📗 REVIEW: Rise & Reign of Mammals by Steve Brusatte

A history of mammals, from the fall of the dinosaurs to the rise of the human. It's a love letter to teeth, & deserving of its bestseller status, tho more technical than most pop science books.

Mar 11, 2026

I picked up The Rise and Reign of the Mammals by Steve Brusatte because it showed up in a Reddit nonfiction book club I lurk in, and I was curious how a dinosaur paleontologist would handle mammals. Fair warning: that’s an affiliate link, and this book is more technical than my usual fare. There were a lot of Latin names, a lot of jaw anatomy, and a lot of phylogenetic reshuffling.

It’s a great book tho, especially if you love passionate nerds talking about their subject of interest. It is essentially a love letter to teeth and the dentary-squamosal jaw joint — the hinge between the lower jaw and skull that defines mammalhood in some senses more than the mammary gland thing. This might sound like an odd focus for a 400-page book, but Brusatte makes a pretty good case that this single anatomical innovation cascaded into everything that makes mammals distinct: precise chewing, larger brains, nursing, better hearing, and eventually the ability to occupy nearly every ecological niche on Earth.

Classification Is Hard

One of the book’s recurring themes is how messy biological classification really is. Brusatte uses the dentary-squamosal joint to define mammals because it’s practical, even though most contemporary paleontologists prefer a “crown group” definition that traces back to the most recent common ancestor of living mammals. The crown group approach would exclude early creatures like Morganucodon from “mammal” status entirely, which matters for reasons I frankly don’t quite understand.

What I liked, tho, is that Brusatte is refreshingly candid about this tension:

Here I’ll be honest: in my scientific writings, I use the crown group definition. In a research paper, I wouldn’t call Morganucodon a “mammal,” but a “basal mammaliaform,” or a “nonmammalian mammaliaform.” As you can see, the terminology quickly becomes unwieldy.

This pragmatic approach appears throughout the book. Brusatte reminds us that “Nature doesn’t put labels on things, people do,” and different classification systems serve different purposes. I really like this kind of nuance — it’s rare to see a popular science book acknowledge that scientific categories are mostly just human constructs chosen for utility.

In that vein, one of the book’s revelations (at least for me) is how completely DNA evidence upended anatomical classifications of mammals. When molecular biologist Mark Springer and colleagues published the first DNA-based genealogies in the late 1990s and early 2000s, “paleontologists were shocked”:

Many of the relationships among placentals championed by Simpson disintegrated, revealed as illusions of anatomical convergence. Genes showed that pangolins are not closely related to anteaters and sloths, but group with dogs and cats. Bats are not next-of-kin to primates, but part of a larger assemblage with dogs, cats, and pangolins, plus the perissodactyls with an odd number of toes (like horses) and even-toed artiodactyls (like cattle).

This reshuffling is a great example of evolutionary convergence — unrelated organisms developing similar features because they face similar environments. Pangolins look like anteaters because both eat ants and termites, even though their genetic family-tree background is wildly divergent. Bats seemed primate-adjacent because both are small and agile, but from a bloodline perspective they’re really closer to dogs.

How Teeth Changed Everything

Anyway, mammals’ new jaw joint enabled precise, controlled chewing — something that’s apparently quite rare in the animal kingdom. Before this innovation, jaws were tenuously attached to skulls, limiting bite strength and precision. The dentary-squamosal joint, operated by newly divided jaw muscles (temporalis, masseter, and pterygoideus), could generate much stronger bites and focus that force on specific teeth at specific times.

Teaching my kids to take bites with incisors and chew with molars has been an adventure, but this is the thing that unlocked mammalian success. Precise chewing allowed mammals to extract more nutrition from food, and better nutrition supported larger brains. The skull cavities of early mammals show “huge cavities” that held much larger brains than their ancestors, with most growth at the front, creating the globular cerebral hemispheres characteristic of modern mammals.

Larger brains required more energy, which likely drove the evolution of nursing. Brusatte argues that mothers “must have started nursing early in mammal history, probably right around the time the first mammals like Morganucodon were scuttling around in the Triassic.” Milk provided a “nutritious, sustainable, readily available food source” to fuel energy-expensive neural tissue. Basically, mammals progressively lost their VIT genes — the genes responsible for producing vitellogenin, the egg yolk protein. As lactation and placentation evolved, egg yolk became dispensable, and these genes decayed into whatever the heck pseudogenes are.

