We humans have left a nasty mark on the Earth, no doubt. We’ve poisoned waterways, incinerated forests, and poured greenhouse gases into the atmosphere. We've critically endangered some species by demolishing huge swaths of their natural habitat, and greedily hunted others to extinction.

Many recent books, notably Elizabeth Kolbert's The Sixth Extinction, have documented the vanished species and explored the idea that we might be entering a new period of human-caused mass extinction — the kind of biological annihilation so colossal, so widespread, that it leaves an obvious scar on the fossil record. As Kolbert put in a recent conversation with Vox’s David Roberts, “the idea that we would eliminate many of the other species on Earth, including our closest relatives (we’re in the process of eliminating the great ape), is a pretty awful legacy.”

Chris Thomas has a somewhat different take. A professor of conservation biology at the University of York in the UK, Thomas does not deny that humans have already caused a "mini mass extinction" — an era of accelerated extinction rates around the globe.

But in his new book Inheritors of the Earth: How Nature Is Thriving in an Age of Extinction, Thomas argues that the usual doom and gloom is only one part of the story. As he sees it, in parallel with all the ecological damage, humans have ignited a great flourishing of life.

We have created scores of entirely new creatures, such as the hundreds of breeds of domestic cats and dogs living alongside us. We have intentionally and unwittingly transported all sorts of organisms — bacteria, ants, rodents, cattle, crops, garden plants, and roadside weeds — across the globe, to places they could never have reached on their own. We have formed numerous new environmental niches in our cities and suburbs, which many hardy critters have used to their advantage. And we have even spurred the hybridization of wild species, resulting in new chimeras, such as the Italian sparrow, yellow-flowered Yorkwort, and apple flies.

Ultimately, humans may be responsible for what Thomas calls a “sixth mass genesis” — a nearly unprecedented branching and blooming of life on this planet.

"The biological processes of evolutionary divergence and speciation have not been broken in the Anthropocene," Thomas writes. "They have gone into overdrive. … Come back in a million years and we might be looking at several million new species whose existence can be attributed to humans. … In the end, the Anthropocene biological revolution will almost certainly represent the sixth mass genesis of new biological diversity. It could be the fastest acceleration of evolutionary diversification in the last half-billion years."

For many conservationists, these ideas are highly controversial, even blasphemous. I spoke with Thomas to learn more about his book and his ostensibly contrarian claims. Our conversation has been edited for clarity and brevity.

Ferris Jabr

Your book has a provocative thesis. Tell us a bit about what inspired it.

Chris Thomas

It all started as a result of a very long process in which I'd been doing research on various animals and some plants in different parts of the world. I came to realize that almost all of the species I had been studying had managed to adjust to the new world that is so altered by humans. And that animals and plants in general are not lying down and waiting to be destroyed.

That is not of course to deny the many very dramatic and upsetting things people are doing to the planet, but to recognize that many species are managing to adjust to new conditions. I realized this story was coming out in dribs and drabs in the scientific literature, but it really needed a book-length treatise to bring it together.

Ferris Jabr

What is the sixth mass genesis?

Chris Thomas

The history of life on Earth is a history of extinctions and ecological failures, but it is also a story of formation of new forms and spread of those new forms around the world. The net result has been a gain in diversity. In the human era we are seeing great losses, but we are also seeing all these biological gains of new animals and plants spreading around the world, new hybrids coming into existence. I am not saying there is yet a balance between the two. I accept the losses, but it is also scientifically, and in terms of our human attitudes to nature, extremely interesting to contemplate the gains simultaneously.

If the processes that are going on at the moment continue for a very long time, it is my expectation that the number of species on Earth will grow enormously. We are moving species of existing animals and plants back and forth across the world, so that they are all arriving in new geographic regions. We know when species have done this in the ancient past, they have turned into new species in those different regions. If you fast-forward a million years or a few million years, all of these introduced species that leave surviving descendants will have turned into new species. And that is going to generate many more species. We have effectively created a massive species generator.

Ferris Jabr

One of the central tenets of your book is that certain species thriving in the Anthropocene will probably become the heirs of the planet. Can you give us an example?

Chris Thomas

One of my favorite examples is the house sparrow, which originally lived on rather prairie-like vegetation in parts of Asia and seems to have become associated with humans when agriculture was developed in the Fertile Crescent. The house sparrow [has] traveled around the world with humans ever since. A bird that started out in the steppes of Western Asia is now a worldwide species in the majority of habitations and towns across the world. Even if humans were to disappear, it's likely we would end up with populations of this species in every country.

It's starting to evolve in different areas as well. In colder areas it's a bit bigger; in Italy it met up with the Spanish sparrow and they hybridized, and there's now a new species called the Italian sparrow as a result.

Ferris Jabr

It's kind of like Darwin's finches on a global scale.

Chris Thomas

Yes, that's an analogy I like to draw. Some people talk about Earth being like a new Pangaea, where all the species are mixed up. But when we move sparrows to North America, or raccoon dogs from Eastern Asia to Europe, it's more like a giant archipelago. The populations of sparrows in North America can now go their own evolutionary way.

