but there it is called the hippocampus. The mammalian hippocampus is a structure consisting of three layers of neurons that are very important in spatial navigation. For a foraging land animal, understanding your environment is important, and the hippocampus is able to link places together into a mental map.
In fact, this function is so useful that it might have formed the basis for a whole range of memory functions. We will explore that story in chapter”
“While the reptilian dorsal cortex is just a thin layer, the mammalian neocortex is organized into a column of neurons across six different layers, which allows for much more complex processing of information between the moments when information comes in and when it goes out.”
“Thus, seeing seems to mean different things at different times. Different parts of the brain use visual information to detect predaters and to avoid obstacles. When the predator detection pathway is damaged, the animal can still avoid obstacles.”
“Based on these types of results, Milner and Goodale concluded that the temporal cortex might be essential for perception of the outside world, for instance, when we have to make judgments about aspects such as color, form, or texture of an object, while the parietal cortex is important for processing visual information needed to guide one's motoric actions. In other words, the parietal pathway is the most important for the type of behavior we identify as typical of primates: visually guided actions.”
“Within the parietal pathway, specific parts of the parietal cortex interact with different parts of the motor cortex to achieve different types of movements, all of which helped our early primate ancestor navigate life in the small branches of trees: leaping from branch to branch, defending the body from harm, reaching and grasping for fruit, and peeling the fruit to get to the juicy insides.”
“Both these simian primates are on a plant-based diet, eating fruits and leaves. Of these two sources of food, fruits are far more nutritious than leaves; however, fruit is not always easily available and the monkeys had to find ways to deal with this eventuality…You can also adjust your digestive tract. This is what
Cool
the howler monkeys did. Their colon is much longer than that of the spider monkey, allowing them to ferment plant fibers for a long time in their digestive system to extract more nutrition.
The spider monkeys followed a different strategy. Instead of growing their digestive tract, they grew their brain. They became more clever in searching out fruit, even at times when it is only scarcely available. Their short digestive tract allows them to digest large amounts of fruit every day, even unripe, low-
- quality fruit if that is all that is available, to extract all the nutrition they need.”
“This type of reasoning, dissociating the knowledge of another from your own, is considered a hallmark of human social information processing. It thus seems to rely on a region in the human brain very similar to the one that other primates use to process information about others' location of attention. Other primates might not quite process the information to the level we do, but the overall brain infrastructure seems very similar.”
“Psychologists call this ability “theory of mind," our ability to have an apprectation of the content of the mind of others—what they know, feel, want, and believe. the content of the mind of others—what they know, feel, want, and believe.”
“But what we do understand now, is that language is of no use to you unless you are already a collaborative species. Language must have begun as a tool for coordination and cooperation…Only a commitment to a communal existence makes it worthwhile to invest in language. With that language, coordination of tasks, collective problem solving and passing knowledge across generations become much easier.”
“Across different primate brains, the larger the brain gets, the more of the brain is occupied by the association cortex. The human brain, in other words, devotes much more space to deeper processing of information.”
“The human arcuate fascicle extends much further into the temporal lobe than that of the macaque monkey or even the chim-panzee. Apparently, our frontal and temporal cortexes have a lot more to say to one another than those of other species. This finding of the expanded arcuate has become an almost iconic example of a major difference between the human and nonhuman primate brain. It has stood the test of time.”
“As the human brain is a primate brain, it is easiest to describe in terms of the common primate we already discussed: a visually oriented brain with different pathways processing different aspects of the world, including a parietal pathway for visually mediating actions originally due to the original arboreal niche, a temporal pathway setting the foraging context, and a frontal cortex expediting information processing to help an animal achieve its goals. These trends all continue in the human brain. As we saw above, the human frontal cortex expanded and possibly contains new regions. These regions further expand the capacity of the frontal cortex to further process informa-tion, re-representing it in more abstract ways and combining more information. The parietal pathway also expanded, as we saw in chapter 3. The system originally used to compute how we transform information from the 2-D representation of the environment on our eyes retina to the guidance of the various muscles needed to effect change in the environment got coS gopted for a much wider range of tasks. Representation and ma-. nipulation of information in different coordinate frames is very. important for what we tend to call "general intelligence" The expansion of the parietal pathway and a dramatic increase in the information exchange between parietal and frontal cortexes facilitated this behavior.”