Even mammalian hearing traces back to jaw evolution. As the jaw joint shifted, some of the old jaw bones became free to evolve into the three tiny bones of the mammalian middle ear. These bones, along with the bulla and petrosal, function as “noise-canceling headphones, allowing mammals to still hear exceptionally well while chewing.”

Size and Senses Matter

Cynodonts — the therapsid lineage that would become mammals — survived the end-Permian extinction while their cousins went extinct or dwindled. Brusatte says cynodonts “took the path of survival and domination.”

In this case, that meant getting small. Shrinking profoundly transformed their biology and evolutionary trajectory — they changed their growth, metabolism, diet, and feeding styles. They already had elevated body temperatures inherited from their therapsid ancestors, but next they developed full-on warm-bloodedness. They gave up size and strength to become specialists with keen senses and precise feeding, which worked out pretty well for them.

Early mammals made another significant sacrifice, essentially giving up color vision in exchange for enhanced smell, touch, and hearing

Brusatte notes that humans are “highly unusual among mammals” in our ability to perceive color, shared with only some of our closest primate relatives. Most mammals cannot see in color, which is why most have drab brown, tan, or gray fur. “Why dress yourself in flamboyant hues—like many day-living, sharp-eyed birds and reptiles do—if your mates or rivals cannot see them?” I knew that humans had better color vision than dogs and bulls (which can’t perceive the red color of the traditional bullfighting cape), but I didn’t realize how rare this was or why we evolved this way.

Turns out most mammals adapted to a nocturnal, scent-and-sound-based existence during the age of dinosaurs. And most mammal lineages never reversed those adaptations even after dinosaurs disappeared.

Some mammals did reverse other adaptations, though. The chapter on whale evolution describes how Egypt’s Wadi al-Hitan (Valley of the Whales) preserves dozens of whale skeletons from when the region was ocean floor:

Many of the skeletons are as pristine as ancient bones can be: enormous bodies preserved in perfect arrangement, with toothy heads connected to gently arching backbones, ribs sticking out sideways. If you follow the serpentine contours of the trunk, flattened flippers emerge from the shoulders, then the back vertebrae transition to the tail, and finally, when the tail has started to taper, some small bones appear, unmoored from the rest of the skeleton. A pelvis, and a leg.

These “walking whales” show the gradual transition from land mammal to fully aquatic whale, complete with vestigial legs. This find was a big deal because, “if anybody tries to claim there are no ‘transitional fossils’ or ‘missing links’ in the fossil record,” we can “tell them about the Walking Whales.”

The evolution of whales also involved sensory tradeoffs. Unlike the early mammals that sacrificed vision for smell, whales went the opposite direction — they “completely lost their sense of smell.” He said that “underwater, scent provides no advantage,” but when I first read this I wondered why sharks don’t have the same problem, since they famously detect blood from great distances. But it’s an apples-to-oranges comparison: sharks are fish with ‘olfactory’ systems completely separate from breathing, specialized for detecting waterborne chemicals. Toothed whales, by contrast, repurposed their nasal passages for echolocation and haven’t needed anything like “smell” to replace those functions.

Reality vs. Fiction

One of the other things I really liked about Brusatte’s writing is that his narrative sections were pretty grounded, especially compared to Tamed by Alice Roberts, which kicked off with a lengthy story that she explains is pure speculation that “could have” happened only after a lengthy scene walking us through how she thinks the first humans domesticated dogs.

The most memorable narrative scenes I remember came from the Ashfall Fossil Beds in Nebraska, which preserved a remarkable snapshot of various animals’ final moments. A volcanic eruption blanketed the region in ash, killing rhinoceros, horses, and camels (which I did not even realize existed in prehistoric America!?).

For example, in this moonscape of death, one hell hound made a final meal:

Haggard and hungry, it was about to give up before seeing the spread of rotting meat along the lakeshore. It walked up to a rhino and took a few half-hearted bites from its flank, spitting out ash as it gnawed. A last meal, before it lay down beside its prey.

Epicyon — and the actual scientific name really is “hell hound” — was a five-foot-long, two-hundred-pound bone-crushing predator that roamed Miocene North America. I didn’t realize those were “real” either, although I’m sure that the urban fantasy books I’ve read featuring hell hounds have remarkably little resemblance to the prehistoric canines.

Brusatte concludes the book with humans and our relationship to other mammals, including the possibility of mammoth de-extinction — currently as fictional as Jurassic Park (did you know it’s based on a book? I keep wanting to like Michael Crichton but keep bouncing off, how about you?). He’s confident mammoth re-creation will happen “and there will be a Nobel Prize in it for somebody.”

