PDF Summary:Becoming Martian, by Scott Solomon

SpaceX and NASA intend to send people to Mars—with plans for rockets, habitats, food systems, and more. What those plans don’t include, evolutionary biologist Scott Solomon argues, is any serious discussion of what Mars will do to the people who stay there. In Becoming Martian, Solomon contends that Mars’s low gravity, intense radiation, and microbial isolation will set evolutionary forces in motion, pushing Mars’s humans to diverge from Earth-based humans until they become a new species.

In this guide, we’ll see that Solomon raises questions that science alone can’t settle, including what we’re trying to preserve by colonizing our neighboring planet and what we owe to the generations who’d have to live with the consequences. We’ll also look at what the science of isolated populations—from Italian wall lizards to Polynesian navigators—tells us about Solomon’s predictions, and why the split he describes would be different from every cultural rupture humanity’s survived before.

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Researchers in space bioethics have specified the problem that this legal gap creates for children: The first generation born on Mars would be involuntary subjects of the largest uncontrolled biological experiment in history, shaped by conditions no one yet knows how to make safe and governed by frameworks assembled without their input. Philosophers have long recognized the underlying puzzle—if a choice brings people into existence who wouldn’t have existed under any alternative, those people can’t claim to have been wronged by it, since without that choice they simply would not be. But this puzzle doesn’t absolve the responsibility of the people making the decision.

Why Evolutionary Change Is Inevitable

The conditions on Mars wouldn’t just make life difficult for the first generation of Mars settlers. Solomon contends that the conditions in a Mars settlement would set evolutionary forces in motion that could transform the humans living there into something new over the span of generations. In this section, we’ll begin with the basic mechanisms of evolution, then examine the specific biological changes that scientists think the Martian environment would drive, and finally trace the force that Solomon argues would push Earth and Mars populations past the threshold of speciation.

The Mechanics of Evolutionary Change

We often imagine evolution as something that happened to our ancestors a long time ago, but Solomon writes that biologists understand evolution as an ongoing change in the inherited traits of a population across generations. All evolution requires is that some individuals in a population survive and reproduce more successfully than others, and that they pass those advantages on to their offspring. In this sense, evolution is happening right now to every population of organisms (including humans), and biologists recognize several distinct mechanisms by which populations change over time.

The most familiar mechanism of evolutionary change is natural selection. When a heritable trait improves an individual’s chances of surviving or reproducing in their particular environment, that trait spreads through the population over generations, while traits that don’t provide an advantage disappear. Solomon emphasizes that natural selection can’t create new traits: It works with the genetic variation a population already has, filtering it in favor of whatever is useful as environments change. The greater the difference between a new environment and the one a species evolved in, the stronger the selection pressure and the faster the change. In an environment as extreme as Mars, the selection pressure would be intense.

The Gene’s-Eye View of Natural Selection

In The Selfish Gene, Richard Dawkins explains that genes replicate—that’s what they do, mechanically, generation after generation. Because replication is imperfect, variation occurs. Some variants produce organisms that survive and reproduce more successfully than others, and those variants get copied more often. Over time, the gene pool shifts. That’s what natural selection is: the outcome of differential survival. Dawkins suggests that this process is best understood not from the organism’s point of view but from the gene’s: Rather than casting genes as tools that organisms use to reproduce, he sees organisms as “survival machines” that genes use to replicate themselves.

That framing clarifies why the intensity of evolutionary pressure matters. On Earth, much of our genetic variation is neutral: Small differences between people don’t produce meaningfully different outcomes in a familiar environment. On Mars, that changes. A gene affecting bone density, radiation repair, or immune function will have clear consequences in an environment where gravity is 38% of Earth’s, and radiation hits at nearly full strength. More of our existing variation will matter, the gap between well-suited and poorly-suited organisms will be large, and the gene pool will shift correspondingly faster, even with no one intentionally directing it.

A second mechanism, genetic drift, creates evolutionary change through chance. In large populations, random genetic variations tend to cancel out, but in small groups, chance events can shift which traits predominate, regardless of whether those traits are useful. A small founding population carries only a fraction of the source population’s genetic variation, and whatever is missing from that founding group will be unavailable to future generations. This is why, drawing on research by space anthropologist Cameron Smith, Solomon estimates that a Mars settlement would need at least 10,000 to 11,000 people to preserve enough genetic diversity to remain viable—far more than most Mars mission proposals have envisioned.

