The multibillion-year story of life on Earth is defined by a handful of epochal transformations, such as the emergence of the cell, the rise of multicellular organisms and the biosphere’s expansion from the primordial seas to the land and sky. Today, with life having crept into practically every possible niche on Earth, it may seem that there’s nowhere else to go. But there is, of course, still one place that beckons as life’s next evolutionary frontier: space. And humans are uniquely poised to propel this great transition by pushing out from our planetary cradle.

That’s the case Caleb Scharf makes in his latest book, The Giant Leap (Basic Books, 2025). Drawing on his expertise as senior scientist for astrobiology at NASA’s Ames Research Center, he argues that our modern spacefaring era is not just a matter of geopolitical competition and technological innovation but also part of a deeper, more fundamental evolutionary imperative. For the first time in Earth’s history, life—with human technology as its agent—can permanently expand from one lonely planet out into the rest of the solar system. Scharf calls this new trajectory the “Dispersal” and notes it’s becoming possible just as our ever growing dominion over Earth pushes the planet toward disruptive tipping points in climate change, biodiversity loss and resource use. This next great evolutionary transition, it seems, can’t come too soon—because the fate of life on Earth may ultimately depend on leaving our planet behind.

Scientific American spoke with Scharf about some implications of this cosmic outlook—from the physical limits of human expansion across the solar system to the dubious quest to build settlement on Mars and the political uncertainties that could stifle the Dispersal before it even starts.

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[An edited transcript of the interview follows.]

What was the genesis of this book?

I’d be dishonest if I didn’t admit that it partly came about because I wanted to nerd out about space. But the deeper genesis was about examining the question of what it looks like, and what it means, when life moves beyond its planetary origins. There are lots of visions out there for our future in space and what happens when we’ve built, you know, space elevators, and so on. They can get more and more fantastical. But they usually don’t tell us about the big picture; they’re attempts to imagine specifics of the future, which is really difficult, and so they’re probably going to get things wrong. So I wondered if, instead, we could look at what might be called the natural history of space exploration—of spaceflight as a sort of planetary phenomenon—and what that would tell us.

Let’s talk about the Dispersal. What is that, exactly?

As an astrobiologist who spends my days chewing over the nature of living systems at many different levels, in both abstract and practical terms, I’m coming to this from a much broader perspective, thinking of our unfolding space age as another sort of evolutionary leap. Forget, for a minute, about the specifics of humans or even the specifics of how we’re getting to space. If you think about it, gravity is glomming stuff together all across the universe to make stars and planets—and eventually more and more complex chemistry. And, at least here on Earth, life arose and evolved. And now, billions of years later, we’ve reached a point where our planet is throwing some sort of perverse tantrum and tossing—dispersing!—materials, machines and organisms back out into space. And I’m interested in extrapolating this process and examining how far it can go.

So the Dispersal is about what might happen to us, and all life, when it encounters scales of space and of resources that are unimaginably large. And just as you can imagine speciation happening when organisms are suddenly scattered across different terrains—like the Galápagos Islands, for instance—when life disperses beyond Earth, it can follow increasingly divergent trajectories. This means, among other things, that we’re not necessarily talking about modern humans anymore in our extrapolations but whatever comes after us. It can seem kind of terrifying and awful that we might not fully recognize or identify with future “dispersed” versions of us, but those transformations aren’t necessarily bad things.

That means, I guess, that whatever the Dispersal is, it’s not only about rocket science—and it’s not really about fantastic visions of space elevators or warp drives or the specific details of any posthuman future either. One thing I like about the book is how you’re approaching this huge topic from a “first principles” perspective, initially focusing on fundamental things such as orbital mechanics that set various sorts of boundary conditions and shape the landscape of possibilities that may exist for us beyond Earth.

Yeah, this concept of boundary conditions is really important—the idea that certain phenomena can only happen within certain constraints, beyond which they can no longer take place. You need that to scope out the “landscape,” as you called it. Imagine a continent where humans have never set foot, and you’re trying to predict what’ll happen when they do—you might say, for instance, they’ll build a city, and that city will have a subway system. But if the whole continent is a waterlogged swamp, no one’s going to build a subway there, and the planning for any city will have to account for the swampy conditions, and so the possibilities are different. It’s the same for the solar system.

There’s a chapter in the book talking about all this, looking at the range of possibilities and challenges that exist for places close in to the sun, such as Mercury, to the other extreme, far-out places such as Neptune and even further afield. The obvious difference is sunlight, which is about seven times stronger at Mercury and more than 1,000 times fainter out at Pluto than at Earth. Usually, when an astrobiologist like me is talking about this, it’s in the context of habitable zones, the physics- and chemistry-based potential for a world to have conditions such as an atmosphere, surface liquid water and other things that would allow life as we know it. But there are additional “zones” to consider if you’re thinking about how technological life might disperse itself in a planetary system. They are similarly related to availability of sunlight, of energy, but also things such as how much radiation you’re exposed to and how easy any destination is to reach.

