# What is the smallest number of humans required for colonization?

There is an estimate at Understanding Genetics that the minimum number of humans required for a colonization process is 160. It also mentions some animal populations that have rebounded from near extinction with much lower numbers. Strangely there is no mention of haplogroups in the answer there, and it seems to assume a random sampling of humans rather than a picked group.

It would seem that if you were trying prevent mutation in all haplotypes you would probably need a larger gene pool. Common sense also implies that it would be easier to get a diverse gene pool from a smaller group by purposely selecting from different haplogroups.

If hand picking with gene profiling, what is the smallest group of humans that can form a viable human colony, where new gene infusion is unlikely?

Clarifications:

• This question is looking for a lower limit number.
• The number should (at least, theoretically) provide population self-sustainability and long-term survival.
• The answer should take two different approaches:

1. An intra-solar-system colony that may have future additions to its numbers
2. An interstellar colony that does not expect future additions to its numbers
• Let's define "long-term" as at least 200 years.
• This question is focusing on genetic/population considerations. Practically, a real colony may require more than the answer here indicates.
• What geneticists mean by that number is that you need an estimated 80 pairs (F/M) of complete DNA, where none of them repeat (so genetic structure of 160 unique individuals), but that doesn't necessarily mean you'd need 160 actual people colonising new planets, but maybe a reasonable number of surrogate mothers, the remaining (out of 80, provided surrogate mothers are fertile human females) number of egg donors, and 80 sperm donors. While this might sound harsh, it's scientific and technological progress like this that makes colonization more feasible, and it will most likely be female only. – TildalWave Jul 19 '13 at 10:57
• @TildalWave, True and given, but that is putting all your eggs in one basket so to speak. Given frozen egg/sperm/embryo you could easily have as many unique pairs as you wanted, 6 women and their decedents could easily bring forth the children of uncountable parents. Given the science and some luck, you could trim back to one woman, or even none. Technical and/or human factors could adversely impact that plan. For redundancy you would want to have included a viable genetic sampling in your settlers. – James Jenkins Jul 19 '13 at 11:13
• I started writing an answer to this but it sounded too much like what Hitler wanted to achieve... so I think I will pass. Realize that with 2 unrelated females and a diverse supply of stored sperm (enough for gen1 and gen2) the gen3+ offspring could reproduce with very small incidence of Inbreeding provided they not breed with children from their own mother. – Chad Jul 19 '13 at 15:21
• The answer is one female, plus lots of stored, viable eggs and sperm. – Mark Adler Nov 7 '13 at 21:51
• @MarkAdler : 1 female plus a lot of sperm is a lot to risk in the case of post partem depression or just going generally crazy from having to deal with raising kids while being pregnant and absolutely no other adult support. Not to mention lack of a midwife or doctor should there be any complications. That combinaton might be possible, but risky ... and you'd want additional skills for other reasons (maybe not mart's 100M, but at least a doctor, engineer, botonist, psychologist, etc ... and likely spares for each skill) – Joe Nov 18 '13 at 13:58

I think for colonization within the Sol system, the genetic diversity question is not a significant problem as new colonists will likely continue to arrive for a long time. Sending additional colonists is easy compared to the initial challenge of sending everything needed to establish a sustainable colony.

For a scenario like interstellar colonization, where the group will potentially be on their own for a very long time, I don't think there is a solid threshold that should not be crossed. It's a matter of degree. More is better. Less is riskier. Relatively small gene pools have survived for long periods as we have seen in some remote island populations, royal lines, or in the Fugates of Kentucky. The smaller your gene pool though, the more you see the emergence of long dormant recessive traits.

Many of these are very benign, but if you're unlucky, you could end up with something life-threatening, like Sickle Cell Anemia. And even if you specifically select for the absence of known dangerous recessive genes, you never really know what's going to turn up, because these recessive genes may have gone unexpressed for many centuries until inbreeding reveals them. So you are basically rolling the dice if your population is very small.

If you are looking for a lower limit, as stated in the comments, one woman and a sperm bank could theoretically populate a world, or if we assume for some reason that we are limited to only natural conception, one man and one woman is enough. But it's very risky. It's really a ridiculous risk to take. An interstellar colonization would already be so expensive and require such a large payload that it's hard to imagine a scenario where you wouldn't want to include many thousands of genomes at least in the form of sperm on ice.

How many exactly should be included as an absolute minimum? Again, it's entirely a matter of degree. There is no threshold value that makes a critical difference. How much risk are you willing to accept for this really really expensive venture? A whole lot? Then two genomes will do it - or even just one male if you're willing to make the female a cloned twin with two copies of the X instead of a Y. Would you rather have zero risk? There's no such thing. Somewhere in between? Well, take more than 2. And don't take so many that they occupy more than 10% of your payload.

Ultimately cost/benefit per pound is the equation you will be working. That's what everything usually comes down to in space exploration.

If space colonization is to unfold, exchange of genetic material between colonies is a very likely scenario. It is quite easy to exchange sperm. Even if we consider stellar colonization, interstellar sperm ships look much more possible than travel (sperm containers can withstand greater acceleration, do not require starship deceleration and can travel for longer time in frozen form).

• +1 for writing "interstellar sperm ships" before XKCD – uhoh Jun 7 '20 at 5:24
• – uhoh Jun 7 '20 at 5:25

The Amish are a genetically separated group. They could survive hundreds of years from a core population of 200 people. Yes, they can have genetic disorders, but it is not so bad as to threaten their long-term survival.

There is a genetic study saying that roughly 25 mouse pairs are needed to build a stable population on the long term. With humans, this number may be roughly in the same order.

I think that the real answer is somewhere between the two.

• This answer seems based on opinion, can you offer any references or studies supporting the assertions? – James Jenkins Nov 14 '13 at 11:24
• I'm afraid that even with a source for the mouse study this answer seems to be making a claim about humans that is pretty speculative. – called2voyage May 11 '19 at 20:33
• @called2voyage I've heard about it from a trustable source, that the estimated size is around 50 (25 pairs) in the case of the humans. The problem is that I can't prove it. The source was verbal, non-English, and about 20 years ago, from a high school biology teacher. My extensive searches for this information were not successful. I suspect, it might be available in academical sources. Maybe I could ask it on the Biology SE. However, the genetical problems of the Amish is well known, I could easily get a lot of reference. – peterh May 11 '19 at 20:39
• @called2voyage (sorry for the late react) Uhoh asked this on the Biology SE and a for me believable answer mentioned this 50 limit. – peterh Jun 8 '20 at 21:44