Not Just How Many—But Who

The 10,000 to 11,000 figure Solomon calculates from Smith’s math is more of a floor: Smith ran the numbers on scenarios involving disease outbreaks, demographic crashes, and the hazards of genetic isolation (including genetic drift) and arrived at a figure closer to 40,000 as a minimum population size. This dwarfs the 100 to 200 settlers that SpaceX’s Starship is designed to carry on each of its eventual flights to Mars. But just as important as the question of how many is the question of whom. The logic of astronaut selection—draw from a narrow pool of the physically extraordinary—turns out to be backwards for a settlement. What makes an isolated population’s survival viable isn’t concentrated excellence, but breadth.

The inhabitants of a Mars settlement would need the widest possible range of genetic variants so future generations have more raw material to work with as conditions change. But that genetic variation isn’t evenly distributed across the population because every migration in human history has removed a subset of one community’s diversity and planted it somewhere new. The effect, over the past 60,000 years, is that people of African descent carry about 25% more distinct genetic variants per person than people of non-African descent. A population that could carry the broadest possible genetic toolkit to Mars would look very different from one assembled by screening applicants from countries that currently have space programs.

A third mechanism of evolutionary change, gene flow, depends on the movement of individuals and their genes between populations. When people from different groups have children together, their descendants carry genes from both groups. Gene flow keeps geographically separated populations genetically connected: Over time, it prevents them from diverging into separate species. When gene flow slows or stops, the populations drift apart. Distance, danger, and biological incompatibility have all been shown to reduce gene flow. Solomon argues that all three factors would come into play if we establish a settlement on Mars, constraining gene flow between the settlement and other humans on Earth.

(Shortform note: Solomon’s constraints on gene flow—distance, danger, and biological incompatibility—create a cascade where each stage sets the next in motion. When distance reduces gene flow, populations adapt independently, which makes moving between them dangerous, which renders interbreeding impossible. In Sapiens, Yuval Noah Harari explains that this is how multiple human species emerged as our ancestors spread out of Africa. Separation let populations diverge, and the intermittent interbreeding of Neanderthals, Denisovans, and early Homo sapiens wasn’t enough to arrest that divergence. On Mars, all three stages of this cascade would be in play from the very beginning, rather than unfolding over geological time.)

Considering all three of these evolutionary mechanisms, Solomon projects that the strong natural selection pressure of Mars’s extreme environment, genetic drift from the small founding population of a settlement, and its decreasing gene flow with humans on Earth would produce noticeable evolutionary changes in the population of humans on Mars within four or five generations. Those changes would compound into significant divergence between Earth humans and Mars humans within as few as 10 generations. By biologists’ standards for the timeline of evolutionary change, that’s an extremely short amount of time for such significant changes to occur.

Does the Evidence Back Solomon’s Timeline?

How confident can we be in Solomon’s projections? The empirical record on isolated populations suggests they’re plausible, even as they describe something unprecedented in human history. In what would become a dramatic demonstration of the potential for fast evolutionary change, five pairs of Italian wall lizards were introduced to an uninhabited island. They gave rise to a population that, within 30 generations, not only had larger heads and stronger jaws than their relatives on a neighboring (and very similar) island, but they also developed a digestive structure that didn’t exist in any of their ancestors. Something like this has happened with humans before—but never as completely.

Each time a human population has been isolated—such as small groups colonizing Pacific islands or populations stranded by glaciers during an ice age—has left a genetic signature. This shows that evolutionary changes do accumulate in isolated humans under the right conditions. What human history hasn’t yet produced is a situation in which all of Solomon’s pressures converge. Disparate Polynesian groups maintained trade networks, and Ice Age populations were reunited as glaciers retreated. Their isolation wasn’t permanent or total, and it didn’t occur in an environment that was radically different from the one they came from.

What Traits Mars Will Select For

Armed with an understanding of how evolution works and knowledge of Mars’s conditions, Solomon makes specific predictions about how Mars will reshape humans. Many of these changes would be driven by natural selection—the Martian environment will favor certain traits, and over generations, the population will shift toward whatever works best there. But other changes will emerge in a less predictable way: Radiation exposure will produce heritable mutations, giving natural selection more raw material to work with. Let’s look at each of the traits these forces would select for.

