When space shuttle Atlantis rolled to a stop in 2011, it did not mark, as some worried, the end of human spaceflight. Rather, as the extinction of the dinosaurs allowed early mammals to flourish, retiring the shuttle signaled the opening of far grander opportunities for space exploration. Led by dozens of ambitious private companies, we are entering the early stages of the dispersal of Earth life into the solar system at large. The European Space Agency has revealed plans for a lunar habitat, and a Mars settlement is the stated goal of Elon Musk’s SpaceX company. The international research team Icarus Interstellar is taking this century to design propulsion technologies and great starships, ultimately to send an entire culture to a distant exoplanet. The human settlement of space is beginning now.

But technology is not enough. If space settlement is to succeed in the long run, we must consider biology and culture as carefully as engineering. Space settlement cannot be about rockets and robots alone—it will have to embrace bodies, people, families, communities and cultures. We must begin to build an anthropology of space settlement to grapple with the fuzzy, messy, dynamic and often infuriating world of human biocultural adaptation. And we must plan this new venture while remembering the clearest fact of all regarding living things: they change through time, by evolution.

Three main concepts shape current thought about space settlement. First is the colonization of Mars. Widely publicized by the peppery space engineer and president of the Mars Society, Robert Zubrin, Martian colonies would be self-sufficient, using local resources to generate water and oxygen as well as to make construction materials. Next is the concept of free-floating colonies—enormous habitats built from lunar or asteroid metals. Popularized by physicist Gerard K. O’Neill in the 1970s, these would house thousands of people, could rotate to provide an Earth-like gravity (as beautifully envisioned in the 1968 film 2001: A Space Odyssey), and could either orbit Earth or hang motionless at so-called Lagrangian points, spots where an object’s orbital motion balances the gravitational pull of the sun, moon and Earth. Finally, we might also consider the concept of the Space Ark, a giant craft carrying thousands of space colonists on a one-way, multigenerational voyage far from Earth. I have been working with the nonprofit foundation Icarus Interstellar to design just such a mission.

Each of these approaches has its merits, and I think they are all technologically inevitable. But we must never confuse space colonization with the conquest of space. Successfully settling space beyond Earth will be an enormous challenge. Space is vast and unforgiving. When humanity begins to make its home in space, it is we who will change.

The pioneers

Who will be the space colonists? Here we must ditch the old concept of crew selection and the comically diabolical tests of chisel-chinned space heroes depicted in The Right Stuff. Space colonists will be ordinary families and communities who will not be on a mission but who are intending to live out their lifetimes. We will need a few Captain Picards, although most early colonists will probably be farmers and construction workers.

Still, early colonists will have to be genetically healthy. In smallish populations, individuals carrying genetic maladies could threaten the future in ways that do not play out in a population of billions. In a Space Ark, the biological fate of the colony is strongly conditioned by the genetic constitution of the founding population—if just a few travelers carry the genes for inherited disease, these genes will spread much more thoroughly.

We now know the details of hundreds of genes that cause disorders, from cancers to deafness. (Researchers can already screen for thousands of such traits prenatally and during a person’s lifetime.) A genetic screening program seems clear—if you are carrying certain genes, you remain Earth-bound—but life is not so simple. Many maladies are polygenic—that is, the result of complex interactions among myriad genes. And even though one might carry the gene or genes for a certain disorder, environmental factors encountered during the course of life can determine whether or not those genes are activated in a healthy or unhealthy way.

For example, the human ATRX gene helps to regulate processes related to oxygen transportation. But ATRX activity can be altered by environmental influences as diverse as nutrient intake or a person’s state of mind. When ATRX function is significantly modified, oxygen transport is impeded, resulting in seizures, mental disabilities and stunted growth. Thus, one cannot simply screen out people carrying ATRX: everyone has it. In some people, though, based on poorly understood environmental factors, ATRX will go haywire. Can we deselect someone for space colonization for something that might happen? After reviewing this issue over the past few years, I’ve concluded that unless space-settling populations were to be very small, on the order of just a few hundred individuals, there would be no real need of genetic screening.

