If a major disaster wiped out most of humanity, how many survivors would there need to be in order for them to rebuild?

Question

I have a story set about five thousand years after a major disaster reduced the world population from billions down to... well, that's the question. How low could it have gone, and still provide the seeds for a genetically stable population able to grow and thrive?

It's an alternate planet, and the majority of the pre-disaster population was dispersed across one large continent. Post-disaster, small pockets of people were left in many different places on that continent. They lost almost all of their technology, and were knocked back to the level of hunter-gatherers and basic agriculture. But, the climate was very favorable, resources were plentiful, and the needed skills/knowledge were available.

They would have started out in small pockets, which eventually established contact with other such groups, sometimes within a generation or two and other times after a longer delay.

Five thousand years later, people have spread, settled, and developed. Ultimately they have reached a population of about 35 million, with tech levels varying by region from hunter-gatherer or base agricultural societies in the most distant , least advanced areas (wildernesses and undeveloped countries) to late medieval technology in the most advanced and heavily populated areas (developed countries).

So, to get from one position to the other, how many survivors of that initial disaster would there need to have been?

*Additional info: the aftermath of the disaster was removed from the equation. For example, there were not millions of dead bodies to be dealt with or to cause disease. There was also no infrastructure left behind, from roads to buildings to power grids; the survivors had to start from scratch, BUT they were given resources to obtain the knowledge and skills they needed to do so. It's complex, but suffice to say this whole thing was staged by very powerful non-humans, as if they hit a kind of "civilization reset button", and a small (compared to the size of the population before) number of people were left to rebuild.

Answer 1

Well, based on the calculations done on this article for a generation ship (A ship which would travel through space with the intent of colonizing other planets)(http://www.sciencedirect.com/science/article/pii/S0094576513004669), it says:

14,000 to 44,000 people would be sufficient to survive such journeys in good health. A safe and well-considered Nc figure is 40,000, an Interstellar Migrant Population (IMP) composed of an Effective Population [Ne] of 23,400 reproductive males and females, the rest being pre- or post-reproductive individuals. This number would maintain good health over five generations despite (a) increased inbreeding resulting from a relatively small human population, (b) depressed genetic diversity due to the founder effect, (c) demographic change through time and (d) expectation of at least one severe population catastrophe over the 5-generation voyage.*

Although this is not exactly related, it does provide some pretty concrete numbers in regards to the size of a population required to keep genetic diversity.

Answer 2

Given that the population of the planet is spread out in small pockets and that there is no pre-existing infrastructure, there would have to be a fair number of people left over for each pocket to last on its own, much less have genetic stability.

Looking at the estimates in this question, it seems, from a societal perspective, that about 420 people would be needed for a small pocket of today's tech level: ~140 children, ~80 engineers, scientists, teachers, doctors, leaders, etc., ~200 farmers, hunters, workers, soldiers, etc. With lower technology levels (like agricultural or medieval), there would be fewer professionals, and therefore fewer farmers and supportive workers are needed. Without any larger groups, I suspect technical knowledge would largely die out due to lack of scientists/professionals to pass along know-how. For minimum societal function (similar to a band of hunter-gatherers), I would estimate the minimum group size to be around 20. Assuming there is a uniform distribution between these group sizes, and there are, say, 100 groups, there would be 22,000 people in all. Of course, change the distribution and/or number of groups to get a figure in line with DevourerOfStars.

Also, in your question, you said that it would take, on average, a generation or two for pockets to link up (approx. 20-40 years). On this timescale, genetic stability within a pocket won't be too much of a problem except at the very low end of the spectrum, but the longer it takes groups to link together, the larger the groups need to be (maybe 1.5 to 2 times larger for each generation) to maintain genetic integrity over however many generations.

Each pocket would invest energy in rebuilding technology to some extent to making their lives easier; instead of having each pocket conducting its own research and having to feed its own group of scientists, it would be more efficient for the entire society to have one single group of scientists, basically pooling resources. This goes back to the number of people required to support said scientists. Another consideration would be that smaller pockets would have fewer resources for scientists, and would re-progress more slowly in technology, setting up the social order in the future.

Answer 3

An MVP (minimum viable population) of 500 to 1,000 has often been given as an average for terrestrial vertebrates when inbreeding or genetic variability is ignored. When inbreeding effects are included, estimates of MVP for many species are in the thousands. Based on a meta-analysis of reported values in the literature for many species, Traill et al. reported a median MVP of 4,169 individuals. Since we are an intelligent species, we might be able to temporarily get by with fewer individuals (recognizing of course, that there is always the random possibility of war, natural disaster or epidemic).