It’s actually really difficult to get to Mercury, for example, because it’s so deep in the sun’s gravity well—the amount of energy you need to get there from Earth is on par with what you need to get to Jupiter and beyond. And because Mercury is so much closer up to the sun, you can be more vulnerable to nasty flares, and so on, whereas the further you get from the sun, the less you have to worry about solar activity but the more exposed you’ll be to cosmic radiation, which has its own hazards.

Accounting for all these, you might imagine a “zone of easiest exploration” that will help you understand where we’re likely to go elsewhere in the solar system and how we’ll get there. That’s how you get clues to what the Dispersal will look like.

So, for most folks, this may be a really novel way of thinking about exploration and expansion in the solar system. But of course, for space nerds like us, lots of old, classic factions and debates pop out of this new framework.

For instance, there’s the choice of prioritizing sending humans to the moon—as the U.S. and other nations are planning—versus doing crewed missions to Mars. And there’s also an argument that humans shouldn’t be going to planets and moons at all and should instead stick to building space stations and other sorts of artificial orbital habitats. You discuss all this at length in the book, but I had trouble discerning which route you actually prefer.

Well, in writing the book, I came to the conclusion that planets can be a real pain in the ass!

We definitely need to study Mars and the moon, and maybe some of us should even try to live there. I mean, there’s all sorts of genuine reasons for doing that. But in the long term, it’s far better to engineer what you really need—to create environments that place fewer stressors on life that evolved on Earth over four billion years. Even if you build fabulous habitats on the moon or Mars, you’ll never have Earth-normal gravity in those places, for example.

Whether you make use of a natural object such as an asteroid or just construct an entirely artificial habitat out there, both would give you far more options than a planetary surface. You can spin your habitat to get Earth-like artificial gravity; you can engineer a lovely atmosphere that precisely fits our biological needs. You can set up seasons, choose where to orbit and have robust protection from cosmic radiation.

Relatedly, I get the sense you’re not enamored with Elon Musk’s dream of building cities on Mars. You even write in the book that if the rationale for Mars settlement is to safeguard humanity against existential risks, it would probably make more sense to skip Mars entirely in favor of building settlements deep underground on Earth. Why don’t you find the “occupy Mars” arguments convincing?

I think it’s too easy to be the sort of the space enthusiast who sits in an armchair puffing on a cigar and saying, “Oh, yes, of course we’ll go conquer and colonize space.” Apart from feeling discomfort at all the sociopolitical baggage and privilege associated with that attitude, I think we simply can’t afford to be so blinkered and blasé—because these are unbelievably difficult and expensive things to do. And they can seem extremely disconnected from the problems that most humans face every day. So you’ve got to have a really sensible, sane, reasonable motivation and explanation for why anyone should want to do this, and I don’t think building a city on Mars is sufficient.

Mars is a very interesting test bed for querying why we humans today, amid all our problems, should be thinking about any of this stuff. But I find today’s focus on the Silicon Valley–style “let’s make a backup for humanity” outlook lacking. The way it’s presented reeks of a utopian “we know best” attitude—“We’re going to build some glorious new society on Mars!”—when, hey, living on Mars would probably be pretty dystopian no matter what.

At the same time, yeah, no one with a big-picture view would seriously argue that life on Earth isn’t subject to a variety of existential risks. So of course it makes sense to have some sort of backup—and one way to do that would indeed be to create a self-sustaining permanent settlement on Mars populated with enough people to avoid everyone becoming inbred idiots. But I don’t think this can be the sole motivating factor.

So what’s your preferred approach to Mars, then?

In the book I tried to examine Mars exploration by ideological camps. There’s the stance we just mentioned, you know, of “let’s immediately get lots of humans to Mars as an existential backup. To hell with the Martian environment or anything like that; this whole planet is just there for us to conquer and take.” Then there’s the pure science camp that says we really should be incredibly cautious but not entirely hands-off about sending humans or anything else to Mars, because we don’t want to disrupt this near-pristine environment where there might be clues we simply can’t get on Earth to the origins and fundamental nature of life. And then there’s this sort of “Earth first” extreme, which says even the scientific stuff is too expensive, and space exploration as a whole is diverting attention from all the very pressing problems we have right here on Earth, so we should just stop doing it. Most debates about space policy tend to revolve around these views and usually end up in a stalemate.

I advocate for a fourth kind of approach that, in some ways, is about reinventing how we do human exploration. We know how to “connect up” and digitize a planet with sensors and satellites, and so on, because we’ve done that on Earth, and we’re learning how to work with our machines in new, incredibly tight-knit ways. If we really put all that to work on Mars, I think there’s an opportunity to do all the things we want and to learn new ways to exist.

And I guess the reason this same logic wouldn’t apply to, say, the moon is that, in comparison with Mars, it offers dismal prospects for breakthroughs in our search for the origins of life.

Right. The moon is also incredibly interesting and beautiful, and it’s certainly much more accessible in terms of its proximity to us. And there are resources there such as water ice that could support interplanetary exploration needs. We could also learn about processes of biological contamination there. But the moon doesn’t quite tick all the boxes the same way as Mars does, yeah.

