Biological Evolution in Interstellar Human Migration
Centauri Dreams is happy to welcome Dr. Cameron M. Smith, a prehistorian at Portland State University’s Department of Anthropology in Portland, OR, with an essay that is the capstone of this week’s worldship theme. Dr. Smith began his career excavating million-year-old stone tools in Africa and today combines his archaeological interests with a consideration of human evolution and space colonization. He is applying this interest in his collaboration with the scientists at Icarus Interstellar’s Project Hyperion, a reference study for an interstellar craft capable of voyaging to a distant star. Recently Dr. Smith presented a paper at the NASA/DARPA ‘100 Year Starship Study’ conference in Houston, Texas. His recent popular science publications in this field include “Starship Humanity” (Scientific American 2013) and the book Emigrating Beyond Earth: Human Adaptation and Space Colonization (Springer-Praxis, 2013). We can look forward to a follow-up article to this one in coming weeks.
by Cameron M. Smith
1. Interstellar Migration: An Insurance Policy for the genus Homo
Planets orbiting distant stars are now being discovered at a rapid pace, with hundreds known and countless worlds implied. As an anthropologist, I take a wide and long-term look at human evolution, and this development is very exciting to me; those almost unimaginably-distant planets are where humanity is headed, in the longer or shorter term. Humanity has been characterized by spreading itself wide across the Earth, and after first colonizing Mars we will surely wish to go farther, just as, in Polynesian legend, the siblings Ru and Hina—having explored the whole Pacific—chose to build a special vessel for a trip to the moon. Although civilization as practiced so far is perhaps its own worst enemy, I am optimistic that humanity’s better side will generally prevail, and that our species will invest in space colonization as an insurance policy for our lineage. Figure 1 indicates the five most recent mass-extinction events in Earth history, and Figure 2 indicates that even in recent times, civilizations have repeatedly collapsed and disintegrated, with no guarantee of recovery [click on figures to enlarge as needed]. In the larger picture, over long time, space migration is the best means of surviving such disasters.
Recently humanity has spent just over a generation exploring the solar system just beyond our atmosphere, sometimes with robotic voyagers, and sometimes with our own bodies; for the past 23 continuous years human beings have already lived off of the surface of the Earth in various orbital stations; cosmonaut Sergei Krikalev has spent over 800 days in space, and his colleague Valeri Polyakov once remained continuously in orbit for well over a year. Despite some close calls, nobody has died in this nearly quarter-century of continuous space habitation, which has taught us some basics of space biology. Clearly, our species’ technical capacities are superb, and the essential technologies for long-term stays in space are being sketched out. We also have realistic destinations in view, and highly-focused, forward-thinking physicists and mathematicians working on how to reach those destinations with unique propulsion systems.
Will developing space technology be enough, however, to pave a way to space colonization? It will be part of the equation, but the equation is not complete. Space colonization will be about humans living off of Earth, but human biology off of Earth is only barely understood, and only in the cases of the individual physiology of adults, for short periods of the entire life course. Space colonization, however, will be about humans living out entire lifetimes—from fertilization to embryo development and so on through adulthood—and in populations (rather than just individuals) and populations over multiple generations, not short stints in Earth orbit. For these reasons, at least, we need an anthropology of human space colonization. Anthropology studies the biocultural evolution of our lineage and some of our closest relatives. In this article I would like to introduce some of the biological issues involved in space migration, specifically among multigenerational, interstellar voyages. In a second article I will introduce cultural evolution in such vessels.
2. Where to Begin?
Space colonization will be a continuation of human evolution. I don’t mean this in the shallow sense of a ‘March of Progress’, but in a deeper, more mature and more useful way. Humans are life forms, and thus we change through time, both biologically and culturally. When such change allows us to live in new places—for example, the Canadian Arctic (over 5,000 years ago), the islands of the Pacific (over 3,000 years ago), or Earth orbit—it is called adaptation. Space colonization, then, will be an attempt at adaptation, and if we want to succeed in this adaptive endeavor we should be informed with everything we know about evolution in general and adaptation in particular.