Ferris Jabr

In your book you argue that although many of the species that thrive in the human-altered world — rats, pigeons, so-called invasive weeds — are vilified, we should encourage their spread and increase their numbers. But why should we value these species over others?

Chris Thomas

If I gave that impression, it wasn't exactly what I was intending. There are species that are convenient to humans and species that are less convenient. We should be rather even-handed with respect to the origins of species when we make decisions as to whether we like them or not, whether we should encourage them or not.

Species are just species, wherever they came from. Throughout the history of life, all species have moved around the planet's surface, so that if you go back far enough in time you find that the ancestors of a certain species used to be somewhere else.

Ferris Jabr

But you do take the view, if I interpreted you correctly, that, as an inevitable consequence of humans having altered the planet so profoundly, a certain set of species — the sparrows, raccoons, deer, and domestic breeds of the world — will flourish and other species simply aren't going to make it. And you argue that in order to make our future ecosystems as robust as possible, we should benefit the survivors. They are the ones that will be the building blocks of future ecosystems.

Related One primate species is driving most of the others to extinction

Chris Thomas

I think what I'm saying is we shouldn't automatically knock them down. In some cases we might want to facilitate them, but the most robust things that can live in all sorts of places, we don't actually need to help them very much. They are doing perfectly well.

Normally when you bring species together in continental areas, all of the species survive. It's very unusual that one of the native species goes completely extinct. So you're getting more diversity. Where all of this goes wrong from a conservation point of view is, first of all, a perception problem. People are prone to equate change to loss. And they see the arrival of a new species as almost equivalent to a loss as well, because it represents a further departure from a previous state.

Ferris Jabr

Is it accurate to say that we are currently experiencing a massive loss of biodiversity, that we are poised to enter a sixth mass extinction?

Chris Thomas

The most commonly given definition of a mass extinction, that is one of the big five mass extinctions that have taken place in the last half-billion years, has been when 75 percent or more of species that have existed up to that time become extinct. In the 50-100,000 years since modern humans started to leave Africa, 5 percent of mammal species have gone extinct, whereas 10 percent of birds have disappeared because of humans.

So if somewhere on the order of 5 to 10 percent of all species have gone extinct, and we're aiming for 75 percent of species to classify as a mass extinction, then we are still a long way off. On the other hand, if we were to keep up the current rate at which we are killing species off, we would get to a mass extinction 10,000 years [from now]. And that is extremely fast. With the exception of the mass extinction that killed off the dinosaurs, that is much faster than any of the other extinctions.

“... we usually take a disliking to new hybrids, think they are somehow impure and try to exterminate them, when actually this is a natural process by which new genetic diversity is created”

Ferris Jabr

A lot of the chapters in your book focus on how human activity has increased the number of species in regional ecosystems as diverse as Belgium, Vermont, and Lake Maggiore in the Alps. But have humans increased biodiversity on the planet as a whole?

Chris Thomas

Since humans left Africa, the number of mammal species on the planet has certainly gone down; the number of bird species has certainly gone down; the number of amphibian species has certainly gone down; and the number of plant species has probably gone down. But we still have all the new species too. There is a very good argument that the rate at which new species are coming into existence could be higher than at any time in the last half-billion years.

Ferris Jabr

So right now there is a net loss of species due to humans, but at some point you see that trend reversing?

Chris Thomas

At some point, the lines of extinction and speciation will cross again. At the moment we are concentrating on reducing the rate of extinction of species, and I am all in favor of that.

But we are also in many respects fighting the origination of new species; we are doing things that slow that down deliberately. In particular I am thinking of the fact that we usually take a disliking to new hybrids, think they are somehow impure and try to exterminate them, when actually this is a natural process by which new genetic diversity is created.

[In his book, Thomas gives several other examples of disparaged hybrids: Rhododendron x superponticum, a hybrid of European and North American rhododendrons that is particularly robust and grows exceptionally well in the forests and heaths of Europe, hybrids of red deer and imported Asian sika deer in Scotland; and beefalo in North America, hybrids of domestic cattle and North American bison that are blamed for destroying native habitats and national parks.]

Ferris Jabr

That leads into a discussion of something at the heart of your book, which is when humans impose value judgments onto the natural world: when we try to decide which species we like — which are "good" and "bad" — and try to define what constitutes a healthy or unhealthy ecosystem. Ultimately these are arbitrary decisions. One could ask, for example, why biodiversity is such an important metric. Why should we focus on that?

Chris Thomas

There is a great argument running through the global shift in conservation thinking toward ecosystem goods and services, where more biological diversity is in principle associated with more good things we get from the Earth, everything from personal enjoyment to conditioning of the atmosphere, retaining soils, keeping water clean. Almost all of these good things we get from biological diversity we get from species quite close to us.

I like the idea that there are giant pandas in the forest in parts of China, but I don't benefit in terms of the direct ecosystems goods and services in and around myself from the fact that there is some rare species at the other end of the planet. The number of species in the region that you live in is much more relevant to the delivery of those good things we get from nature. And that is the element of biodiversity, how many species in each region, that seems to be going up around the world.

