Fossils don’t just reveal the past. They help predict the future.

A fossilized dinosaur skeleton at the headquarters of CAPPA, a paleontology center in Brazil. In his book Otherlands, Thomas Halliday writes that fossils provide clues about the past and future of the planet. | Carl De Souza/AFP via Getty Images

A paleontologist on the wonder of life on Earth — and the ways that climate change will transform it.

When Thomas Halliday was a young lad in the village of Rannoch, Scotland, he loved exploring the Caledonian Forest. The pinewoods were like living fossils, a remnant of the last glacial period and a bygone age when the west coast of Scotland was covered in trees. “It was such a diverse and wonderful place to explore as a 7-year-old,” Halliday told Vox. “I essentially had free rein to go and run about, and I became very interested in the natural world.”

Fittingly, the boy who explored the ancient forest went on to become a paleontologist. In his new book, Otherlands: A Journey Through Earth’s Extinct Worlds, out this week, Halliday writes about primordial history as though we could witness it first-hand, bringing life to prehistoric geese that were as ornery as their modern-day cousins, towering forests that transformed our planet, and dinosaurs that lived before the evolution of flowers.

“By visiting extinct sites with the mindset of a traveler, a safari-goer, I hope to bridge the distance from the past to the present,” Halliday writes. He dives into the fossil record and invites readers to “see ancient life forms as if they were commonplace visitors to our world, as quivering, steaming beasts of flesh and instinct, as creaking beams and falling leaves.”

I recently spoke with Halliday about the clues the past leaves for us. He told me that “temporal wanderlust,” or a hunger to understand eras that were different from our own, can teach lessons about the future of the planet and the grave dangers of human-caused climate change. Our conversation has been edited for length and clarity.

You describe this book as a travelogue through time. What inspired that idea?

When people think of paleontology, they tend to think of skeletons in a museum. It’s very separated from the living creature. And when it is presented as a living creature, it’s usually in some sort of monster film, out for human blood. This isn’t really how creatures behave in reality. The past isn’t this barbaric age, you know? It was a real, functioning, biological system. So I thought, “If we can visit the Cairngorms [a mountain range in Scotland] and talk about their wonderful biology, then why not interpret the fossil record in such a way that it becomes sort of like visiting those worlds?”

The chapters of your book read like vivid descriptions of a day in time — say, a Tuesday in the Pleistocene. How did you do that? How did you collect the details of the weather or the behaviors of animals?

The behavioral side of things and the climatic side of things are obviously not directly observed, they are inferred. If you look at sedimentology, there are patterns of grains in the rock that tell you something about the environment. For example, in the Miocene chapter, about 4 million years ago at Gargano [present-day Italy], the Mediterranean Sea has dried out and we’re on this island. There’s a giant, flightless goose there. It has a bony spur on its wing, which is an anatomical feature that you can directly observe [in fossils]. And we know birds today have this kind of spur on their wing for fighting. We can then reconstruct that this is a behavior that probably happened among these geese on Gargano. It’s probably something that is happening within the flock, between birds of the same species, rather than defense against predators, because of what we can see in today’s biology.

Many of your chapters are set on the cusp of disaster, either right before or right after. Why are these events so useful when you’re looking for stories in the past?

Part of it is because they are incredibly important in telling the story of life. There have been several mass extinctions, and the way that life responded to them is very important — not just for telling us what’s happened in the past, but what’s going to happen in the future. Everything in my chapter on the Oligocene is represented as fossils in what’s called a lahar, which is what happens when a volcano has erupted and you have this layer of ash that turns into a sort of slurry, which goes down mountainsides at horrendous speeds and buries everything. There’s little chance for things to escape. In Tinguiririca [present-day Chile], we see the remnants of mammals in this lahar, so I’m talking very specifically about particular individuals.

At one point you describe one of the earliest species that might have been lost to our human ancestors, or hominins. How far back can you see those impacts?

Right, so this is about 4 million years ago in what is now Lake Turkana. This is sort of a cradle of African fauna. There are several species of relatives of elephants, and there are a couple of giraffe species, and there’s early wildebeests, antelopes, and the ancestors of the domestic cat. All sorts of creatures that are very familiar to us now.

There are these bear otters, which are lion-sized otters that used to live alongside early hominins. And they have no living relatives, so this is a group that has gone extinct. Some people have suggested — although this is a little controversial — that because they had a similar sort of generalist diet to humans, perhaps the bear otters were out-competed and essentially sort of lost their place in the ecosystem.

We’re not talking about Homo sapiens. We’re talking about three-foot-tall Australopithecines. They’re some of the first species that we can confidently say are on the human lineage.

When you begin to talk about humans directly impacting ecosystems, that comes much later. There’s good evidence for people managing fire and using fire to clear ecosystems and to change the forest layout tens of thousands of years ago. And even in what we would today think of as relatively undisturbed ecosystems, like the Amazon jungle, there’s been thousands of years of very active management by people. Even though it’s not been done in the sort of open plantations we’re used to in Europe and North America, it’s a landscape which has been highly modified by humans.

Throughout your book, you write about animals evolving to adapt to changes. Why can’t the natural world adapt to what’s happening to the planet now?

