Of all the books I have read about climate change, Snowball Earth, by Gabrielle Walker, is definitely one of the best – and it’s not even about the current climate change.

Part of what makes it so good is the style of writing. As the Los Angeles Times said about her later book, An Ocean of Air, “Walker has a Ph.D. in chemistry, but she writes like a poet.” And, indeed, after an education at Cambridge, Walker has spent most of her career as a science journalist. It’s sort of sad that this doesn’t happen more often. Usually, those who understand a subject best are not the ones who communicate it. Walker is the exception to this rule.

Take, for example, this passage about the history of life on Earth:

Stretch your arms out wide to encompass all the time on Earth. Let’s say that time runs from left to right, so Earth was born at the tip of the middle finger on your left hand. Slime arose just before your left elbow and ruled for the remaining length of your left arm, across to the right, past your right shoulder, your right elbow, on down your forearm, and eventually ceded somewhere around your right wrist. For sheer Earth-gripping longevity, nothing else comes close. The dinosaurs reigned for barely a finger’s length. And a judicious swipe of a nail file on the middle finger of your right hand would wipe out the whole of human history.

Another impressive aspect of Walker’s writing is her characterization. Wacky, stubborn, and exuberant scientists are brought to life. Instead of just hearing about their work and accomplishments, you feel like you’re getting to know them as people. She writes about arguing scientists particularly well. Arguing scientists are so much fun to read about – that’s one reason I loved The Lost World by Arthur Conan Doyle.

However, the best part of this book, by far, is the subject matter. The theory of Snowball Earth is possibly the most awesome thing I have ever heard about. Here’s how the story goes:

From what paleontologists can see preserved in fossils, complex life arose at a very specific point in prehistory: the end of the Precambrian. For several billion years before that, the only thing that lived on Earth was unicellular goop. But then, suddenly, all at once, complex organisms burst onto the evolutionary stage.

Something must have caused this dramatic appearance, and a series of scientists from the 1940s on – most prominently, Paul Hoffman – likely have discovered what. At the end of the Precambrian, there are signs of ice in rocks all over the world – scratches, rock deposits, everything that led Agassiz to discover the ice ages.

Because plate tectonics moves everything around so much, though, rocks were not necessarily formed at the location they sit today. Their magnetic field is what discloses their birthplace. Tiny bits of magnetic material, such as iron, line their field up with the Earth’s. The Earth’s magnetic field is perpendicular to the surface at the poles and parallel to the surface at the Equator, like this:

So, if a rock’s magnetic field is vertical, it was formed at the poles. If it is horizontal, it was formed at the equator. Incredibly, scientists found Precambrian rocks, with signs of ice, with horizontal magnetic fields. During that period of prehistory, the equator was covered in ice – and, therefore, the whole planet, because it’s not really possible to freeze the equator without freezing all the other latitudes too.

The scientists determined that, for several instances on the Precambrian, the continents were arranged in a way that was very conducive to ice-albedo feedback. With the smallest trigger, ice from the poles would creep across the temperature zones and meet at the equator. Frozen oceans, frozen land, the whole bit.

And now CO2 comes into the story. Volcanic eruptions naturally release carbon dioxide, but the amount is so small that the oceans have no trouble soaking them up – unless they’re frozen on the surface and cut off from the air. CO2 would gradually build up, in that case, and millions of years later, the greenhouse effect would be so strong that all the ice would melt and the planet would plunge into a state referred to as Hothouse Earth. Then the oceans would start absorbing all the extra CO2, and ice would reappear at the poles, and the cycle would begin again.

Many scientists believe that these Precambrian cycles of extreme heat and extreme cold provided such a strong pressure on organisms that natural selection was pushed to new boundaries. Complex life had an advantage in these extreme conditions, and it flourished. The most catastrophic climatic event our planet has ever experienced, in our knowledge, was what led to the evolution of multicellular organisms, and eventually, us.

It makes me feel very small, the same way that attempting to comprehend the vastness of the universe makes me feel very small. The life we see all around us only exists because of a series of coincidences. Human beings, one of the youngest of the millions of animal species that have ever existed, are alive because of continental drift lining things up in the right way. And who knows what would have happened if things had been slightly different?