Steve Brusatte’s The Rise and Reign of the Mammals is an ambitious attempt to tell the evolutionary story of one of the most successful groups of animals on Earth, while ultimately exploring our own origins. Following his earlier book, The Rise and Fall of the Dinosaurs, Brusatte turns his attention from the animals that dominated the Mesozoic to the lineage that ultimately inherited much of their ecological legacy. The result is an engaging synthesis of palaeontology, evolutionary biology, anatomy and natural history, written with the accessibility of popular science but grounded firmly in contemporary scientific research. For a zoologist such as myself, the attraction of the book lies not simply in its account of fossils and extinct species, but in the way it connects extinct mammals with their living descendants. Mammals are often introduced through familiar characteristics: hair, milk, endothermy and specialised teeth, but Brusatte demonstrates that mammalian evolution is much more complicated and considerably more interesting than such a checklist suggests. The modern mammalian fauna is the product of more than 200 million years of evolutionary experimentation, extinction, ecological opportunity and adaptation. The Rise and Reign of the Mammals succeeds particularly well when it makes that deep history relevant to the mammals that zoologists encounter today.
One of the most important strengths of the book is its treatment of the origins of mammals. From a zoological perspective, it is tempting to regard mammals as a clearly defined group that appeared at a particular point in geological time. Evolution does not work in quite such a tidy fashion. The transition from early synapsids to mammaliaforms and eventually to mammals was a prolonged evolutionary process involving changes to the skull, jaw, teeth, middle ear, posture, locomotion, metabolism and sensory systems. Brusatte communicates this complexity effectively. The earliest members of the mammalian lineage did not look like modern mammals, and many of the features that distinguish mammals evolved at different times and in different combinations. The familiar mammalian jaw and ear, for example, have a history extending deep into the fossil record. Bones that once formed part of the jaw were gradually incorporated into the middle ear, producing the sophisticated auditory apparatus characteristic of modern mammals. This is an especially useful perspective because it reinforces a fundamental principle of comparative anatomy: structures should not be considered in isolation. The mammalian middle ear cannot be properly understood without considering the evolutionary history of the jaw, just as mammalian teeth make more sense when viewed in relation to diet, skull mechanics and ecological specialization. Brusatte repeatedly demonstrates how apparently familiar anatomical features have long and complex histories.
The same is true of mammalian locomotion. The ancestors of mammals were not simply miniature versions of contemporary shrews or rodents. Their limbs, vertebral columns and gaits changed substantially through time. Some early synapsids retained sprawling or semi-sprawling postures, whereas later mammaliaforms developed more erect postures and increasingly specialised locomotor abilities. These changes were intimately connected with changes in ecology. The central narrative of the book is that mammals spent much of the Mesozoic living in the shadow of dinosaurs. This is an effective narrative device, although it needs some qualification. Dinosaurs did not simply prevent mammals from becoming ecologically diverse. Mammals occupied numerous ecological niches during the Jurassic and Cretaceous, and the fossil record increasingly reveals a much more diverse Mesozoic mammalian fauna than the traditional image of small, nocturnal, shrew-like animals suggests. Brusatte is particularly good at dismantling this old stereotype. Mesozoic mammals included animals adapted for climbing, burrowing, swimming and predation, as well as forms with highly specialised teeth and feeding strategies. Some were surprisingly large. Others possessed unusual anatomical adaptations that demonstrate that mammals were already experimenting with different lifestyles long before the extinction of the non-avian dinosaurs. This matters because the conventional story of mammalian evolution has often been too teleological. It can be tempting to imagine that mammals were patiently waiting for the dinosaurs to disappear before beginning their evolutionary ascent. The fossil evidence instead suggests a more complicated story. Mammals were already diverse, and the end-Cretaceous extinction created opportunities that some lineages were able to exploit.