“Let's look at some of the regions involved in culturally transmitted behaviors a bit more closely. Reading activates a group of regions in the left hemisphere of the brain. A number of these regions are important for language in general, a skill that did evolve over a long enough time to have dedicated regions evolve, but one temporal visual stream region seems particularly active in response to the written word. It responds to letters, but not to equally simple shapes or more complex shapes such as houses or faces. Dehaene and his colleagues called it the visual word form area. The activation of the region is related to your exper: tise in reading. It is more active in response to familiar than less familiar script, It is also more active to familiar letter combina tions than to unfamiliar combinations. When human volunteers were asked to read letter strings that differed in how well thes obeyed the rules of their language, the more the string followed the rules, the more activation the region showed.”
Brain evolved in response to the practical challenges of survival, as a foraging device to find food, avoid predators, navigate environments, and live alongside others. Rather than placing humans above the rest of the animal kingdom, Mars highlights the continuity between species and shows how many of our most distinctive cognitive abilities have deep evolutionary roots.
I enjoyed this insightful introduction to brain evolution that combines neuroscience, evolutionary biology, and animal behaviour to reveal what makes us human.
Cool Facts:
“Not all animals have neurons. Neurons probably evolved at least twice, once in the lineage leading to comb jellies and once in the lineage leading up to Cnidaria (anemones and jellyfish) and Bilateria (which include all vertebrates). Exactly when and why they appeared is hard to determine. The most likely scenario is that early animals benefited from electrically connected cells that could communicate the presence of a sensory stimu-Tus outside the body to other cells. The first neurons, including chemical transmission at synapses, are thought to have appeared at a similar time as simple muscles did, helping to create a stimulus-detection-to-movement-execution pathway in the body. When animals started to engage in proper predator-prey arms races, muscles that allowed faster contractions appeared.
Neurons guided the coordinated contraction and relaxation of these muscles. Once synaptic transmission was established, neurons could start forming connections with one another, allowing much more complex information processing.”
“Brains seem to have evolved independently in arthropods (insects, spiders), mollusks (ranging from oysters and slugs to octopuses), worms, and chordates (the group including the sea squirt and vertebrates such as the lamprey. How brains evolved is different in each of those different groups; evelution does not always follow exactly the same path.”
“In the reptile brain, toward the top and the middle part of the brain is a large space normally filled with cerebrospinal fluid that provides mechanical and immunological protection for the brain. Even more to the middle is a layer of brain cells that is termed, because of its position, the medial cortex. If we look at how this part develops and which genes are expressed here, we can find a very similar structure in the mammalian pallium, but there it is called the hippocampus. The mammalian hippocampus is a structure consisting of three layers of neurons that are very important in spatial navigation. For a foraging land animal, understanding your environment is important, and the hippocampus is able to link places together into a mental map.
In fact, this function is so useful that it might have formed the basis for a whole range of memory functions. We will explore that story in chapter”
“While the reptilian dorsal cortex is just a thin layer, the mammalian neocortex is organized into a column of neurons across six different layers, which allows for much more complex processing of information between the moments when information comes in and when it goes out.”
“Thus, seeing seems to mean different things at different times. Different parts of the brain use visual information to detect predaters and to avoid obstacles. When the predator detection pathway is damaged, the animal can still avoid obstacles.”
“Based on these types of results, Milner and Goodale concluded that the temporal cortex might be essential for perception of the outside world, for instance, when we have to make judgments about aspects such as color, form, or texture of an object, while the parietal cortex is important for processing visual information needed to guide one's motoric actions. In other words, the parietal pathway is the most important for the type of behavior we identify as typical of primates: visually guided actions.”