Generally speaking, while it is clear that humanity has adapted to many ecological niches over time, it is equally clear that our adaptation differs significantly from that of most other life forms. This is because humanity adapts more culturally and technologically than biologically; witness our essential biological similarity across the globe, despite a few regional variations in skin color, hair texture and so on. Rather than adapting largely by body, then, we adapt largely by mind, which generates complex behavior (such as adjusting our kinship rules and sex taboos to match our distribution across landscapes) and complex technologies (including everything from inventing sailing vessels and stellar navigation in the Pacific to walrus-hunting watercraft and harpoons in the Arctic) allowing our species to flourish in places never dreamed of by our earliest bipedal ancestors. Having said this, humanity continues to evolve biologically even today, and will continue beyond Earth, and that evolution must be considered.
While technology is a cultural invention, in this article I will leave the technologies of space colonization to the engineers, and focus on the biological issues involved in interstellar voyages. These have been sketched out before in disparate literature; here I would like to update the material and consider it specifically in the case of current draft studies of interstellar vessels.
3. A Thoughtscape for Planning Interstellar Voyages
Current plans for interstellar voyaging essentially envision a gargantuan starship hurtling on a one-way voyage from Earth towards a distant star system. Interstellar craft would have to be very large—perhaps kilometers on a side—to house populations in the thousands, as I will discuss below. The most distinctive characteristic of these colony ships is that they would essentially be closed systems, with no opportunity for bringing in new genes, maintenance or repair materials, or consumables, such as water or breathing gases. These ‘Space Arks’ would have to be regenerable and self-contained, a fascinating challenge not just in engineering but also concerning biology and culture.
Presuming that we could build such vessels, the main engineering challenge is going to be propulsion (an issue being tackled right now by the scientists at Icarus Interstellar). Our galaxy, the milky way, contains an average density of .004 stars per cubic light year, and the closest star to Earth, Proxima Centauri, is 4.2 light years away. At light speed that is 4.2 years of travel time, but today, attaining that speed (or, technically, something very close to it, as light speed is not thought to actually be attainable by anything other than individual photons or similar particles) is unrealistic. How fast can we go? The fastest humans have moved is about 24,790mph (just over .00003% the speed of light), on Apollo X in 1969. At that pace, Proxima Centauri is about 140,000 years away. Maybe we can do much better, though. In the last 150 years we have increased human travel speeds almost a thousandfold up from the locomotive’s 30mph (the Voyager spacecraft speed along at about 1,000 times the speed of a locomotive). If we can do something similar in the near future, travel times become significantly more manageable. A 100-fold increase in speed from Apollo X (bringing us to just under 3,000,000 miles per hour) would take us to Proxima Centauri in something under 1,400 years. This is getting manageable, but is still a long time; imagine launching the ship during the Dark Ages (just after the collapse of Rome, about 1,400 years ago) and having it arrive at its destination in the time of such oddities as desktop computers. If, however, we manage to make a 1,000-fold increase in speed (bringing us to about 30,000,000mph) in the next century or so, then the heavenly wonders of Proxima Centauri (actually we don’t know what it is like there, yet, but I am being optimistic) could be achieved in about 140 years (see Figure 3). This is a very manageable timescale, a distance only that from today’s world to that of 1872–less than five generations distant and essentially comprehensible.
This, then, is the thoughtscape I am exploring in this article; voyages on the order of under two centuries, carrying some thousands of people (a number discussed below) before reaching another solar system. What biological and cultural issues need to be addressed on such voyages? While the answer will be some time coming (I’m currently investigating it in research for my forthcoming textbook on the anthropology of space colonization), I can sketch out some below.
4. The Biology of Interstellar Voyaging
While evolutionary biology is currently undergoing significant revision in the light of new genomic data, the principles of evolution are largely intact. These include the four main processes (genetic mutation, selection, migration and drift) that change the properties of the gene pool over time. Each is discussed below.
4.1 The Biology of Interstellar Voyaging: Mutation
Mutation is ultimate source of new variations, such as coarser hair, or darker skin than one’s fellows. In common speech a mutation is thought of as being deleterious, but scientifically speaking mutations could be advantageous, disadvantageous or neutral. Some mutation is a result of radiation, and interstellar voyage designs will have to consider trapped particles, radiation energy near Earth and constrained within the Van Allen belt (and presumably about other planets with analogs of the Van Allen belt; cosmic rays derived from many sources outside this region, and solar radiation blasts out of suns. Radiation can degrade biological tissues and cell functions, and it can also be a powerful mutagen, altering DNA; recently it has been suggested that cosmic radiation could increase the incidence of brain disorders.