Denser Bones and Smaller Bodies

Some of the most significant changes will affect our bones. As we saw earlier, reduced gravity weakens bones over time. In lower gravity, where people lose bone density throughout their lives, natural selection will favor individuals born with naturally denser bones—those who can lose more before it becomes dangerous, and women whose bones can survive the demands of pregnancy and childbirth. Denser bones made of the same amount of tissue will be smaller, so Mars humans might gradually become shorter in stature.

The Genetic Raw Material

Solomon’s prediction that natural selection will push Mars humans to develop denser bones hinges on the fact that differences in bone density are substantially genetic, not just a consequence of lifestyle factors. Research places the genetic portion of bone density differences between 60% and 85%. This variation isn’t concentrated in one or two genes, but is distributed across dozens of variants, each contributing a small nudge in one direction or the other. That means natural selection doesn’t have to wait for a single lucky mutation to appear; it can work simultaneously on many small differences that already exist.

There’s a wrinkle that ties bone density biology to Solomon’s concerns about population diversity. The genetic variants associated with higher bone density (the ones that give the most buffer against bone loss) aren’t evenly distributed. People with ancestry in Sub-Saharan Africa carry these variants at higher rates than people of European or East Asian descent, and this creates differences in children’s bone development that hold true even when accounting for differences in diet and physical activity. This suggests that a genetically narrow Mars population would arrive with a lower starting point for the trait that matters most in low gravity, and with fewer of the variants that natural selection could work with.

Larger Skulls

Changes in medical practice will also shape selection. Because bone loss will make natural childbirth increasingly dangerous, cesarean sections may become standard practice on Mars. This carries an evolutionary consequence: If babies no longer need to fit through the birth canal, the constraint that’s historically placed on skull size disappears. Absent that limit, heads will evolve to be larger. But if skull size eventually outgrows what natural birth can accommodate, the entire population may become dependent on the colony’s surgical capability to reproduce. A subsequent disruption of that capability could have catastrophic consequences.

(Shortform note: Solomon is describing a shift that’s already begun. Research by evolutionary biologist Philipp Mitteroecker shows that the daughters of women who gave birth by C-section because their baby’s head was too large to pass through the birth canal are 2.8 times more likely to face the same complication in their own pregnancies. Before surgical delivery existed, a mismatch between the size of the baby’s head and the size of the birth canal was typically fatal to mother and child—a grim evolutionary check on the genes contributing to the mismatch. C-sections have removed that check, allowing those genes to spread more freely: Mitteroecker calculates that the rate of disproportion has increased by 10% to 20% since the mid-20th century.)

Radiation-Resistant Skin

Higher radiation levels will also drive natural selection to make changes to our skin. Solomon argues that humans on Mars will likely evolve enhanced protection against radiation damage—perhaps through increased production of melanin, the group of pigments that gives skin its color and absorbs harmful radiation, or possibly through the evolution of novel protective pigments that don’t exist in any humans today.

A Trade-Off That Doesn’t Exist on Mars

The scientific consensus is that skin pigmentation clearly illustrates how natural selection can shape a human trait. The pressures driving it are measurable, and the genetic signatures are exactly what the theory predicts. Dark pigmentation is our ancestral condition, protective against the intense UV radiation from the sun at the Earth’s equator. But melanin creates a tradeoff: By blocking UV at the skin’s surface, it also intercepts the wavelengths needed to synthesize vitamin D. As people migrated into regions with weaker sunlight, lighter pigmentation was favored because it let enough UV through for vitamin D production. The color gradient across humans is the outcome of these two opposing pressures.

On Mars, one of those two pressures disappears. Settlers living underground in habitats with artificial lighting wouldn’t rely on sun exposure to synthesize vitamin D and would face no counterpressure against protective pigmentation. Selection would have every reason to push toward darker skin, and none to push back. But there’s a complication worth noting: The radiation that makes Mars dangerous—cosmic rays and high-energy solar particles—passes through the body rather than being absorbed at the skin’s surface. Pigmentation changes would address part of the radiation problem, but the more penetrating threat would likely pull evolution toward enhanced DNA repair, too.

More Variation and Faster Change

Radiation will also steer change in a less-predictable way: by generating new variations for natural selection to work with. As NASA saw in its Twins Study, radiation damages DNA and produces mutations. Scott Kelly’s mutations occurred in his body’s ordinary cells and couldn’t be passed on. But heritable mutations, which occur in the cells that produce sperm or eggs, can be transmitted to the next generation. On Mars, with its much smaller population, heritable mutations will accumulate faster than on Earth, accelerating the pace of evolutionary change.

