(Courtesy of NASA Ames Research Center) A city in space, housed in a 500-m (about a third of a mile) diameter Bernal sphere, rotating at 1.9 rpm to produce a full Earth artificial gravity at the sphere's equator. View with cutaway shows the interior, which resembles a large valley around the sphere's equator.
Space Colonization — Populating a Planet
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JULY 19, 2012
April Holladay, WonderQuest

Q: How many and what ratio of men and women will be required to create a healthy human population on another planet?
Andre, Mumbai, India
The lander touched down some 200 years after it left Earth, 20 light years away. Descendants of the original pioneers looked out at a new planet ? a dark alien world. Some despaired; others stared with excitement at the challenge. More than a few wondered, 'What had their ancestors gotten them into?'
A red-dwarf Sun, fifty times dimmer than Sol, filled much of the sky. Low scrub vegetation, dark in color, almost black, crowded the landing site. The broad daylight seemed too dark to support plant-life, but shrubs absorbed nearly all light that hit their fuzzy leaves and flourished. A large, deep ocean shimmered in the dusky sunlight, lapping the shores, awaiting seafaring folk...
The voyagers comprise a healthy population able to populate the planet. Their forebears had done well.
* * *
A: How many? About 160 original pioneers can do the job. Actually a mere 15 can work if they're lucky. Five hundred might be enough. NASA's proposal of 10,000 is a more comfortable number. You pick 'em.
What ratio of men to women? Perhaps half men and half women or possibly two men for every three women.
That's the basic answer. Now let's delve into how we arrived at the numbers ? sex and genes. Also, what is a "healthy population?"
A healthy population is one:
• big enough to minimize inbreeding, (although this is not much of a problem if the original pioneers don't have genes that could cause harmful conditions, like hemophilia)
• small enough for space travel.
Genetic diversity may be helpful the "first several hundred years", as long as it doesn't interfere with the social organization of the colonists, says biochemist Lawrence Moran of the University of Toronto. After that, most genetic diversity will "disappear."
Sex
Anthropologist John Moore of the University of Florida thinks the traditional family is the best organization, since it has served us well for a million years. A ratio of one man to one woman works. He suggests about 160 people as a minimum number, which allows around ten potential marriage partners. He bases his population estimate on mathematical modeling and studies of small migrating groups of early humans.
A much smaller initial population is possible with the help of human embryos (and the technology to support in vitro embryogenesis and fetal development). Just two human females could manage the job, Moore says. Sperm banks can provide genetic diversity in years to come.
However, "don't expect human nature to change," Moran says. "Men and women will still want to pass on their own genes."
Genes
Genetic drift or random catastrophes can leave a population either without certain genes helpful to survival or with nearly identical genetic makeup, and therefore identically vulnerable.
Present-day cheetahs are an example of what can happen to a species struck by a calamity of some sort. About 10,000 years ago most cheetahs died out ? we don't know why. The population has since rebounded in size but their genetic structure, based on the few who managed to survive the catastrophe, is almost identical. Consequently, says biologist John W. Kimball author of Biology, cheetahs will accept skin grafts from each other just as identical twins do. Kimball wonders if a population so much alike has the potential to adapt to a changing environment.
On the other hand, Moran says, "The idea that you need to have lots of variation within a species in order to prepare for changing environment is a myth that doesn't stand up to scrutiny."
"Evolution is a very long-term process. It highly unlikely that there will be some rapid drastic change in the environment that will wipe out all existing cheetahs but would have saved some of them if they had more diversity."
Does natural selection or does genetic drift dominate evolutionary changes in a population? That debate rages among biologists these days.
Biologists striving to save threatened animal species from extinction have developed some rules of thumb for estimating population sizes to overcome genetic problems:
• A size of 500 to 5000 individuals may ensure overall genetic diversity (Frankham).
• Another figure (based on work by Soulé and Foose in 1986) states a founding population of 20 to 30 individuals that are unrelated and non-inbred would preserve 90% of the original genetic diversity for 200 years.
• An initial population size (the founders) of 50 (Frankham and Franklin) avoids inbreeding problems.
Rules of thumb, however, rest on theoretical modeling, without much experimental evidence to support the predictions. Unfortunately, this is especially true of projecting populations needed to colonize space. So, let's consider a bit of history. What worked on Earth?
A case study
Starting in 1816, fifteen British immigrants settled the most remote island on our planet, Tristan de Cunha ? located almost half way between Cape Town, Africa and Buenos Aires, South America in the Atlantic Ocean. We know much about the islanders' founders and their lonely struggle for survival. Colonizing a remote island is similar to colonizing a remote planet.
It all started with Napoleon, imprisoned on St Helena, about 2,430 kilometers (1,510 mi) to the north. The Brits had exiled Napoleon to this tiny windswept island in 1815. But now, rumors of rescue plans reached the halls of Parliament. What if the French launched a rescue of Napoleon from Tristan? In August 1816, Britain annexed Tristan de Cunha to thwart the possibility.
The Royal Army sent five officers and 36 soldiers from South Africa to take possession of Tristan da Cunha in 1816. Corporal William Glass from Kelso in Scotland, with his South African wife and two children, asked to stay when the troops withdrew a year later.