Certainly radiation is an issue to be addressed, but it does not seem to be a showstopper for human space colonization. First, many shielding schemes have been devised. In interstellar craft some kind of radiation shielding will be necessary, but because shielding is heavy, designers will probably try to get away with as little as possible. How thin is too thin? This is difficult to answer; aside from clear cases of lethal exposure to high doses of radiation, its long-term effects are mysterious. One 1995 study tracked the health of over 70,000 children of parents who were within a kilometer of the nuclear bombings at Hiroshima and Nagasaki, Japan, during World War II. Surprisingly, there was no statistically significant difference between the study population and populations of children of non-irradiated parents, in terms of malignant tumors in early age, differences in sex of offspring, chromosomal abnormalities and other mutations. On the other hand, recent studies have shown highly elevated mutation rates in people living near the Chernobyl disaster site. We have plenty to learn.
The second reason that radiation will not be a deal-breaker for interstellar voyaging is that mutagenesis itself has recently been found not to be largely the result of such ‘one-off’ ‘zaps’ from space, but rather more the result of the failure of DNA-repair mechanisms on the molecular level itself (over 300 such mechanisms and processes have been identified in the human genome alone). Thus, DNA repair therapy will probably be a large part of mitigating radiation issues in interstellar voyages.
In short, anywhere beyond Earth’s natural radiation shielding, under which we have evolved for millions of years, the mutagenic environment will be new and therefore a candidate for affecting our genome. Obviously we will use technology to mitigate such effects, including shielding and management of DNA repair, but we must remember that we do not know everything and that it will take time to adjust—both biologically and culturally–to new environments.
4.2 The Biology of Interstellar Voyaging: Selection
Natural selection occurs when life forms are prevented from reproducing—or simply have less offspring than others of their generation—due to their genetic characteristics. For example, a fly born without wings is unlikely to have offspring, or at least less offspring than its cohort (because of its reduced health and reduced ability to find mates) such that ‘wingless’ genes are likely to be ‘selected out’. Concomitant to most ‘selection against’ certain characteristics is selection ‘for’ alternatives. It’s often thought that humanity has halted natural selection with modern medicine and technology, but this is an illusion; many people continue to die before giving birth because of their genetic properties, particularly in populations without access to modern health care; and several recent studies have shown natural selection to be underway in modern populations. Even so, by the time of interstellar travel health technology will be much advanced…and yet selection will continue, for at least three reasons.
First, conditions off of Earth will differ from the conditions on Earth, where a relatively narrow environmental envelope of temperatures, atmospheric pressures and elemental compositions, nutrient supply and gravity have prevailed. The conditions in interstellar craft might well approximate these conditions, but much experimental data indicates that even small alterations in such variables as breathing gas composition and pressure can negatively effect gene expression (the switching on and off of genes on very complex schedules) and development of vertebrate embryos during the critical phases of early formation (e.g. gastrulation). We will of course try to control such variables, but it is unlikely that we will be able to anticipate everything. It seems certain that there will be a degree of increased infant mortality as the human genome adjusts to new conditions in interstellar colony craft, however carefully we design them. I am currently researching the genetic aspects of human development and the life course in environments slightly different from those on Earth.
Second, selection will likely play out in the cases of sweeps of novel diseases through interstellar craft populations. Again, we will be very careful, but it is impossible to anticipate all biological change, and in smallish populations (e.g. the 10,000+ that I suggest for interstellar craft; see below) inhabiting closed environments, sweeps of new disease, I believe, might well occur. Whether such sweeps structure the interstellar craft genome structurally is impossible to know, but we must be prepared for this possibility.
Finally, most interstellar voyage plans head not for other stars per se, but for their planets, where the resources and landscapes of alien worlds will allow humans to once again take to a planetary surface. In such a case, it is certain that new environmental conditions will be encountered. We will use technology to mitigate environmental selection, of course, but we must remember that we are only just appreciating the significance of epigenetics, the throwing of genetic switches by environmental factors. How will new planetary conditions shape the human genome? We simply can’t say, but we can be sure that it will occur. For example, even the Apollo lunar walkers commented on the lunar soil they inadvertently tracked into their lunar modules; you can bet that they breathed it in. They stayed on the moon only for days…but what would be the effect of such close contact of the human body with new chemistries of different worlds over the course of a lifetime? What genetic switches could be activated in such conditions? We don’t and can’t know; we can estimate and model, but we can’t be certain, and it is likely that selection will occur on our genome again in new planetary environments, at least until we control it with technology.
