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Repopulating the Earth from a few people

Number Six

JREF Kid
Number Six
Joined
Sep 5, 2001
Messages
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The story in the Bible says everyone was killed during the flood except for Noah and his family (I don't know how mny people that left total....eight adults and a few kids maybe?). Anyway, suppose something happened so that only a few people were left. And suppose there was no technology to manipulate reproduction or genes and that all people could do was reproduce the old fashioned way.

Could a small number of people reproduce the Earth to it's current population and diversity today? Or would the inbreeding monster get them first?
 
I remember reading that the human line has gone through more than one bottleneck - perhaps down to only a few thousand individuals. Sobering thought, eh? I'll try and look it up later - must get some sleep now.
 
Number Six said:
Could a small number of people reproduce the Earth to it's current population and diversity today? Or would the inbreeding monster get them first?
Click to expand...

Depends on which people. In the case of noah the wives would'nt be related so you would probably have just about enough gentic diversity to get by unless noah had some serius problems with his y chromosome. The answer is that other species can survive going down to tiny numbers so humans should be able to too. Sure at first you are going to get some problems with interbreeding but the people with serius problem are unlikly to breed so that will reduce the number of problem genes in the population.
 
I seem to recall somebody calculated the minimum number of people required to prevent inbreeding, but it included things like all adults producing offspring with all adults of the opposite gender. So at a base it'd sure throw monogamy out the window if you wanted to survive the bottleneck.

Given sufficient time I'd think you'd end up with as diverse a population, but probably diverse in a different way than currently since you'd have to rely on random mutation to introduce some variations.
 
I suspect genetic analysis could reveal if there had been a severe bottleneck 5000 years ago (or whenever the alleged flood incident occurred). I've heard there have been bottlenecks, but I'm not sure how severe they were; a bit more than a boatload, I think.
 
Originally posted by pupdog

I suspect genetic analysis could reveal if there had been a severe bottleneck 5000 years ago
Click to expand...
Mitochondrial DNA evidence (for one thing) suggests that the entire worldwide human population fell to perhaps some few thousands of individuals at around 74k years ago (not 5). A population of, say, a few dozen individuals would face considerable risk of extinction from inbreeding, but this would probably not actually be the greatest risk they would face. An aggressive virus, for example, could wipe them out a lot quicker.

A little on the dynamics of small populations here
 
OK, making a REALLY SIMPLE model, I allowed that we started with 2 people, and that the population doubled each 30 years. This is an underestimation - most humans produce more than 2 kids, and usually before they are 30 years old. However I had to allow for die-off of the older folk, so an average of double per cycle seemed OK.

Results: (population, years from zero)

<table border="1"><tr><td>2</td><td> 60</td></tr><tr><td>4</td><td> 90</td></tr><tr><td>8</td><td> 120</td></tr><tr><td>16</td><td> 150</td></tr><tr><td>32</td><td> 180</td></tr><tr><td>64</td><td> 210</td></tr><tr><td>128</td><td> 240</td></tr><tr><td>256</td><td> 270</td></tr><tr><td>512</td><td> 300</td></tr><tr><td>1,024</td><td> 330</td></tr><tr><td>2,048</td><td> 360</td></tr><tr><td>4,096</td><td> 390</td></tr><tr><td>8,192</td><td> 420</td></tr><tr><td>16,384</td><td> 450</td></tr><tr><td>32,768</td><td> 480</td></tr><tr><td>65,536</td><td> 510</td></tr><tr><td>131,072 </td><td>540</td></tr><tr><td>262,144</td><td> 570</td></tr><tr><td>524,288</td><td> 600</td></tr><tr><td>1,048,576</td><td> 630</td></tr><tr><td>2,097,152</td><td> 660</td></tr><tr><td>4,194,304</td><td> 690</td></tr><tr><td>8,388,608</td><td> 720</td></tr><tr><td>16,777,216</td><td> 750</td></tr><tr><td>33,554,432</td><td> 780</td></tr><tr><td>67,108,864</td><td> 810</td></tr><tr><td>134,217,728</td><td> 840</td></tr><tr><td>268,435,456</td><td> 870</td></tr><tr><td>536,870,912</td><td> 900</td></tr><tr><td>1,073,741,824</td><td> 930</td></tr><tr><td>2,147,483,648</td><td> 960</td></tr><tr><td>4,294,967,296</td><td> 990</td></tr><tr><td>8,589,934,592</td><td> 1,020</td></tr><tr><td>17,179,869,184</td><td> 1,050</td></tr></table>


This suggests that the current population of about 8.5 billion people could be reached in under 1000 years. It also suggests that 200 years ago, when the USA was born, the population of the Earth was just under 70 million people. And that King Richard the Lionheart probably sat on the throne of England with a only few dozen people to cheer him (so who did he crusade against, then?).