(Shortform note: It’s tempting to read “more radiation means more mutations” as good news for evolution—more raw material means quicker adaptation. But Daniel Dennett notes in Darwin’s Dangerous Idea that mutations are random and purposeless, and most are either neutral or outright harmful. He points out that mutations don’t “try” to be useful—the environment just sorts through them over generations, discarding the ones that don’t work. What higher radiation on Mars means, then, is higher stakes in both directions: more variation for natural selection, but also more broken proteins, developmental errors, and genetic damage, all in an environment with little margin for error.)

How the Microbiome Will Drive Divergence

Of all the forces that would reshape humanity on Mars, Solomon argues that the one most likely to push Earth and Mars humans past the threshold of becoming separate species is the disruption of the third evolutionary mechanism: gene flow.

The mechanism of that disruption will be how life in a Martian colony would impoverish the human microbiome, the vast community of bacteria, fungi, and other microorganisms living in and on the human body. These microbes help us digest food, regulate our mood, and train the immune system to identify threats. The health of a person’s microbiome is closely tied to the diversity of microorganisms they encounter in childhood. Broader exposure produces a better-calibrated immune system, one that knows what to attack and what not to. Too little exposure leaves the immune system prone to attacking the body’s own tissues.

On Mars, the microbial environment will be very sparse. The only microorganisms will be those that arrive with the settlers—a tiny fraction of Earth’s vast microbial diversity. Children raised on Mars will develop immune systems shaped by that narrow range of life. They’ll be without defenses against the wide variety of microbes that Earth humans encounter every day: the bacteria in soil, the fungi in outdoor air, and the microbes passed through ordinary human contact. For a person born on Mars, those microbes could be dangerous. For an Earth visitor to Mars, the reverse would apply: Microbial life that evolves in that closed environment will be foreign to immune systems trained on Earth’s diversity.

(Shortform note: That Mars would be a biological blank slate may seem strange from a terrestrial perspective. On Earth, biologists have found microbes inside Antarctic sandstone, volcanic vents, nuclear reactor cooling water, and deep below the seafloor. The working assumption is not that life occupies certain niches but that it occupies nearly all of them. Mars wasn’t always so different: The planet had water for hundreds of millions of years, and whether anything alive persisted after Mars lost its atmosphere and its surface grew hostile is unknown. The answer could change everything: Carl Sagan wrote in Cosmos that even a microbe would be enough: If anything lives on Mars, the planet belongs to it.)

The Cost of Microbial Poverty

The human microbiome’s impoverishment on Mars sounds like a purely physical problem that would make settlers more vulnerable to Earth’s bacteria, fungi, and viruses. But the consequences may run deeper. When a child’s immune system doesn’t encounter a diversity of microbes, it never learns what’s safe to ignore. The result is a system that stays on low-level alert, generating a persistent drip of inflammatory proteins even when there’s no threat. Under normal conditions, the brain is shielded from what circulates in the blood, but that barrier weakens under sustained inflammation. Once inflammatory proteins begin crossing into the brain, they disrupt the chemistry that regulates mood and motivation.

This isn’t a matter of the brain visibly swelling. The effect is more insidious and looks like depression: fatigue, flat mood, and difficulty sustaining effort. A 2024 study found evidence that the relationship between elevated inflammatory markers and depression may be partly causal (not just correlational), though the picture is still incomplete. There’s also a compounding effect for humans on Mars. If most births there become C-sections, as Solomon predicts, those children would miss even the initial microbiome a baby normally inherits during the passage through the birth canal. The confinement and isolation that Solomon catalogues as psychological hazards of Mars are real, but the biological foundation that helps people tolerate those stresses would also be a protection Mars can’t provide.

Solomon argues that this growing biological incompatibility will ultimately stop gene flow between the two populations. As generations pass and immune profiles diverge, contact between Earth and Mars humans will become ever riskier. Interplanetary reproduction will eventually become too dangerous to attempt, and without reproduction between populations, gene flow stops. Without gene flow, the forces of natural selection, genetic drift, and accumulating heritable mutations will operate independently on each group of people. Given enough time and separation, Earth and Mars humans will cease to even be capable of interbreeding. They will, in the biological sense, become separate species.