Ferris Jabr

But aren't there dangers in fixating on any one metric over another? Just because there are technically more species in a new area, that doesn't mean there are more individual creatures or that they are living well together.

Let's say you bring over five new exotic species to an island, and as a result of competition one native species is wiped out and a bunch of other native species diminish in population. So technically there are now more species on the island, but the number of individual creatures has gone way down.

Chris Thomas

Yes, there are many different metrics, and they are all valid in certain circumstances. At the moment some people are emphasizing metrics that appear to be going down, and I am emphasizing some that are going up.

Suppose we were to double number of plant species on the Hawaiian Islands, which is approximately what has happened. Then on average, the number of individuals per species is going to go down. That's an inevitable consequence. But we should ask ourselves why this is a bad thing. If we go to Amazonia, we find extremely large numbers of extremely low-density or rare species. When we go there, we think it's absolutely brilliant, the most biodiverse place on the planet. When the same change happens nearer to us, we take the perspective that it's a negative change to the region.

You're quite right to make the point that when new species arrive, they can change things, and sometimes a native species might die as a consequence. But in Britain, we've had nearly 2,000 species of non-native plants and animals established in the last couple of thousand years. Some species have become extinct over the same period but not because of the arrival of new species. Some have been hunted to extinction or their entire habitat has been plowed up. When new species arrive, what normally happens is not very much. They get added to biota of the region. And some species change in their abundances, but it is extremely rare for native species in continental regions to become entirely extinct because of new arrivals.

Ferris Jabr

One of the case studies in your book that I thought was especially interesting was the Monterey pine. You present this tree as an example of a once-rare species that became widespread due to human interference and discuss how this is both an extension of ancient evolutionary trends and a good thing. Can you tell us more about that?

Chris Thomas

The Monterey pine is just the sort of species that you might think is useless. It's a rather beautiful, novel pine tree that grows on a few rocky hillsides in California and on a couple of islands off Baja. And this species was taken by some foresters to New Zealand, where it grew spectacularly well, and they developed new races of it. Since that time, it has become the most important commercial forestry tree in New Zealand, Australia, Chile, and Argentina. It's actually endangered in California because of climate change, and under attack from a canker disease. But outside its native range, this is now an extremely successful species. Not only is it grown in plantations, it has also gone wild in some areas, establishing new pine forests in new parts of the world.

Here we have a species that starts off rare, becomes widespread in the world and simultaneously important to humans. And this is an example in recent times of what has long been the story of life on Earth, where the conditions on Earth change, particularly when climate changes, and what were previously rare species turn out to be the important species that construct the ecosystems of the next age. Indeed, all of the forests in Europe are constructed of species that have only migrated to those areas within the last 10,000 years.

Ferris Jabr

Another section of your book I really liked is when you start digging a trench in your backyard and inspecting the cake layers of geological history. You talk about how if you take a really big-picture view, the history of life on this planet is a series of massive extinctions followed by exuberant reflowerings of life, and that the types and numbers of species on Earth, as well as where certain species live, have changed continuously and dramatically over time.

Chris Thomas

When I dug my hole in the field, I could visit geological deposits from the last 15,000 years, going back to the Ice Age: There was a lake, and it was filled with one set of species; then it became sand dunes that had mammoths and woolly rhinoceroses; then it became forest, then it became farmland, and now it's my hay meadow; and with climate change, a whole new set of species has arrived.

We shouldn't take the existing state of the world, or how we imagined the world was a few centuries ago, as some correct state of the world. We are changing the world so dramatically, and species have moved into all sorts of new environments — well, that is what has always happened in the geological past. It's no longer realistic to think we can stop the change. It's like nature's healing process. When we see things changing, one attitude is, “Oh, the whole world is going wrong.” The other attitude is that nature is adjusting to a new situation.

Ferris Jabr

Or that nature is itself change.

Chris Thomas

Yes, exactly. You can't treat humans and nature in isolation. We should look on change for exactly what it is: change, not loss. We should ask is it objectively worse in some way than it was previously? Britain has 1,500 extra species of plants growing in the wild. Large numbers of conservationists regard that as a bad thing. I find it rather difficult to understand why having so many more wild plants is in any objective sense a bad thing. On something that I can actually measure, is it worse? Nature survives by being flexible and moving, and we should facilitate that flexibility if it makes ecosystems more robust and potentially more useful to us.

My bottom line is that there are biological losses going on on the planet. It is very rational to fight those losses, particularly when it is the loss of an entire species that may be difficult or impossible to get back in the future and which may have some unknown future value to us. But we are also living in a world in which there are biological gains. It is equally valid to celebrate biological life forms that are doing well in the presence of humans, rather than simply to resent these species and somehow prevent them from becoming the new biological success stories of the human epoch.

Ferris Jabr is a writer based in Portland, Oregon. He has written for The New York Times Magazine, Outside, Scientific American, and Slate, among other publications.