“Within the parietal pathway, specific parts of the parietal cortex interact with different parts of the motor cortex to achieve different types of movements, all of which helped our early primate ancestor navigate life in the small branches of trees: leaping from branch to branch, defending the body from harm, reaching and grasping for fruit, and peeling the fruit to get to the juicy insides.”
“Both these simian primates are on a plant-based diet, eating fruits and leaves. Of these two sources of food, fruits are far more nutritious than leaves; however, fruit is not always easily available and the monkeys had to find ways to deal with this eventuality…You can also adjust your digestive tract. This is what
Cool
the howler monkeys did. Their colon is much longer than that of the spider monkey, allowing them to ferment plant fibers for a long time in their digestive system to extract more nutrition.
The spider monkeys followed a different strategy. Instead of growing their digestive tract, they grew their brain. They became more clever in searching out fruit, even at times when it is only scarcely available. Their short digestive tract allows them to digest large amounts of fruit every day, even unripe, low-
- quality fruit if that is all that is available, to extract all the nutrition they need.”
“This type of reasoning, dissociating the knowledge of another from your own, is considered a hallmark of human social information processing. It thus seems to rely on a region in the human brain very similar to the one that other primates use to process information about others' location of attention. Other primates might not quite process the information to the level we do, but the overall brain infrastructure seems very similar.”
“Psychologists call this ability “theory of mind," our ability to have an apprectation of the content of the mind of others—what they know, feel, want, and believe. the content of the mind of others—what they know, feel, want, and believe.”
“But what we do understand now, is that language is of no use to you unless you are already a collaborative species. Language must have begun as a tool for coordination and cooperation…Only a commitment to a communal existence makes it worthwhile to invest in language. With that language, coordination of tasks, collective problem solving and passing knowledge across generations become much easier.”
“Across different primate brains, the larger the brain gets, the more of the brain is occupied by the association cortex. The human brain, in other words, devotes much more space to deeper processing of information.”
“The human arcuate fascicle extends much further into the temporal lobe than that of the macaque monkey or even the chim-panzee. Apparently, our frontal and temporal cortexes have a lot more to say to one another than those of other species. This finding of the expanded arcuate has become an almost iconic example of a major difference between the human and nonhuman primate brain. It has stood the test of time.”
“As the human brain is a primate brain, it is easiest to describe in terms of the common primate we already discussed: a visually oriented brain with different pathways processing different aspects of the world, including a parietal pathway for visually mediating actions originally due to the original arboreal niche, a temporal pathway setting the foraging context, and a frontal cortex expediting information processing to help an animal achieve its goals. These trends all continue in the human brain. As we saw above, the human frontal cortex expanded and possibly contains new regions. These regions further expand the capacity of the frontal cortex to further process informa-tion, re-representing it in more abstract ways and combining more information. The parietal pathway also expanded, as we saw in chapter 3. The system originally used to compute how we transform information from the 2-D representation of the environment on our eyes retina to the guidance of the various muscles needed to effect change in the environment got coS gopted for a much wider range of tasks. Representation and ma-. nipulation of information in different coordinate frames is very. important for what we tend to call "general intelligence" The expansion of the parietal pathway and a dramatic increase in the information exchange between parietal and frontal cortexes facilitated this behavior.”
“Let's look at some of the regions involved in culturally transmitted behaviors a bit more closely. Reading activates a group of regions in the left hemisphere of the brain. A number of these regions are important for language in general, a skill that did evolve over a long enough time to have dedicated regions evolve, but one temporal visual stream region seems particularly active in response to the written word. It responds to letters, but not to equally simple shapes or more complex shapes such as houses or faces. Dehaene and his colleagues called it the visual word form area. The activation of the region is related to your exper: tise in reading. It is more active in response to familiar than less familiar script, It is also more active to familiar letter combina tions than to unfamiliar combinations. When human volunteers were asked to read letter strings that differed in how well thes obeyed the rules of their language, the more the string followed the rules, the more activation the region showed.”