The simple answer is that it’s far too fast. Some degree of warming and cooling is absolutely a natural cycle, but the way we’re doing it now is entirely unnatural. When we talk about the changes that occur on geological timescales, they’re typically extremely slow. The fastest-known increase in carbon dioxide concentration is happening now.

When you get rapid changes in climate, however temporary, you often then get a big transition in what life is doing. At the end of the Permian, 250 million years ago, was what’s known as the Great Dying. It is the worst mass extinction that has ever happened. There was a huge outgassing of things like methane and other greenhouse gases from volcanic activity. In Siberia, 95 percent of life was wiped out by this radical change in global climate. There were huge problems with ocean acidification, with these gases going out into the atmosphere, and a loss of oxygen in the oceans. And a lot of these things are problems that we are seeing now.

Lucas Heinrich (cover design) and Chris Wormell (cover illustration) for Penguin Random House

“Otherlands: A Journey Through Earth’s Extinct Worlds” was published on February 1, 2022, by Random House.

Are there any other organisms that have changed the biosphere in the way that humans have?

Yeah, absolutely. One of the classic stories is the first photosynthesizers. Photosynthesis is the process that turns carbon dioxide and light into oxygen and sugar and energy. This was first done by single-celled organisms billions of years ago, and before then there wasn’t really much oxygen on Earth. When the photosynthesizers started producing oxygen, it completely changed the atmospheric composition. Most of the microorganisms that lived on Earth were not really able to tolerate oxygen, and so it caused problems for them.

More recently, 360 million years ago or so, we have the scale trees. This is in the period that’s called the Carboniferous, when we really get the first big plants. These scale tree forests formed in sort of swampy conditions, there’s a huge growth in plant material, they’re absorbing lots of carbon dioxide from the air. All of this carbon that was in the atmosphere was absorbed by growing plants, the plants that died fell into the swamp, and their bodies were converted into peat and then into coal and then buried, and so all of this carbon was captured. All of this served to change the global climate. It made the world cooler.

Very shortly after, you get what’s known as the Carboniferous rainforest collapse. The plants have changed the world’s climate such that swamps are no longer a common ecosystem, and scale trees and their kin go extinct. In a sense, they sowed the seeds of their own destruction.

How are humans different from the other organisms that have changed the planet?

Well, we as conscious beings are able to reflect on our actions. We are able to predict what the outcome of our actions will be, and therefore to choose an appropriate path. The first experiments that showed that carbon dioxide caused air to warm faster were done by a woman called Eunice Foote in the late 19th century, about two years before the first oil well was dug in the US. And for various reasons, despite having known about the warming effects of greenhouse gases for well over 100 years, little has been done so far.

I am always hopeful, though. There is now a movement to choose the right path and to recognize what we’ve already lost forever, and what we can salvage. Every day we go on without changing things, things are going to get worse, but there’s never going to be a single point at which all is lost. We can always, as a society, choose the right path.

In your last chapter, you write that human-induced change is not new, and can even sort of be considered natural. How is the intervention natural? And how should we think about it going forward?

It’s natural in the sense that we are part of the biological world and that we should not try and consider ourselves apart from it. We have been part of this world as a species for 200,000 years and as a genus for 2 million years. There are so many species that we have evolved alongside. We depend on that biological world that we are tightly integrated into. It’s a very unusual time in Earth’s history, in that all the ecosystems of the world, from the bottom of the sea to the tops of mountains, are affected by the actions of this single species.

Are there periods in the past that you think are particularly important parallels for us to pay attention to as we look to the climate-changed future?

If we’re talking about climate change, the important periods are the five major mass extinction events. The Ordovician is the only one which was caused by global cooling, and I think is an important parallel here. People have an assumption that warmth is somehow what is bad here. But in fact it’s not the warmth itself, it’s the rate of change.

At the end of the Ordovician, you get this onset of glaciers that expanded out across the whole of Africa and South America, which at that time were joined. And when that happens, we see a big extinction event in marine organisms as they are forced into deeper water, which perhaps they can’t survive in. But then you get this rebound, and the world begins to warm again. The ice begins to melt, and there’s a second pulse of extinction.

Earth has two roughly stable states. You’ve got the icehouse world, which we are in at the moment, where there is permanent ice at the poles. And then you have greenhouse Earth, where there is no permanent ice at the poles. Life is currently not really adapted to a greenhouse world. Humans — and I mean all hominins, all great apes — have never experienced the greenhouse world.

You write that we shouldn’t become despondent, which is easy to do when you’re faced with forces that affect life on a planetary scale. What should we do instead?

The problem with despondency is it leads to inaction. There’s a poem which I really like by Piet Hein:

Eradicate the optimist
who takes the easy view
that human values will persist
no matter what we do.
Annihilate the pessimist
whose ineffectual cry
is that the goal’s already missed
however hard we try.

The point is that we cannot sit back. It’s never too late. The sooner we act, the more we save, but there’s always something else to save.

If this is a mass extinction that we are causing right now, life will rebound and eventually be as diverse as it is today. But this is our world. We are here, and there are wonderful creatures around today. There are wonderful landscapes and wonderful plants. And I think it’s a shame to throw it away. If we undergo a huge period of transition, usher in a new age where life is fundamentally different, then we’re less likely to be a part of it. So we should protect the world that is our land, our part of geological time.