To succeed in migrating from Earth we are going to have to accept some risk. We do this on a daily basis; in the U.S. many of us take a daily commuting gamble, with nearly a hundred losing that bet—dying in car crashes—daily. If we can take that risk, surely we can adjust to the return of a degree of selection in our dream to colonize space and supply our genus with an insurance policy against extinction or even ‘just’ civilization collapse.
Ultimately, natural selection can be strongly mitigated with technology and is unlikely to strongly structure our genome in the 140-year ‘thoughtscape’ I am currently exploring. But natural selection should not be discounted in plans for interstellar colonization, particularly because it will ultimately involve generations of humanity adapting to new environments.
4.3 The Biology of Interstellar Voyaging: Migration and Drift
Migration is the flow of genes into and out of gene pools (populations) and it is a major factor on Earth, where humans have vast travel networks and today mate even across different hemispheres of the globe. However, in interstellar voyaging craft populations will be relatively fixed (rather than expanding, until new planets are reached), and migration will be between rather small sub-populations of the interstellar craft, an issue returned to below.
Drift, on the other hand, is the result of chance events in the history of a species; a good example, and one most suitable here, is the founder effect in which the genetic composition of a population is strongly conditioned by its founding members. This factor will be of critical importance in interstellar colony voyages because they will be closed genetic systems—as just mentioned—whose genetic structure will be largely established by the founding populations.
Regarding populations, we should take a minute to consider the size of interstellar colony groups. Should we send tens, thousands, millions of people? One way to tackle this issue is to consider our species’ MVP or ‘minimum viable population’, the figure required to avoid the deleterious effects of inbreeding. This figure has been much debated; anthropologist John Moore has suggested a figure of about 150, while others have suggested closer to 500. Such small populations, however, are highly vulnerable to single catastrophes, and my own calculations have suggested an MVP of about 3,000, multiplied by a ‘safety factor’ of 4 to 6 for an interstellar colonization population ‘reference figure’ of 12,000-18,000, which I consider significantly capable of surviving both biological disasters such as disease sweeps and a number of significant technological failures, over the low-centuries figure to reach, for example, Proxima Centauri.
From 12,000 to 18,000 people, then, as a ballpark figure for a founding population for interstellar migration; how do we pick them from the human population? Evolutionary ecologists measure s species’ health by its genetic diversity because a diverse gene pool allows for adaptation to new, unexpected conditions; thus our colonists should be biologically diverse, representing the human genome worldwide (which includes variations adapted to low and high altitudes, for example). However, an over-inclusive approach could endanger future populations if certain genetic maladies are allowed among the founding population. The screening process—determining that certain humans should not and could not participate in off-Earth colonies—seems to go against the very Enlightenment values of equality and freedom at the heart of Western Civilization, but if we are going to succeed in human space colonization we cannot ignore genetics. This is nothing new: over thousands of years human cultures have already devised many and elaborate kinship systems and sexual regulations that prevent the genetic disorders associated with close inbreeding; a survey of Yale’s Human Resources Area Files ethnographic database indicates that most cultures ban marrying or mating between parents and siblings of parents, siblings themselves, grandparents, and first cousins. Humanity has been looking after our genome for a very long time.
The main issue in genetic screening is the detection of genetic disorders that might send a biological ‘time bomb’ into future populations, particularly small and closed populations. But even this ‘simple’ issue includes moral and practical hurdles. In practical terms, this is evident in a depressing poster generated by the federal Genomic Science Program, a rather gruesome document pointing out the location–on each of our chromosomes–of many hundreds of genetically-controlled disorders, from cancers to deafness (we should remember that genes don’t exist simply to cause problems, and that in most cases they build healthy individuals!). Screening for interstellar suitability here seems simple enough: people carrying certain genes would have to remain Earthbound. The significant complication, though, is that while many genetic disorders are known to be simply correlated with certain genes—these are called Mendelian traits—modern genetics finds that more disorders are not so easily ‘pinned’ onto just one easily-spotted genetic marker. Indeed, in a recent paper Professor Aravinda Chakravarti of the Johns Hopkins School of Medicine noted that the textbook concept of simple Mendelian inheritance—’evolution basics’ that I teach in my own classes—seems to be melting away in light of his ‘genetic dissections’ of the real mechanisms of genetic disease, replaced by far more complex models. For example, many disorders are polygenic, the complex result of the interactions of many genes. And, single genes can be pleiotropic, affecting multiple characteristics of the individual organism! To further complicate matters, 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 such a way as to ‘express’ the genetic disorder.