But he also acknowledges the ethical problems, and honestly I’m pretty skeptical of the whole project. Debates about the causes and extremities of global warming aside, “it will soon be much warmer than any mammoth ever experienced.” This is not a world in which they can thrive, even without humans hunting them. Where would we even put them, the Arctic to mess up that ecosystem even more? If we’re going to bring something back, I’d rather bring back something that has a chance at surviving, like the dodo or the thylacine.

The mammoth extinction itself remains controversial. I’d encountered the “blitzkrieg hypothesis” — the idea that humans hunted mammoths and other megafauna to death — before reading this book, and I’d assumed it was basically true. Brusatte convinced me otherwise, which might be the single most practically useful thing I got from this book; it saved me from confidently repeating something that’s not well supported. The evidence is thin, which matters because as he points out: “If we hunted dozens of large mammals to death, then the Near Time fossil record should be riddled with butchered mammoths and sabertooths with stab wounds.” The few examples of mammoth butchery are “far outnumbered by tool-marked bones of another large North American mammal: bison, a survivor!”

Brusatte favors a more complex explanation involving climate change, habitat disruption, and selective pressure on large slow-breeding animals. Bigger animals have lower reproduction rates and produce fewer offspring that take longer to develop. “Any forces that disrupt population structure and heighten juvenile mortality can thus topple these big-bodied slow breeders.” Not all of those forces are our fault!

Interesting Modern Mammals, aka Relevant Scientists

Speaking of fault, I want to talk about the people whose “fault” it is that we know all this stuff at all.

I don’t always enjoy the “human stories of science” sections in popular science books — I recently read a book about bats where the anecdotes about researchers felt off-putting rather than charming — but Brusatte’s work well. Walter Kühne, imprisoned by the Nazis during World War II, spent his detention assembling and describing the cynodont Oligokyphus. His meticulous work while imprisoned culminated in a 1956 monograph on the specimen.

Father Rigney had a worse experience. While studying a fossil mammal in China, the Communist secret police arrested him and imprisoned him for four years. Unlike Kühne, Rigney couldn’t continue his paleontological work in prison. The new government was “serious about purging China’s religious institutions.” After being freed through diplomatic channels, he eventually reunited with the skull specimen, which had been “secretly whisked away from the Communists.” But first he set aside the fossil to write his autobiography, Four Years in a Red Hell (and I believe it’s in the public domain, if you’d like an electronic copy of Rigney’s autobiography).

The obsessive dedication to research under impossible circumstances reminded me of The Perfectionists by Simon Winchester, which as I said in January was a great history of precision engineers: another group of people so consumed by their work that prison would barely slow them down.

The imprisoned paleontologists are hardly the only interesting human stories, tho. There’s the “Bone Wars” rivalry between Edward Cope and Othniel Marsh, which is one of those historical episodes that reads like fiction. Cope named more species than Marsh largely because the actual fossil hunters liked him better and gave him first access to their finds — a good reminder that people skills matter even in paleontology. The rivalry accelerated the field enormously, but also produced sloppy work and bitter feuds that lasted decades.

And when Darwin speculated that whales might have evolved from bears that swam with mouths open, skimming insects from water — a notion “so ludicrous he removed it from later editions of the Origin of Species” — Brusatte includes it not as a gotcha but as a reminder that wild theories that turn out to be wrong are a very important part of doing science. You have to be willing to be risk being wrong when you are trying to get closer to the truth.

Life Gets Grander Every Year

Brusatte ends with a reflection that captures why studying deep time matters:

The blue whale is not merely the largest mammal alive today, but the largest living animal, period. Nobody has ever found a fossil of anything bigger, which means that the blue whale is the all-time record holder, the heavyweight champion of the history of the world... It’s a simple but profound statement that bears repeating: the biggest animal that has ever lived is alive right now. Of all the billions of species that have lived during the billions of years of Earth history, we are among the privileged few that can say such a thing. How glorious is it that we breathe the same air as a blue whale, swim in the same waters, and gaze at the same stars?

This is the kind of perspective that makes this book such a lovely read. We tend to think of the past as grander, stranger, more extreme than the present. But the largest animal in Earth’s entire 4.5-billion-year history is swimming in our oceans right now. The age of mammals is still happening, even as we reshape the planet in ways that will determine which animal lineages survive into the next geological epoch.

Manuscriptions

Discussion about this post

Ready for more?