Complicating matters, we must also ensure broad genetic diversity of the gene pool. If all members of a population are genetically identical, a single sweep of disease could wipe everyone out. (This consideration demolishes the concept of a genetically engineered superrace of space travelers, as depicted in the 1997 film Gattaca.)

Once screened, what should be the population of space colonies? In a Mars colony, populations can grow and expand into new territory. But in a Space Ark, the population will be relatively low, and inbreeding becomes a concern. For example, in a study of Amish, Indian, Swedish and Utah populations, infant mortality was roughly double when matings occurred between first cousins than when they occurred between unrelated people.

Ship shape: Interstellar spacecraft will have to carry thousands of people, plus the plants and animals needed to support them. Credit: Steve Summerford Icarus Interstellar–Project Hyperion

To avoid these issues, we will have to consider the minimum population needed to maintain a healthy gene pool. Our minimum viable population has been much debated, but several anthropologists have suggested a figure of about 500. Because small populations are always at greater risk of collapse, in the past I have suggested founding populations of about 2,000 in a spacecraft that gives this population ample room to grow. For humans away from Earth, safety will indeed be found in numbers. (Even interstellar voyages will focus on reaching another solar system and inhabiting its planets, where populations can grow again.) Over the past year, my colleagues and I have used computer models and statistical investigation to verify these numbers. We found that multiple village-size populations, perhaps 1,000 or so each, could serve as a basis for even multiple-century interstellar voyages. Ten of these flying in parallel, connected so that people could visit other “towns” for friendship and to find mates, might be ideal.

We will also have to carefully consider the colonists’ demographic structure—the age and sex of colonial populations. Simulations by my colleague William Gardner-O’Kearney show that over a few centuries, populations that begin with certain ratios of young to old and males to females persist better than others.

Early colonial populations, then, should be individually healthy and collectively diverse to give future populations the best chance of having genes on hand that might be adaptive in new environments. But we cannot control everything. At some point we will have to roll the genetic dice—which we already do every time we choose to have children on Earth—and set out from our cradle planet.

Space-based selection

No matter how carefully we prepare our colonial populations, life off planet Earth, at least at first, will be more dangerous and perhaps shorter than life here. Away from Earth, people will be exposed to forces of natural selection that we have technically all but removed from modern life. Little of this selection will play out in the dramatic ways we might expect from science-fiction movies, which tend to focus on the lives of adults. Instead it will occur during critical periods of tissue development in embryos and infants, when life is most delicate.

How could such selection play out? As one example, consider that the human body has evolved close to sea level under an atmospheric pressure of roughly 15 pounds per square inch (psi) for the past several million years, breathing a mix of roughly 80 percent nitrogen and 20 percent oxygen. Yet space travel requires pressurized habitats that grow more expensive and laborious to build the more pressure they need to hold. To ease the engineering requirements, atmospheric pressure in any off-Earth structure will be lower than on Earth.

Fair enough—Apollo astronauts survived just fine at 5 psi—but if you lower atmospheric pressure, you must increase oxygen as a percentage of what you are breathing. (Those same astronauts breathed 100 percent oxygen on their lunar voyages.)

Unfortunately, lower atmospheric pressure and elevated oxygen levels both interfere in vertebrate embryo development. Miscarriages and infant mortality will rise—at least for a time. Inevitably, selection will preserve the genes suitable for extraterrestrial conditions and remove those that are less suitable.

Infectious disease—to which small, dense populations such as space colonies are particularly vulnerable—will return as a significant concern, imposing new selection pressures as well. However careful we are with immunization and quarantine, plagues will eventually sweep through colonies, resulting in selection for people more capable of surviving the disease and selection against those less capable.

Finally, we must remember that we bring with us thousands of domesticates—plants and animals for food and materials—and that selective pressures will act on them as well. Ditto the millions of microbial species that ride on and in human bodies—invisible genetic hitchhikers that are critical to our health.

Based on a few calculations, I think it is reasonable that within five 30-year generations—about 150 years—such changes will be apparent in the extraterrestrial human body.