How Biological Incompatibility Would Look (or Smell)

When Solomon predicts that microbial incompatibility will stop gene flow between Earth and Mars, it's easy to picture this as a problem of safety, where contact becomes dangerous. That picture is incomplete. Reproduction requires the immune system to do something strange: accept what it would ordinarily reject. A fetus is half-foreign since it carries genetic material from the other parent, and the immune system’s job is normally to attack foreign tissue. To make pregnancy possible, the immune system has to stand down in precisely calibrated ways. When that calibration is disrupted, pregnancies fail at higher rates—miscarriage and implantation failure aren’t purely anatomical problems, but can be immunological ones.

Human reproduction appears to be shaped by the immune system even before conception. A cluster of genes called the major histocompatibility complex, which is responsible for the immune system’s ability to tell self from non-self, also seems to influence physical attraction through scent. Many vertebrates prefer the body odor of partners with immune profiles that are meaningfully different from their own, a bias that produces children with stronger immune defenses. Research is ongoing on how this signal works in humans and how directly the microbiome contributes to a person’s scent profile. For people living on Earth, those on Mars might not just be more difficult to safely reproduce with; they might even smell wrong.

How Can We Take Control — And Should We?

Solomon argues that evolutionary change in humans living on Mars will be inevitable—but it might not need to be entirely unguided. For decades, scientists have been developing tools to deliberately edit human DNA, raising the possibility that future space settlers could be biologically prepared for the conditions they’ll face before they arrive. This section examines what those tools could do, the ethical questions they raise, and the paradox that Solomon sees haunting the project: that whether humans on Mars change through natural evolution or deliberate engineering, the result may be the same.

What Genetic Engineering Could Do

Natural evolution works, but at a cost. It produces adaptation through survival and reproduction, meaning generations of suffering and death take place before beneficial changes spread through a population. Genetic engineering offers the ability to equip people for an alien environment rather than waiting for the environment to select among them. The most significant tool available is CRISPR, a molecular editing system that can locate a specific sequence in the genome, cut the DNA there, and disable a gene or insert a new one. Scientists have a growing list of genetic targets relevant to survival on Mars: genes involved in bone density, radiation resistance, and immune function.

The genetic toolkit of organisms that have already evolved to thrive in extreme environments may become the raw material for equipping humans to do the same. Solomon reports that one early result offers a striking proof of concept: When researchers inserted a gene from tardigrades—microscopic creatures capable of surviving the vacuum of space—into human cells, those cells became significantly more resistant to radiation damage.

From Water Bears to Whales: Borrowing Nature’s Answers

The tardigrade experiment sounds improbable—why would genetic instructions from a microscopic “water bear” work inside a human cell? The answer is that all known organisms share the same biochemical operating system: the same chemical letters and the same molecular grammar for translating those letters into proteins. This means a gene copied from one species is legible to the cellular machinery of another. Tardigrades produce a protein called Dsup (short for “damage suppressor”) that appears to intercept the kind of chemical damage that ionizing radiation causes. Experiments transferring the Dsup gene into human cells found those cells tolerated roughly 40% more X-ray exposure before breaking down.

Nature has been working on the problem of suppressing chemical damage across many different lineages. The puzzle of why large, long-lived animals don’t develop more cancers than small ones—despite having many more cells at risk—has more than one answer. Elephants accumulated extra copies of a tumor-suppression gene. Bowhead whales, which routinely live past 200 years, appear to have evolved cancer resistance through entirely different mechanisms that researchers are still working to understand. This suggests that engineering equivalent protection into humans isn’t a matter of finding the right gene to copy, but of understanding a problem that evolution has solved at least twice already.

Additionally, none of this translates into something deployable in a living person’s DNA, in part because genes don’t operate in isolation. The effect of editing two genes can’t be predicted by adding together their individual effects, and the interactions multiply with each edit. The math turns daunting quickly: The number of possible interactions among a genome’s roughly 20,000 protein-coding genes already runs into the hundreds of millions, and three-gene or four-gene combinations are orders of magnitude more complex still. Comprehensively modeling even a fraction of that would require datasets larger than the entire human population could supply.

Solomon says the ethical territory we’d enter by making genetic changes is fraught. Edits to the body’s ordinary somatic cells affect only the person treated. Edits to germline cells, which develop into eggs and sperm, are heritable: Every descendant carries the change. Germline editing is controversial because it means making irreversible biological decisions for people not yet born. The eugenics movement casts a long shadow over the idea of directing human evolution. Its proponents aimed to improve the species through selective breeding; the movement led to forced sterilizations in dozens of countries and contributed to Nazi policies. Others nonetheless argue that the obligation to preserve human life justifies genetic editing.