Glass and his family became the first settlers of Tristan. In all, just 15 people (seven founders and eight men) founded the colony, says geneticist Himla Soodyall from the University of the Witwatersrand, South Africa. They arrived at various times between 1816 and 1908. Colonizing over the decades undoubtedly increased the colony's genetic diversity and helped them survive.
We have traced the mitochondrial DNA of the present approximately 300 people back to just five female ancestors, including one pair of sisters, says Soodyall. The population grew and fell with the flow of immigrants and disasters:
• By 1827, shipwrecked sailors, brides from St. Helena and South African settlers boosted the total from 15 to 24 (7 men, 6 women and 11 children)
• 1832: population of 34 with 6 couples and 22 children
• 1852: 85 people
• 1856: population reached 96, then fell to 71 when 25 left for Massachusetts after Glass' death
• 1857: population crashed to 28 (only four families) because 46 left on the rescue frigate HMS Geyser, to avoid starving to death.
• 1887 started off well with a population of 107, but then 15 out of the island's 19 adult men died at sea trying to intercept the ship, West Riding, and barter for food. Their potato crop had failed, so, despite poor weather, the men launched their longboat and were never seen again. This left only 92 people (4 elderly men and 88 women and children)
• 1890: 34 emigrated to South Africa, leaving 58 on the island.
• 1987: population of 296. [Data from Steve Mack and Arnaldo Faustini.]
Only seven family names are in use now, corresponding to the original eight male founders (minus one). Any pair of the 300 islanders is as closely related as first cousins. But it's a healthy human population since it survived. The people (taken as a group) were able to cope with the hardships they encountered, and the few inheritable diseases were not life-threatening.
By and large, this is an impressively successful (and lucky) colonization still viable almost 200 years later.
A final thought ? the roaming space city
Suppose our generational spaceship arrives at an uninhabitable planet. What then?
Perhaps, instead of populating a planet, we should roam space essentially forever by creating a space city of 10,000 as NASA tentatively plans. Whenever the traveling, multi-generational city finds a good planet, the city could drop off a founder's population with supplies and then move on, gradually, in this fashion, populating the galaxy.
Initially, the space city could continue to get new technology information from Earth (arriving at light speed) and then build its own state-of-the-art machines and equipment to maintain city life and explore the galaxy. For needed resources, city peoples could mine asteroids and comets at first, and then, eventually, inhabit and propel an asteroid. A succession of rich asteroids would give them needed raw materials to colonize the galaxy.
"The space city of 10,000 will eventually become as inbred as the inhabitants of Tristan de Cunha," Moran says, "but it probably won't matter very much." Indeed, the effective population size of our species was probably about 34,000 about one million years ago. Furthermore, small groups of colonists founded almost every species on Earth. "It's the normal way of evolving. (Of course, we need to keep in mind that 99.9% of all species become extinct!!!)"
* * *
The newly-arrived pioneers watch the space city recede ? the only home they had ever known ? as it moves toward another planet in the Milky Way. The bright spot in the dim sky shrinks to a point of light and then winks out. A child waves goodbye.
Readers' Answers to the current question: How many and what ratio of men and women will be required to create a healthy human population on another planet?
• Ratio: Here is the appropriate response to the "what if?" question about the repopulation ratio of men and women: http://www.youtube.com/watch Ilan gives us a scene ("Mein Fuhrer! I can walk") from the film Dr. Strangelove with Ilan's and Dr. Strangelove's joint answer.
Ilan, mathematician, Ecole Polytechnique, France
• Size: I feel that a working independent colonial size would be about 1,000,000 ? smaller than New York City (8 million) ? provided the planet, planetoid or asteroid has at least the first 92 elements of the periodic chart and enough light for photosynthesis.
Wayne D., Richmond, Virginia, USA
• Fifty fifty ratio. So one does not feel lonely populating another planet.
Victor, Victoria, Canada
• 100,000 men and women, ratio; 4 women to 1 man
Jonkobol, Trinidad
• I say about 50 men and 50 women. The 50 couples will have kids which will then have a large selection of other kids to mate with, preventing inbreeding. The population will then be healthy, for a while. Eventually, we will need to send up more people to help combat inbreeding after years and years. Then, you will have to send about 500, half men and half women. Sorry, this is based on no information in particular, but this is what I think.
PotatoPancakes, Pennsylvania, USA
• It was done in the bible with one man and one woman. If you wanted to take a non-believer standpoint, the more women, the better. For the purpose of population, one man could impregnate several women. 10 women for every one man, for example, would be ideal.
John, Atlanta, Georgia
Readers contributed answers to this question. The next question is:
When we sleep on a bus trip, why are we often able to wake up just before our intended stop?
Click here to give April your answer: Answer the question.
Definitions and discussion
What is inbreeding and how harmful?
Genetic diversity: what it is
Genetic drift: how and why it matters
Further Reading:
Polymorphisms, founder effect and genetic drift, 20 Sep. 2007, John W. Kimball, Kimball's Biology Page.