June 18, 2026
Delightful, accessible, and up to date (2026) book on the evolution of brains by neuroscientist Rogier Mars. Tracing the evolution of the brain from sea squirts up to modern Homo sapiens, Mars’s central theme was that each new development in brain evolution related to one central reason, the brain’s importance as a foraging tool. Brain developments in mammals are the main focus, particularly the development of the complex, distinctive neocortex and how primates became social, all related to foraging. Along the way there is a nice overview of relevant aspects of the evolution of life on Earth as well as other species of humans. Writing was fun, light, explaining complex aspects of brain evolution, anatomy, and function in easy-to-understand terms for the interested layperson.
Seven chapters: first appearance of vertebrates, brains in sea squirts (which lose their brains as adults) and lamprey, the importance of predictive power in foraging, brains in bees, discussed the telencephalon; brains in reptiles, the mammalian neocortex versus the reptilian dorsal cortex, how mammal brains evolved as nocturnal insectivores in the Mesozoic, the hippocampus (important in memory and navigation), brain mapping, and the importance of the anterior cingulate cortex or ACC region of the brain; brain differences in squirrels and squirrel monkeys, primate brains evolved for vision and grasping, how different parts of the brain interpret visual information for say avoidance of obstacles and avoidance of predators, the parietal pathway and its importance in interpreting visual information, the parietal cortex; how fruit selection lead to development in learning abilities especially in simians, important in drier climates, the temporal cortex’s role in establishing categories in foraging, the prefrontal cortex’s role in foraging goals; how social cue reading in dogs, chimps related to foraging goals, importance of neocortex in reading social cues, how reading what conspecifics know or want is a foraging tool; other human species, human and chimpanzee differences, how human brains are not just scaled up monkey brains, the theory of mind (understanding others have different goals and beliefs); how brain evolution used neural recycling, using existing structures rather than creating new ones, discussion of episodic memory and cultural transmission in birds (thought only to be mammalian), discussion of mammalian neocortex versus analogous structures in bird brains, introduced grid cells and place cells, more discussion of the hippocampus’s importance.
Seven chapters: first appearance of vertebrates, brains in sea squirts (which lose their brains as adults) and lamprey, the importance of predictive power in foraging, brains in bees, discussed the telencephalon; brains in reptiles, the mammalian neocortex versus the reptilian dorsal cortex, how mammal brains evolved as nocturnal insectivores in the Mesozoic, the hippocampus (important in memory and navigation), brain mapping, and the importance of the anterior cingulate cortex or ACC region of the brain; brain differences in squirrels and squirrel monkeys, primate brains evolved for vision and grasping, how different parts of the brain interpret visual information for say avoidance of obstacles and avoidance of predators, the parietal pathway and its importance in interpreting visual information, the parietal cortex; how fruit selection lead to development in learning abilities especially in simians, important in drier climates, the temporal cortex’s role in establishing categories in foraging, the prefrontal cortex’s role in foraging goals; how social cue reading in dogs, chimps related to foraging goals, importance of neocortex in reading social cues, how reading what conspecifics know or want is a foraging tool; other human species, human and chimpanzee differences, how human brains are not just scaled up monkey brains, the theory of mind (understanding others have different goals and beliefs); how brain evolution used neural recycling, using existing structures rather than creating new ones, discussion of episodic memory and cultural transmission in birds (thought only to be mammalian), discussion of mammalian neocortex versus analogous structures in bird brains, introduced grid cells and place cells, more discussion of the hippocampus’s importance.
July 19, 2026
The problem with light reading about brains is that I have deliberately chosen not to pursue a career that requires me to remember which lobe is where and I get lost.
But also this book was fun and interesting and I'm still mad about the wet-nosed/dry-nosed monkey distinction.
But also brains are just cool, man.
But also this book was fun and interesting and I'm still mad about the wet-nosed/dry-nosed monkey distinction.
But also brains are just cool, man.
September 23, 2026
Read aloud with Elijah. A little disappointed as the focus of this book is heavily on the “you” part and not so much on the other animals. However, this book is dense with information on how human brains are organized and have changed through evolution (other animals used for comparison).
June 22, 2026
Perfectly fine but too short to say anything new or go into any detail.
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