When humans go extinct: How life will evolve after we're gone

The sixth extinction could wipe out up to half of Earth's species. Michael Tennesen tells us what might happen next

Published March 14, 2015 8:00PM (EDT)

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The sixth mass extinction is nearly upon us. Species on Earth are dying out at a rate one thousand times greater than they were before humans began altering the environment. By the end of this century, scientists warn, anywhere from 20 to 50 percent of the species on Earth could be lost forever.

And among those who might not make it out the other side, says science writer Michael Tennesen, are humans.

It's to be expected: No species lasts forever, and in our relatively short existence, humanity has done an impressively good job of undermining the forces necessary for our survival. But while we're already taking a number of species out with us, Tennesen argues, nature is resilient: the end of man won't necessarily mean the end of life itself. "Plants, animals, and microbes will survive, adapt, diversify, and proliferate," he writes of life after man. "New plants will evolve to vanquish our monocultures of corn, wheat, and rice. With far fewer animals around, those species that survive the bottleneck of extinction will move into newly abandoned spaces. With little competition, they will thrive and rapidly evolve."

We know that, he adds, because this sort of thing has happened in the past.

In "The Next Species: The Future of Evolution in the Aftermath of Man," Tennesen looks to the previous five extinctions for clues as to what we can expect from the sixth. He spoke with Salon about how recognizing our place in nature might help us last just a little bit longer, and how an eventual Earth without man, at least in our current incarnation, won't necessarily be such a terrible place. Our conversation, below, has been lightly edited for length and clarity:

A lot of us look at these studies about pollution and climate change and extinction on a very day-by-day, headline basis. What was the value for you of stepping back and taking a more pulled-back, planetary perspective on these issues?

I was influenced by a paper that Anthonky Barnosky from the University of California at Berkeley wrote, about his idea that we are entering a mass extinction event. People who study life on Earth think that extinction has a dual side: it could be a catastrophe or it could be an opportunity. The comet that fell out of the sky at the end of the Cretaceous period knocked out the dinosaurs, but made way for mammals and man.

So I’m trying to look at what can happen next. And to get an idea of what can happen next, I kind of had to pull back and look at the history of life on Earth with the idea: how does life recover from catastrophe? What things can you see in both events that might possibly be repeated in the future?  I wanted to look at the whole concept. There was a book by Allen Weissman, "The World Without Us," where he talked about what it would be like tomorrow if man disappeared and how long it would take for man’s infrastructure to come down, for New York to fall.  I just wanted to look at it from more of a reality standpoint: What would the biology be like in such an event?

When you’re looking back at some of these lessons we can learn from past mass extinctions, what are some of the most important things you came across, that we should be paying attention to?

If you look at the past, the driver of four out of the five mass extinctions has been carbon dioxide. I went to Guadalupe National Park and took a hike with the national park biologist Jonena Hearst to Capitan Reef, which was just this explosion of life that existed back in the Permian Era, 250 million years ago, just before the Permian extinction. It showed just how susceptible life is to chemicals in the environment, and the litany of things that was going on during the Permian extinction, which was the greatest extinction we’ve ever had: 90 percent of life was knocked out of the ocean; 70 to 75 percent on land. The high CO2 content and greenhouse gases and other problems -- sulfur dioxide release, major changes in the ocean currents -- these are some of the things we’re dealing with now. I don’t know if we’re going to be heading into that massive of an event, but there are lessons there. A lot of people want to go, "Well, what’s CO2? What’s the big deal?" It’s 400 parts per million. That’s a lot.

As you said, there is sort of a more optimistic way of looking at mass extinction, because there are some positive potential outcomes...

In an extinction event, you’ve got a new playing board. I went up to Mt. St. Helens and looked at the land around that volcano. They’ve actually separated a portion of the volcanic area as a natural experiment to see how life would come back. Nature actually does a pretty fabulous job pretty quickly.

I looked also at after the eruption of Krakatoa -- we’re talking the late 1800s, the whole island was almost leveled and it knocked out islands all around it, just a massive explosion. And yet today, not only have plants returned to Krakatoa and existing remains in that area, but so has wildlife. In about 150 years; that’s pretty amazing.

There would be a resilience to nature if man could just be a part of nature and the natural environment, rather than trying to be the dominant force on Earth. I guess my take-home message of the whole thing is that we can’t start thinking of ourselves as the be-all and end-all of natural history. We’re just a dot on a continuum that’s been ongoing for 600 million years. We really need to get a better perspective of ourselves. We are not the most important thing on the planet. We won’t last forever. Nobody I talked to thought we would. You, I and Homo sapiens have a limited life span, but if we could slow down and pull back a bit and start acting a little bit more like a part of life on Earth rather than the whole reason for its existence, we might just last longer and enjoy what we have left.

I wonder if that could be a different way of framing climate change, and other problems humans are causing. We like to say that we’re destroying the planet, but if nature is resilient, it’s more that we’re destroying ourselves. Earth is more likely to recover than we are.