Exactly what biological adaptations evolve will depend heavily on the atmospheric and chemical environments of the habitats we build. We can control these to a large extent. Yet we cannot easily control two other important factors that will shape humanity in space: gravity and radiation.

Mars colonists will feel just a third of Earth’s gravity. Over generations, without Earth’s gravity, a heavier build will probably be replaced with a lighter, lither Martian stature. In Space Ark and other free-floating scenarios, gravitation might remain about Earth normal, so Earth-normal statures might persist.

Radiation causes mutations, and any space colony will be unlikely to provide the protection from radiation that Earth’s atmosphere and magnetic field provide. Will increased mutations create physical errors—repeated parts like an extra finger or malformed parts like a cleft palate? Certainly, but we cannot know what kind. The only thing we can predict with confidence is selection for increased resistance to radiation damage. Some people have better and more active DNA-repair mechanisms than others, and they will be more likely to pass their genes on.

Could more efficient DNA-repair mechanisms have any visible correlate—such as, say, a particular hair color? Again, we do not know. But it is also possible for beneficial genetics to spread when they have no such visible correlates. Among Hutterites of South Dakota, who interbreed among a relatively small number of small communities, anthropologists have found that people appear to be strongly influenced in their mate choice by body aroma—and the better the person’s immune system, fascinatingly, the better the aroma.

On a moderate, five-generation timescale, then, human bodies will be subtly reshaped by their environment. We will see the origins of adaptations on the order of those of the natives of the high Andes and Tibet, where a more efficient oxygen-transport physiology has evolved, resulting in broader and deeper chests. Each alteration is a compromise, however, and these high-altitude populations also sustain higher infant mortality when giving birth at altitude. One cultural adaptation to this biological change has been for mothers to descend to oxygen-richer altitudes to deliver children. We can expect similar biocultural shifts off of Earth, and we should plan for the most likely of them. For example, on Mars, birthing mothers might shuttle to an orbiting station where delivery could happen in a rotating, 1-g facility with a more Earth-normal atmosphere, but I bet that eventually they would not bother and that distinctive Martian human characteristics would evolve.

A space-based culture

Cultural change will be more apparent than biological change on a 150-year time span. Studies of human migrations have taught us that while migrating peoples tend to carry on some traditions to maintain identity, they also devise novel traditions and customs as needed in new environments. For example, the Scandinavians who first colonized Iceland after A.D. 800 continued to worship Norse gods and speak the Viking language but quickly developed a distinctive cuisine—heavy on meat (whereas rye and oats were grown in Scandinavia) and on preserved foods to survive the harsh winters—as they explored the resources of an unknown land.

Space life: We cannot predict how the culture of a space outpost will change over hundreds of years—only that it will. Credit: Steve Summerford Icarus Interstellar–Project Hyperion

On Mars, this acculturation will play out in innumerable ways. There, in low-pressure, oxygen-rich atmospheres contained in unique architectural materials and arrangements, sound might propagate differently—even if subtly—perhaps affecting pronunciation and even the pacing of speech, resulting in novel accents and dialects. The lighter gravity could influence body language, an important element of human communication, and would influence performance arts of all kinds. Cultural divergence occurs as just such small, innumerable differences accumulate.

More profound cultural change could occur in Space Ark scenarios, where life would have less to do with Earth at each moment that the starship speeds away. Here basic concepts of space and time could well be transformed rather quickly. For example, how long would Space Ark cultures use Earth timekeeping? Without Earth’s days and nights and years, civilizations might invent a base-10 timekeeping scale. Or they might decide to count time down until a distant solar system is reached rather than up from some event in the past (such as the departure from an Earth to which they will never return).

Long-term genetic change

Significant genetic change occurs when new genes become widespread in a population. An example from prehistory is the spread of genes that resulted in lactose tolerance in adults, which appeared independently in both Africa and Europe not long after the domestication of cattle. This genetic equipment allowed more energy to be derived from cattle, and in these populations, it quickly became nearly universal, or “fixed.”