So, you know, it’s not that the moon should be ignored—and it may play a pivotal role in the next several decades—but it’s definitely a downgrade from Mars in my opinion.

I ask about the moon for practical reasons. As you mention, it’s much easier to reach, so in some respects maybe it’s like a set of training wheels for more ambitious trips to Mars, and so on. This is, of course, another resurrection of a timeless debate—some people would say it makes more sense to go to the moon first before making the bigger leap to Mars.

That is to say, it feels like, in the book, you’re laying out the wonders of the solar system— “look at all these amazing worlds that await us!”—without much focus on what the path could be to get there. You’re talking about the boundary conditions that exist, and the resulting zones where we might explore or even live, but less about the plausible routes by which those situations can become reality.

You mentioned evolutionary leaps earlier, and I guess that’s what I’m getting at: it’s not really clear to me how we’re going to go from our comfortable, well-adapted lives here on Earth into, say, dwelling in city-sized space stations carved out of asteroids between Mars and Jupiter. That seems like, well—a giant leap!

That’s a good observation. And it comes back to me not wanting to fall into the trap that’s gotten so many people before, where someone will declare, “Clearly, this is the trajectory that things are going, and so this and that will absolutely happen!”

Imagine, for a moment, being back in 1968, watching three NASA astronauts on their way home to Earth after orbiting the moon for Apollo 8. You’d think that the future was becoming clearer, right? We were about to send more astronauts back there—to land. Maybe the Soviets would do it as well. There’s going to be a lunar land rush, passenger flights to the moon and a flurry of new rockets and space stations, and then we’ll be going to Mars in the 1970s. But of course, the complicated reality of human civilization got in the way, and most of that stuff didn’t happen. We left that imagined trajectory for a different one—if we were ever really on it in the first place.

It’s undeniable that “space” is at another inflection point today. We’re rapidly approaching the point where there will be at least one rocket launch per day into orbit, which is astonishing. There are now around 10,000 space-oriented companies around the world, which definitely wasn’t the case 10 years ago. And I think it’s fair to say even the growth in the value of Earth data obtained from space has shot up. So all the curves seem to be following this exponential upward slope. That’s not a guarantee it keeps going. Regression is possible. Or maybe just a flattening of the curves into a plateau might happen—which would still be interesting because then the question would be: Is there still something in the future that could occur to create more exponential growth?

I don’t know if that gets to your question exactly, but I feel there’s sufficient evidence that something new is happening right now that we should pay attention to—and it may presage the beginnings of the Dispersal. The point is: I don’t think it’s likely that we’ll just repeat history, right? There will be surprises. It’s no longer as simple as it was in the Apollo era, when you could really see most everything through the lens of “the U.S. versus the Soviets” in space.

The situation we have today is a much richer, more complicated set of motivations, actors and capabilities. So it becomes more relevant to consider the ultimate end points in those vast scales and resources of the solar system and what those limits might mean—rather than the limits of earthly thinking about what will happen next week.

Overall, it seems you’re presenting a pretty optimistic big picture of where we’re going from here. But to zoom in on smaller details for a moment, you’re also saying this while being furloughed from your NASA work because of the latest U.S. government shutdown while the space agency is facing steep budget cuts and workforce reductions—things that I’d imagine someone like you would be pessimistic about. How do you reconcile this?

Let me start by saying that finding the bigger perspective always helps in times of uncertainty! In that context, I think the Dispersal is a hugely positive idea, with relevance no matter what the current earthly circumstances are. What life often does when it disperses is find opportunities to do better. The human species is no different. And the universe seems to be tilted towards making that happen. So, to some degree, I think it’s out of any one group’s control.

Space exploration is happening; there’s just a critical mass of financial interests, of people’s pet interests, of individuals with resources who are interested in this, of countries that still see the enormous value in having access to space for all sorts of reasons—economic reasons, security reasons or maybe just for national pride. With all these players, I think we’re crossing a threshold that we didn’t quite get to in the 1960s and 1970s. And having so many players in the mix also means that who gets to do what first is an open question.

That is relevant to your point about NASA—I’m speaking personally here and not for the agency in any way. It’s irrefutable that the pioneering efforts of NASA have been key for getting space exploration to where it is today. We’ve learned that when you solve the hard problems of space with long-term investments in science, engineering and people, it paves the way for others to further innovate. That’s great because the hard problems will keep coming. We just need to figure out how to keep this special recipe working.

Where I’m not optimistic is the idea that we’ll get together as a species and draw up some unified master plan for humanity and space. Forget it. That’s not happening, right? But I think that’s okay. What we’re going to see are multiple ideas and efforts and innovations all at once at a level that hasn’t existed ever before. And in retrospect, it makes a lot of sense that it’s happening like this rather than how it looked like it might for most of the 20th century. Throughout human history, there have been moments where advances just “come together,” whether it was the printing press or mass transport or telecommunications or computing and the Internet. These are invariably fueled by science, as well as by competing interests in realms of economic or commercial opportunity, driving investments in multiple places.

I feel like that’s where we’re at now with space. And that, to me, is new and pretty amazing.