(Shortform note: Genetically engineering a Mars founding population would mean letting a small group of technologists, funders, and scientists decide which traits become permanent features of an isolated population. Bioethicist Rosemarie Garland-Thomson argues that genetic selection operating via markets and individual choice can create the same outcome as eugenics programs—narrowing the range of who gets to exist—when access is determined by wealth and power, as it would in a commercial Mars settlement. Whoever controls access to the program picks the founding gene pool. Add germline editing to that, and the same forces that decide who can afford a seat on the rocket will also decide what kind of people the future should contain.)

Where Scientists Draw the Line

CRISPR has already proven effective for targeted uses like treating sickle cell disease, where red blood cells form in an abnormal shape, blocking blood flow and causing pain and organ damage. To treat it, scientists extract a patient’s stem cells, edit the relevant gene with CRISPR, and reinfuse the corrected cells. This is possible because sickle cell is caused by one gene in one cell type. The traits that would help people survive on Mars—even the radiation resilience that scientists aimed to enhance by adding tardigrade DNA to human cells—aren’t controlled by single genes in specific tissues. They’re properties of how a whole body develops through the interplay of many genes across many cell types.

The only way to engineer such traits is to edit the genome before any of the body’s systems have taken shape. Jennifer Doudna, who co-invented CRISPR, also organized a moratorium on germline editing because we don’t yet have the tools to fully know (or control) what we’re doing. The problem is twofold. First, most genes don’t have just a single job: Edit one to perform better at one task, and you may impair something else it was doing. Second, CRISPR searches for a sequence in a genome three billion characters long and sometimes cuts at the wrong location. In a treatment for a single patient, a mistake can be caught. In an edit made before birth and carried by each subsequent generation, the changes travel forward.

To Be or Not to Be Human

Solomon contends that the deeper problem is that genetic engineering can’t resolve the tension between preservation and transformation, which he sees as the paradox at the heart of Mars colonization. Any adaptations that help people survive on Mars, whether via natural selection or deliberate genetic engineering, create a population that’s less biologically compatible with life on Earth. Genetic engineering might reduce suffering and accelerate useful adaptations, but it would simultaneously accelerate the divergence between populations. Adapting humans for Mars might mean, at the same time, making them into people who can never come home.

(Shortform note: Solomon’s paradox is a result of what historian Yuval Noah Harari calls “techno-humanism,” a belief that humanity should use genetic engineering and biotechnology to upgrade itself into something better. Harari coined the term “Homo Deus” (which literally means “god-human”) for the upgraded species techno-humanists aim to produce. He argues that the aspiration to preserve humanity by improving it is self-defeating, because each upgrade moves the species incrementally farther from what it was. Solomon’s contribution is to show that you don’t need CRISPR to trigger this paradox: On Mars, natural selection will do it just as well.)

Solomon’s conclusion isn’t that we should never go to Mars. The case for becoming a multiplanetary species strikes him as compelling—keeping all of humanity on one planet leaves us vulnerable. But he contends we aren’t ready yet: Too many questions about reproduction, child development, and microbial separation remain unanswered to proceed responsibly. We’re contemplating sending humans on a mission that would transform them permanently without yet understanding what we’d be asking them to become. In trying to preserve the human species by spreading it beyond Earth, we’ll guarantee that it will never be quite the same. That, Solomon suggests, is something we owe it to future generations to understand.

A Different Kind of Change

Human history has always involved change, but beneath the turbulence, one thing has remained constant: the biological compatibility connecting every human group to every other. The people who built Stonehenge and the people reading this sentence could, in principle, have had children together. That thread is what has let every generation, across every rupture, enter the same story. Solomon describes colonizing Mars as the first scenario where it might not hold.

Mars humans might be human in all the ways that feel essential—curious, moved by beauty, and shaped by the drive to make meaning. They might share our cultural memory, reading our literature and speaking languages descended from ours. But what they might not be able to do, after enough generations lived in isolation, is merge their futures with ours. This would be new. If we set that in motion, we may find ourselves asking questions we’ve never had to before: What do we owe to our descendants if they’re no longer quite our kind? And what does it mean to be human if “human” is no longer one thing?