We need Earth. We need nature. New York needs the microbes in the soil and the roots of plants and trees in the Catskill mountains to clean its water. The eastern coast of central America needs the coral reefs, the mangroves and the salt grasses to deaden the force of the hurricanes and the large waves that come in. Nature is really important for insect control: the birds and the bats have a lot to do with controlling insects and they’re a really important part of pollination. Nature actually plays a role in the creation of oxygen. We get a majority of our oxygen from the rainforest and the plankton in the ocean. There are a lot of really important things that nature does for us that we don’t respect.

And the reasons why we should respect them are, in a lot of ways, selfish.

Yeah, we're looking after our own health if we look after the health of nature.

You write about all these things that could potentially cause man to go extinct. Is there any, from what you've seen, that's most likely to hit us first?

People talk about the principle driver being climate change, but I really think it's population growth. I mean, in 1800 we had a billion people, and right now we have 7 billion -- only 200 years later. We’re expecting to have 9 billion by the mid-century and 10 or 11 billion out there by the end of the century. This has just happened in our lifetimes: the big growth was after World War II in the United States. It’s the driving force, but we’re not looking at it, and I really believe that climate change would not be such a horrible problem right now if we only had 1 billion people on Earth. It’s something that we don’t want to look at. That’s even more taboo than climate change.

The argument goes that that’s the underlying factor driving climate change, and also driving the emergence of infectious disease, right?

Well infectious disease, Rick Ostfeld at the Cary Institute of Ecosystem Studies has been studying how Lyme disease comes up in New England and the Northeastern area. He points out that what we’re doing is by eliminating species, we’re actually producing an environment that is conducive to disease. An environment that has a multiplicity of species has different carriers of the disease, some being good carriers like rats and chipmunks, while others like possums are not such good carriers. Essentially, by having the full conduit of species and by having a multiplicity of species, we dilute the transmission of disease. That’s another selfish reason to protect the rest of nature.

I was really intrigued by the part of the book where you talk about different scenarios for mankind's future -- you know, if we don’t go extinct, some other things that might happen. Did anything strike you as the most likely, or perhaps most enticing, scenario?

I had a variety of scenarios in the book. And you know, I’m kind of a nature lover, so the path that goes on after man actually sounds kind of good to me. There are a couple of tar deposits under Los Angeles where they have little representatives of what life was like 30 thousand years ago to what life was like 40 thousand years ago. People talk about what life was like before the coming of the Europeans, and I started running into more people who liked to look at life before the arrival of man. It’s really exciting to me, that L.A. could be a basin of mastodons and camels and saber-toothed tigers and things like that.

Then there's the Palmyra Atoll, which exists between Hawaii and Australia, so it’s way out in the middle of the ocean, and it's unoccupied. Today the Nature Conservancy actually protects it. It’s remarkable. If you run around with science journalists, everyone wants to go there, just to see what life would be like without the influence of man. I’d like to see that.

There’s also the idea of what could life be like if man continued? What would the next species of humans be? Genetic manipulation could produce a race of super-humans that would basically want to isolate themselves from us. That's a possibility. Then there are more fanciful things. I visited Oxford professor Nick Bostrom -- he had the idea that at some point, man is going to start uploading his mind into a computer. It would be based on the fact as you reach the end of life, there’s a way to extend it, the idea being that you could live on as either a robot or as an avatar in a virtual world. Second Life, where huge amounts of people spend 10 or 20 hours a week in virtual lives, is an example of that already. James Barrat with "Our Final Invention" talked about what could happen if A.I. could replicate itself or learned how to advance its intelligence on its own, and how soon it would be before it passed us.

Another big possibility is if we go to Mars. If we were to go to another planet -- and I chose Mars because it’s the most likely -- living there for a period of time would be conducive to developing into another species. You have one-third of the gravity you have on Earth plus you have to make your own oxygen. There’s high UV radiation and this can cause disruptions to the genetic system. These are all good ways to become another species. It doesn’t go beyond a few generations of life on Mars before you could have the definition of another species. It isn’t without reason that in a few generations, that could create another species of man that might come down and visit us again. And whenever we have the next species, part of Darwin’s idea is that one species develops out of the next one and then outcompetes the other.

You seem to feel that the most optimistic futures don't include man. Can you see any role for humans as we are now -- perhaps if there were just fewer of us?

Yeah, that would help. Toward the end of the book I talk about the fact that there have been some examples of that, some major changes. We did get rid of slavery. That was a major thing; we needed a major change of thought. Women’s suffrage is a major change in the way society runs. Today, our growing acceptance for homosexuality could also another change in society. So we could have a major change in society, but what it requires of us is to pull back from the dinner table, essentially. There can’t be as many of us. Maybe we could have 1 billion, and I don't know how that happens.

That’s the next question.

I think if we looked at it more often and if we studied it, if that entered the conversation, that would be a really good thing. And there’s promise in that. If we start really addressing population, whatever that means, and we actually start discussions of it, that would be a good sign.

By Lindsay Abrams

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