Although we cannot predict which mutations will arise, population genetics enables us to estimate how long it would take mutations to become fixed in the genome of space-based explorers. My calculations—based on model Mars populations of 2,000 people of certain age and sex structures—indicate that it could occur in just a few generations and certainly within 300 years; we can expect subtle original off-Earth physical characteristics in human populations on this timescale. These changes will be on the order of the broad geographical variation we see in humans today—variation in statures, skin colors, hair textures and other features.

On Mars, there might be further, internal divergence as some populations elect to live most of their lives sheltered in underground habitats, while others prefer to take the increased radiation risks to live in surface habitats offering greater mobility. In the limited-population, closed-system Space Ark scenario, gene fixation could happen much more rapidly, perhaps driving a greater uniformity than on Mars.

Whereas there will be some biological change, long-term cultural change will be more profound. Consider that in the three centuries from the early 1600s to the early 1900s, the English language changed so much that comprehending 17th-century English texts today requires special training. Three centuries hence, the language spoken on a Space Ark might be profoundly different.

Larger-scale cultural change is also quite likely. Exactly what divides one culture from another is a topic of tremendous debate in anthropology, but I believe that anthropologist Roy Rappaport made the distinction clear. Different cultures have different “ultimate sacred postulates”—core concepts, usually unquestionable and unquestioned, ingrained by tradition and ritual, that shape a population’s essential philosophical and moral codes. For Christianity, for example, one such postulate is that “In the beginning, God created the heaven and the Earth.” How long it will take for such foundation beliefs to change off of Earth—and in what direction—is impossible to say, but several centuries is certainly enough time to allow new cultures to arise.

The evolution of Homo extraterrestrialis

When will we see even more fundamental biological change—that is, speciation? Small populations can change quickly, as evidenced by the unusually large mice that roam the Faroe Islands 1,200 years after Viking ships dropped off ordinary house mice. But anatomically modern humans have gone more than 100,000 years—migrating from Africa into a wide variety of environments, from desert to open ocean—apparently without biological speciation. (Our nearest hominin relatives, such as the cold-adapted Neandertals and the apparently miniaturized “hobbit” humans of the island of Flores in the western Pacific, split from our common ancestor substantially earlier.) This is largely because we use culture and technology to adapt more than biology alone. It would take, then, significant natural and cultural selection to reshape extraterrestrial humans to such a degree that they could no longer productively mate with earthlings.

Unless, of course, humans devise their own speciation. It seems inevitable that off-Earthers will eventually harness the staggering power of DNA to tailor their own bodies for many conditions. Our distant descendants on Mars might biologically engineer gill-like structures to split the oxygen from atmospheric carbon dioxide—a process nasa is currently investigating—and skin tissues that could endure low surface pressure. They might tailor themselves into a new species, Homo extraterrestrialis, by conscious choice.

Where to begin?

Human space colonization will require plenty of engineering and technical advances. We must also improve our understanding of how human biology and culture adapt to new conditions and use that knowledge to help space colonization succeed. I suggest beginning immediately with three courses of action.

First, we must abandon the technocrat’s essential revulsion of humanity and begin procreating off of Earth, giving birth there and raising children there, to understand critical issues of human reproduction, development, and growth in new radiation, pressure, atmospheric and gravity environments. Bureaucrats will recoil at the risks involved—children exposed to risk beyond that of a bicycle-helmeted, First World suburbanite!—but concerns will diminish as space access is privatized. Still, at times the adaptation to space will be painful—but so is birth.

Second, we must experiment with growing and maintaining the health of domesticated species off of Earth. We are going nowhere without our microbes, plants and other animals.

And to promote these first two goals, an X-Prize should be awarded for the first functional, livable human habitat off of Earth: not a sterile orbiting laboratory (as important as those are) but a home where people can grow plants, raise animals and even have children. Many would shudder at the prospect of staying in such a place, but at the same time, there will be no shortage of volunteers.

Finally, we must reengage the proactive approach that has made human survival possible up to the present and use that capacity to shape our own evolution beyond our home planet. We must be immensely bolder than our bureaucracies. Failing that, in time we will become extinct, like everything else on Earth. As H. G. Wells wrote about the human future in 1936, it is “all the universe or nothing.”