The extinction event at the end of the Cretaceous forms one of the pivotal moments in Brusatte’s narrative. From a zoological perspective, this is more than a story about extinction. It is a demonstration of how ecological communities can be radically reorganised. The disappearance of the non-avian dinosaurs removed major components of terrestrial ecosystems. Mammals that had previously occupied relatively restricted ecological roles subsequently diversified into environments that had become available. This process, often described as adaptive radiation, resulted in the evolution of many of the major mammalian groups familiar today. Yet Brusatte avoids portraying the post-Cretaceous world as an empty landscape waiting to be colonised. Ecosystems were damaged, but they were not erased. Plants, invertebrates, reptiles, birds and surviving mammals all contributed to the rebuilding of terrestrial communities. Mammalian diversification therefore occurred within an existing ecological framework rather than in a biological vacuum. This is particularly valuable for understanding modern zoology. Ecosystems are communities rather than collections of independent species. The evolutionary trajectory of one group can affect opportunities available to another. Extinction therefore does not merely reduce biodiversity; it can reorganise ecological relationships and create new evolutionary possibilities. The distinction is important. Evolution does not have a predetermined destination. The fact that mammals eventually became ecologically dominant in many terrestrial environments does not mean that this outcome was inevitable.
One of the book’s major achievements is its account of the diversification of mammals following the Cretaceous–Palaeogene extinction. Brusatte introduces readers to a remarkable succession of extinct forms and lineages, many of which are unfamiliar outside specialist palaeontological literature. The Paleocene and Eocene were periods of considerable mammalian experimentation. Mammals diversified in body size, diet, locomotion and habitat use. Forests covered extensive areas, and many mammals evolved arboreal lifestyles. Others became increasingly specialised for life on the ground. The later evolution of grasslands produced another major ecological transition. As open habitats expanded, mammals evolved adaptations for cursorial locomotion, grazing and browsing. Teeth became increasingly specialised for processing abrasive vegetation. Limbs became adapted for efficient running, and body sizes changed dramatically. This is where the book is particularly strong from a zoological perspective. Mammalian evolution is not presented simply as a sequence of increasingly sophisticated animals. Instead, changes in anatomy are linked to environmental change. Teeth tell us about diet; limb bones reveal aspects of locomotion; skulls provide clues about feeding behaviour; body size influences metabolism, predator avoidance and reproductive strategies. Such connections are at the heart of zoology.
Brusatte also tackles one of the more difficult aspects of mammalian evolutionary history: explaining how living groups are related to one another. Modern mammals comprise three broad evolutionary divisions (monotremes, marsupials and placental mammals) but the relationships within and between these groups have been the subject of considerable research. The book demonstrates how fossil evidence and molecular phylogenetics have transformed our understanding of mammalian relationships. DNA provides an extraordinary source of information for living species, but fossils provide the essential historical context. Neither approach is sufficient by itself. This integration of palaeontology and molecular biology is one of the book’s strongest scientific themes. Zoology increasingly operates across traditional disciplinary boundaries. A modern understanding of mammalian evolution requires morphology, genetics, developmental biology, ecology, behaviour and palaeontology. Brusatte’s narrative reflects this interdisciplinary approach.
The discussion of modern mammals is particularly effective because it brings the evolutionary narrative back into the present. Elephants, whales, bats, primates, carnivores, rodents and other groups are not evolutionary endpoints but surviving branches of an ancient and continually changing tree. The evolutionary history of whales provides an excellent example. Modern whales are highly specialised aquatic mammals, but their ancestors were terrestrial artiodactyls. The transition involved profound changes to the skull, teeth, limbs, pelvis, spine and respiratory system. Whale evolution illustrates the explanatory power of evolutionary thinking. The anatomy of a modern whale can initially seem bizarre: the nostrils have migrated to the top of the skull, the hind limbs have become vestigial, the forelimbs have been transformed into flippers, and the tail has become the principal propulsive structure. Yet these features become comprehensible when considered historically. Similarly, bats demonstrate how evolutionary history can produce one of the most unusual locomotor adaptations among vertebrates. Their ability to fly is associated with profound modifications of the forelimbs, digits and integument. Primates provide another example, with their sensory adaptations, grasping hands and complex brains reflecting an evolutionary history shaped by arboreal and later terrestrial environments. Brusatte’s achievement is to show that these animals are not merely interesting because they are alive today. They are interesting because their bodies are records of their evolutionary past.