Do you think I should adjust my model parameters at all? :D

[Edit: numbers formatted - I hope]
 
Ugh, sorry here Zep, but either you or I have totally missed the point of this thread.

Number six's issue is (IMHO) what is the minimum number of people required in a breeding population that could lead eventually to a viable human population. Two is the answer only if inbreeding and lack of genetic diversity aren't important.

I know the risk of inbreeding has been exagerated but I don't know with a small population it could be ignored. I have heard estimates for the number in the few thousand range but I'm not sure where that comes from.

I know also that the caucasions and orientals are believed to have been derived from a very small population but I don't know how small that population is believed to have been.
 
Funny, I was thinking about this the other day. In the Rama sequels (the crap ones where Arthur C. Clarke worked with Gentry Lee) one female and two male characters end up stuck on a spaceship for years and years. The woman has a child with one man, who then disappears for years, so she ends up having a kid with the other guy. Then, in a bizarre piece of writing, she starts talking about pairing off the kids so they can breed - I say bizarre because none of the characters appear to have any moral isssues over it (I know the subject has been covered here on the forums before, with differing opinions, but to completely ignore all moral issues was sloppy, IMO).

Anyway, it got me wondering - can you reach a sizeable population starting from two males and one female, with no closer pairings than cousins?

David
 
Number Six said:
Could a small number of people reproduce the Earth to it's current population and diversity today? Or would the inbreeding monster get them first?
Click to expand...

The danger of inbreeding is directly proportional to the extent of the culling. If offspring with defective traits are prevented from procreating the subsequent generation is in fact more genetically healthy than the previous.

(If two carriers (-x) have children then they have an equal chance of -- (clean), -x (carrier), -x (carrier), xx (culled), then the percentage of bad genes in the breeding stock has dropped from 50% to 33%).

Lack of variation, however has a big impact on the adaptibility of a species - a small change could cause the entire species to die out, rather than to evolve to the new environment.

If you're interested, you might want to look into the debate surrounding the lack of genetic diversity among cheetahs.

Disclaimer: Everything I Ever Knew I Learnt From Google.
 
Originally posted by davefoc

I have heard estimates for the number in the few thousand range but I'm not sure where that comes from.
Click to expand...
It is extrapolated from the amount of variation currently found in the human population, especially in mitochondrial DNA. Because mtDNA is passed only along maternal lines, the rate of change over time is not affected by recombination as is the case with chromosomal DNA. The regular rate of mutation in mtDNA (about one per 300-600 generations) has been considered a reasonably reliable 'clock'. Though questions have recently been raised about the exact rate of mtDNA mutation, there is more variation within sub-populations of some other primates than in the entire human population as a whole.
 
Dynamic,
Thanks for your reply, I was actually aware about the necking down of the human race to a few thousand individuals. I thought that I had seen an estimate that a few thousand individuals is required to reestablish the human race also.

But the guy in DangerousBeliefs link ( http://www.mwilliams.info/archives/001580.php )
thinks you can do it with about 50. But he is hypothesizing a considerable informed cooperative effort on the part of the participants to reduce inbreeding and improve fertility. It sounds like from that link that 50 routinely horny, randomly mating humans may have difficulties with reestablishing the human race.

I also noted that some guy wrote in and said that his beliefs would make him uncomfortable with the out of wedlock birthing and polygamous relationship required to make the 50 person group a success. Even if a few volunteers could be coerced into taking up the slack that this guy is creating, his reluctance to cooperate is going to reduce the diversity of the gene pool and make successful repopulation more at risk.

I can also imagine that some guys are going to be over enthusiastic and may do more than their share in the repopulation effort and this may lead to less than friendly relations amongst the original goup and the premature removal of gene pool contributors from the group by others who don't appreciate their enthusiasm.

Edited to add:
I just realized after I wrote this that Pitcairn Island might serve as a real world experiment for the issue in question. Somebody might take a look at that and see how it relates to the question.
 
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