We must address such issues because, as geneticist David Altshuler of the Harvard Medical School recently noted in the New York Times, “Even if you know everything about genetics, prediction will remain probabilistic and not deterministic.”
And what if we could identify, say, a ‘gene’ for deafness? Just after thirty years of age, Beethoven became deaf; it this were due to a genetic disorder, should he have been ‘selected out’? Should Beethoven’s deafness have been ‘pre-emptively corrected’? He completed much fine music even after his deafness. And what about Stephen Hawking’s genetic disorder, amyotrophic lateral sclerosis? Would screening out someone carrying a certain likelihood of expressing that disorder be a good idea? We already make mate choices, some based on actual or perceived health and future of our partners, and even the fates of some of our embryos. For the health of off-Earth populations, we must be willing engage in these complex discussions to determine what levels of probability we are comfortable with in terms of deciding whether or not a given person can participate in space colonization. Philosophers of morality could be of great help in clarifying the issues in a secular way, and designing real-world solutions.
On the surface, we might think that a solution to these issues would be to encourage the breeding of a master ‘Space Race’, as in the science fiction film Gatacca. But this idea is counter to nature and all of population genetics. If all are identical, all are subject to the same evolutionary ‘sweep’–for example, a single devastating disease. This is why ecologists measure—as mentioned–the health of a species not by its sameness, but by its genetic diversity, which is a well of untapped variation that might provide for a changed future. Any ‘super-race’, then, would be genetically imperiled in the manner of the closely-inbred royal families of Europe, who, according to Dr. Alan Rushton, author of Royal Maladies: Inherited Diseases in the Royal Houses of Europe, have suffered statistically more than their share of genetic disorders.
All in all, from a genetic perspective we have plenty of both moral and genetic reasons to begin studying the genetics of space colonization today. And, critically, we will have to ensure the genetic health of our domesticates and symbionts as well: we will be taking many domesticated plant and animal species off of Earth, some as food, some as companions, some as providers of such things as fibers. A good way to proceed would be to set clear milestones for what we want to know before we can leave the Earth, and work towards meeting them; otherwise, the endless, question-generating process at the heart of science might keep us here too long–and it is only a matter of time before a civilization- or species-destroying event will occur again on Earth.
In the end, if we are going to migrate from the Earth we are going to have to grapple with the probabilistic world of the genome in order to make smart—and moral—choices about the genetic health of our descendants. Regarding the important issue of preserving genetic variability, this could be maintained by ensuing gene flow among sub-populations of the interstellar craft as well as such technological means as carrying along from Earth ‘novel’ genetic material in the form of stored sperm and egg, as well as artificial mutagenesis. All of these measures are under investigation.
5. The Biology of Interstellar Voyaging: Final Comments
Over the course of several generations, as on voyages to nearby stars with propulsion systems that are beginning to seem reasonable, interstellar voyaging is entirely possible from a genetic perspective, with two provisions. First, we will have to ensure the genetic health of the colonist population as it will be under strong founder effect. Gene therapies, carrying genes from Earth in the form of stored eggs and sperm, and even the artificial induction of mutations can all be used to mitigate such effects, but at some point it will cease to be desirable to keep ‘pushing’ a human Earth genome into interstellar space. This brings us to the second proviso, and that is that natural selection will in fact return as a significant concern in human evolution, particularly when the unknowns of new planetary environments are encountered (even if they are surveyed by reconnaissance vehicles first).
We should note that, in the currently-considered timelines and populations, according to what we know about human biology it is unlikely that humanity will undergo speciation in less than a few thousand years (Figure 1, lower right).
These lessons remind us that adaptation is a continual process of the adjustment of the genome to environmental conditions. In non-humans that evolve reactively, with no conscious effort, this equilibriating process is slow and uncentralized and results in many extinctions over time. In humanity, consciousness can be used to help proactively shape our evolution, but we must remember that the only way to stop evolution is by extinction. We should accept and learn from the fact that if things live, they evolve and adapt. We should plan our adaptation to space as students of evolution. We must internalize the truth that the nature of the universe is change, not fixity, and allow this truth to condition our plans for the human colonization of space.
In the next article I will address cultural evolution, and some issues in the coevolution of DNA and culture, in biocultural evolution.
