Mammals have a surprising secret history – forget everything you thought you knew

Imagine if the Isle of Skye fell on your head, Cuillin and all. As Portree pummelled into the Earth at 20 kilometres per second, life as much as 2,500 kilometres away would be blasted, smothered in ash, and drowned by tsunamis. Dust and debris blown into the atmosphere would blacken the skies for months, disrupting seasons, destroying habitats and causing a global mass extinction. You wouldn’t have to worry, because you would have been instantly reduced to vapour. Small mercy.

This is the moment where most authors begin their tale of mammal origins. Just 66 million years ago – virtually yesterday for those of us who measure time in rocks – an asteroid of Hebridean proportions did hit our planet. It led to the loss of the non-bird dinosaurs, flying reptiles and marine reptiles (including Nessie), as well as a host of other organisms on land, sea and sky. Famously, it was our tiny, furry forebears, the mammals, who were quickest off the mark in the aftermath. Within 10 million years they had recovered and begun to evolve into the major groups we see around us today. The so-called Age of Mammals had begun.

But if the history of mammals was a movie, beginning at the asteroid impact would be like watching Jurassic Park from the moment they all got onto the helicopter and flew to safety. The exciting parts of the story happened much earlier, and unlike that movie, most people have never heard them before. 

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As a student palaeontologist, I was stunned to discover that mammals can trace their beginnings over 300 million years ago, to a time called the Carboniferous. It was hot and humid back then, the continents covered in thick swamp-forests. Pushing through the undergrowth were the earliest backboned, four-limbed, egg-laying animals, making their living by feasting on the crunchy lunch of giant insects. A fundamental split developed, resulting in two epic lineages: the reptile line and the mammal line. We did not evolve from reptiles, but we do share a common ancestor with them. This is the true beginning of our story.

The mammal line are called synapsids, and for the next 90 million years they grew large, evolved into the first herbivores, carnivores, burrowers and even tree climbers. Long before the dinosaurs were even a twinkle in our planet’s eye, synapsids were experimenting with all that life could offer.

You may be familiar with dimetrodon, an animal with the huge sail on its back, held up with spikes. You may have been told it was a kind of reptile, but this beast was firmly on our branch of the tree of life. Other synapsids include estemmenosuchus, an omnivore from Russia that only a mother could love. Nearly three metres long and built like a nightclub bouncer, it had horns erupting from its forehead and cheeks, as though a firework had exploded in its skull. Preying on such animals were the gorgonopsids, named for the petrifying Gorgon of Greek myth. Fast predators with sabre-teeth and powerful forelimbs, they make tigers look like lap cats. 

It wasn’t to last: around 252 million years ago volcanic eruptions in Siberia caused the most brutal mass extinction of all time, wiping out around 85 per cent of all life on Earth. This put an end to the first flourishing of synapsids, and cleared a path for reptiles to take over in the Triassic, Jurassic and Cretaceous eras. This marks the start of the time of dinosaurs. But it’s also when mammals as we know them took shape.

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Far from being losers in this period, the predecessors of mammals found a spectacularly successful new niche to exploit. They evolved to be extremely small, insect-eating creatures of the night. As dinosaurs swelled to become giants, the first mammals shrank, and honed their bodies into finely tuned sensory marvels. They developed excellent night vision, widened their hearing range, turned their smell receptors up to 11 and utilised an exquisite whiskery sense of touch. Their brains grew larger. This was one of the most fundamental moments for mammals as we know them. They had hit upon a way of life so successful that most of them still live this way: 90 per cent of mammals today are small (most are rodents), and the majority are nocturnal.

In my book, Beasts Before Us, I reveal the incredible triumphs and twists of mammal evolution, including the emergence of defining mammal traits like fur and milk production. Many of the key fossils that tell the tale come from Scotland, including those found by my team during fieldwork on the Isle of Skye. Uncovering their remarkable history has inspired me as a researcher, and I hope you too will revel in their amazing untold story.

Beasts Before Us: The Untold Story of Mammal Origins and Evolution by Elsa Panciroli is out now (Bloomsbury Sigma, £12.99). You can buy it from The Big Issue shop on Bookshop.org, which helps to support The Big Issue and independent bookshops.

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