The evolution of the mammalian brain receives substantial attention, reflecting the importance of nervous-system evolution in the eventual success of many mammalian lineages. However, this is also one of the areas where a zoologist may want to approach some of the narrative more cautiously. There is a tendency in popular accounts of mammalian evolution to associate increasing brain size with increasing evolutionary success. While larger brains have evolved repeatedly in mammals, brain size alone is an imperfect measure of behavioural complexity, intelligence or ecological success. Small-brained mammals can exhibit remarkably sophisticated behaviours, while large brains do not necessarily guarantee ecological dominance. The diversity of bats, rodents and insectivores demonstrates that successful mammals can have radically different neurological and behavioural adaptations. Nevertheless, Brusatte’s broader argument is convincing: sensory biology and behaviour have played major roles in mammalian diversification. Nocturnality, echolocation, olfaction, visual adaptations, social behaviour and parental care have all contributed to the success of different mammalian groups.
The book inevitably culminates in humans, but one of its strengths is that Brusatte places our species within mammalian evolutionary history rather than treating humanity as the inevitable culmination of the story. Humans are primates, primates are mammals, and mammals are one branch of the larger vertebrate tree. From a zoological perspective, this is a useful corrective to anthropocentric narratives of evolution. Our distinctive characteristics (large brains, upright locomotion, complex social behaviour, language and technological culture) are extraordinary, but they evolved from a particular mammalian and primate background. We are not separate from the history described in the preceding chapters. Indeed, the human story raises one of the book’s most important contemporary themes: evolutionary success can be temporary. Mammals have survived several episodes of environmental upheaval and extinction, but modern human activity is itself transforming ecosystems on a global scale. Habitat destruction, climate change, overexploitation, pollution and introduced species are altering the evolutionary and ecological context in which mammals now exist. Many modern mammalian species are declining or threatened. The irony is striking. A book about the rise and reign of mammals ultimately becomes a reminder that evolutionary dominance does not provide permanent security.
As a popular science book, The Rise and Reign of the Mammals has considerable strengths. Brusatte is an excellent storyteller, and his enthusiasm for fossils is infectious. He has a particular ability to make technical discoveries understandable without removing their scientific significance. The book also benefits from its broad taxonomic coverage. Rather than concentrating exclusively on charismatic megafauna, Brusatte introduces readers to lesser-known mammals and extinct lineages. This gives a much more accurate impression of mammalian diversity. The breadth of the book inevitably means that some subjects receive less attention than they would in a specialist work. The complexity of mammalian phylogeny, for example, cannot be fully explored within a general-audience narrative. Similarly, evolutionary debates and uncertainties are sometimes necessarily compressed. There is also an unavoidable tension between storytelling and evolutionary complexity. Evolutionary history is branching, contingent and full of dead ends, whereas books require a narrative. Brusatte generally handles this tension well, but readers should remember that the sequence presented in the book is a narrative reconstruction rather than a simple chronological unfolding of predetermined evolutionary progress. This is not a criticism unique to Brusatte. It is an inherent challenge of writing popular palaeontology.
From the perspective of a zoologist, The Rise and Reign of the Mammals is an impressive synthesis of palaeontology and zoology. Its greatest strength is its ability to demonstrate that mammals cannot be properly understood by studying living species alone. Every mammalian skull, tooth, limb and behavioural adaptation is part of a much longer evolutionary history. The book also succeeds in overturning the idea that mammals were insignificant during the age of dinosaurs and simply inherited the Earth after the Cretaceous extinction. Instead, it presents mammalian evolution as a long history of experimentation, diversification and survival. Mammals evolved alongside dinosaurs, survived catastrophic environmental change, diversified into newly available ecological niches and eventually produced the extraordinary diversity seen today. The most valuable message is perhaps that evolution connects disciplines that are too often considered separately. Comparative anatomy leads naturally into palaeontology; palaeontology connects with ecology; ecology informs our understanding of behaviour; genetics reconstructs relationships; and all of these disciplines contribute to our understanding of biodiversity. Ultimately, The Rise and Reign of the Mammals is less a celebration of mammalian supremacy than an exploration of evolutionary contingency. Mammals did not inevitably inherit the Earth. Their history was shaped by environmental change, competition, extinction, innovation and chance. Their present diversity is the surviving product of a vast evolutionary experiment stretching back more than 200 million years.
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