‘Dinosaurs are not us’: book reveals how mammals came to rule the world

Steve Brusatte writes of the evolutionary twists, catastrophes and luck that led to the warm-blooded animals of today

With a head like a horse, a body resembling a giant bear and possessing huge, clawed knuckles upon which it walked like a gorilla, Anisodon looks like a figure from Greek mythology.

But it is not a beast of the underworld or a monster of nightmares. Instead it is one of a bizarre group of animals called Chalicotheres that roamed Earth from 46m years ago, with the last of the creatures surviving long enough to have been encountered by human ancestors. What’s more, Anisodon was a mammal. Just like us.

King Kong may have easily bested a T rex in the 1933 film, but since then our interest in dinosaurs has conquered any fascination with mammals. While the reptiles have been propelled into the public eye by films such as Jurassic Park, early mammals have been the underdog – with mammoths and sabre-toothed tigers among the few garnering fame.


Yet the mammal family tree is bristling with jaw-dropping creatures, from Anisodon to the biggest creature that has ever lived – the blue whale.

“I don’t think we appreciate this enough,” says Steve Brusatte, a palaeontologist at the University of Edinburgh and author of The Rise and Reign of the Mammals, which sets out to bridge the fascination gap. “Just imagine if whales were extinct, and all we had were their bones. I mean, they would surely be as famous, as fascinating, as dinosaurs.”

As a science consultant for the forthcoming film Jurassic World Dominion, Brusatte has nothing against dinosaurs, and the shelves of his office are teeming with sketches, plastic models and even origami creations of the beasts.

The effusive American even began as a T rex expert before branching out into studying mammal fossils. But there’s a simple reason why he’s so passionate about the latter. As he says in his new book: “Dinosaurs are awesome, but they are not us.”

The Rise and Reign of the Mammals is nothing short of a thriller, revealing the luck, evolutionary twists and near-apocalyptical catastrophes that have led to the mammals of today, us included.

Fascinating revelations come thick and fast, from the discovery that ancient rodents and monkeys crossed the vast distance from Africa to South America on rafts, to the fact whales have belly buttons and elephants recognise themselves in the mirror.

Along the way, Brusatte brings readers face to face with our distant ancestors, including the last common ancestor of mammals and reptiles: a small, scaly, swamp-dwelling creature that lived about 325m years ago.

At some point, two populations of these lizard-like creatures became separated from each other. And the rest is history.

As natural selection got to work, one population accrued adaptations that would eventually give rise to mammals. Chief among them was a single opening behind the eyes – allowing for bigger, stronger jaw muscles – and teeth specialised for different purposes.

“A lot of our biological superpowers come from our teeth,” says Brusatte. “Something like a T rex, or a lizard, just basically has all the same type of teeth, they can just chomp up and down. Mammals, we have all these different varieties of teeth, we basically have a Swiss army knife in our jaws and the teeth do many things.”

Dimetrodon. Photograph: Todd Marshall

The early ancestors of mammals are a far cry from our fluffy pets. About 290m years ago the huge, sail-backed Dimetrodon, dubbed “something of a Frankenstein creature,” by Brusatte, was stalking the landscape with its sprawling limbs and sharp teeth, and about 255m years ago an intrepid time traveller could have met Inostrancevia, a group of monstrous sabre-toothed beasts. “These things were nasty flesh-eaters,” says Brusatte.

Soon, hair began sprouting, brains grew in size, and higher metabolisms developed. “When you look in the fossil record, you see there was this long story [over] tens of millions of years, when mammals were essentially assembled by evolution, piece by piece,” says Brusatte.

Then, about 252m years ago, volcanoes erupted in what is now Siberia. The upshot was runaway global warming and the death of about 90% of the planet’s species – an event called the end-Permian extinction, or “great dying”.

Most of the forerunners of mammals bit the dust. But, against the odds, some survived, including a hairy, cat-sized creature called Thrinaxodon that could not only burrow but rapidly grow and reproduce. It was the ultimate “disaster species”.

Thrinaxodon. Photograph: Todd Marshall

“It seems like that just by the dumb luck of evolution most [mammal ancestors] died, but a small number of them turned out to be particularly suited to a world of chaos,” says Brusatte.

These survivors garnered new adaptations: their lower jaw changed from having a collection of bones to just one, and a new type of joint emerged – long thought the hallmark of true mammals. The vestigial bones were repurposed, becoming tiny bones in the middle ear commonly known as the hammer and anvil – a radical development that super-charged hearing. At some point they started feeding milk to their young, and became truly warm-blooded.

But another type of creature was also on the rise: dinosaurs. And as these beasts went big – a diplodocus was roughly the length of a basketball court – mammals went small. Brusatte is keen to stress that the pressure went both ways. “You never saw a triceratops the size of a mouse. And that’s because the mammals were keeping the dinosaurs big,” he says.

Their diminutive form was to be mammals’ trump card when, about 66m years ago, a six-mile-wide space rock collided with Earth. The dinosaurs, with the exception of the ancestors of birds, died out. So too did a vast array of mammals, perhaps as many as 90%.

But some lived. “Those that did survive happen to be the ones that were smaller, the ones that could burrow or hide more easily, and the ones that had very generalist diets that could eat lots of things,” says Brusatte.

Mammals soon grew larger. And while some laid eggs, like platypuses today, others gave birth to live young – either nurturing them via a complex placenta in the womb, or in a pouch.

Down the corridor at Edinburgh University, Dr Sarah Shelley, a palaeontologist who illustrated Brusatte’s book, unveils the jaw of a creature that lived a few hundred thousand years after the space rock struck.

Upper-side 3D rendering of content inside a burrow: thrinaxodon and broomistega.
Upper-side 3D rendering of content inside a burrow: Thrinaxodon and Broomistega. Photograph: Creative Commons Attribution LicenseTodd Marshall

Periptychus was about the size of a border collie, but chunkier, with a big head, massive cheek muscles, a small brain and teeth like lemon juicers. And it was hairy, and had five digits and fingernails. “Its hands look freakishly human,” Shelley adds. “They’re not yet hooves, but they’re more than claws.”

But Brusatte is not only enthusiastic about showcasing bizarre mammals of the past. He wants greater appreciation of what is here now. To illustrate his point, he notes that besides birds and pterodactyls, only one creature has evolved the ability to fly by flapping its wings: bats.

“Imagine if they were not around any more and all we had were fossils. I mean, we would marvel at something like a bat,” he says.

Humans, too, offer much to marvel at: as Brusatte points out, we are sentient apes that have changed the world. But we are only a chapter in a far bigger story.

“I want people to come to appreciate our evolutionary history – where we come from, why we look the way we do, why we behave the way we do, why we have hair and feed our babies milk and we have the teeth we do and we have big brains and keen senses, and all of these things,” says Brusatte. “This all comes from evolution.”


Nicola Davis Science correspondent

The GuardianTramp

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