If 2 men and 158 women are the last people on earth, how long would it take to repopulate the world?

I would suppose that would depend on a great many things. Let's assume our terrified group of survivors has ample resources including food and shelter, and medicine approaching the current level of technology, and seeks to repopulate the world as quickly as reasonably possible. We further assume our intrepid band is hardworking, peaceable, and cooperative.

Two men can easily perform the reproductive, ahem, duties of ensuring that the women do their part. We assume the men are healthy and fertile, and of an age where they can both continue until some of their male offspring are old enough to do their part.

The women are the bottleneck. No matter how many women you assign to the task, as they say, it still takes 9 months to produce a child. Whereas the male record for reproduction is over 1,000 children (set by a 17th century sultan with a large harem), no woman has given birth to more than 69 children. That's in recorded history, and I cannot imagine any woman having more children than that in prehistoric times.

But as that particular Russian peasant was prone to multiple births (several sets of twins and triplets), it would be ridiculous to expect every woman to accomplish as much. For an idea of what an AVERAGE might be, we must look to the era of the "revenge of the cradle" in French-speaking Canada after the loss of what we in the US call the "French and Indian War".

For over 3 generations, French-Canadian women AVERAGED 13 live births per married woman. Average age of marriage was just shy of 17, and of course modern techniques like embryo transfer were not available as women neared menopause. Allowing that some women will simply have incurable infertility, it seems reasonable that each woman will produce on average, 10 children during all her reproductive years. That's an average over the entire population, and yes, I chose 10 because it's a nice, round number.

A woman's reproductive lifetime is some 30 years, from about 15 to 45. I'm sure many of you know a woman who has given birth outside that range. In particular, my mother birthed me at age 13. But again, we are talking averages here. 10 children per 30 years, it's a reasonable assumption based on current infertility rates, historic reproduction rates, and modern medicine.

In 30 years, generation 2 will be complete, 1580 children. Half will be female (I won't quibble over a percentage point or two, or some 790 women.
60 years to finish generation 3, numbering 7900.
90 years, generation 4, 39,500.
120 years, generation 5, 197,500.
150 years, generation 6, 987,500. Almost to the first million!
180 years, generation 7, I'll round down to 4.9 million.
210 years, generation 8, 24.5 million.
240 years, generation 9, 122.5 million
270 years, generation 10, 612.5 million, only China and India are larger.
300 years, generation 11, I'll round down to 3 billion.

It is clear that at this rate, we'd pass 7 billion before generation 12 is complete. We must also add the population of the previous generation, as many of those people will survive to age 75 and beyond.

Now, all this assumes no logistical problems from feeding and housing this massively growing population. It assumes no massive disaster or catastrophe decimates the population in the early generations. It assumes that being bred back from 2 males doesn't induce serious genetic problems. And most of all, it assumes that everyone will contribute the massive personal sacrifice to have large families. Remember, 10 is the average for ALL women...for fertile women, family size will be closer to 13 or 14, and some women will have families of 17 or more. And once you're past the first generation, each male will still be responsible for providing for those 13 or 14 hungry mouths...children will start working at an early age, and must still receive enough education to maintain the technology base. It would be a very harsh life, much harsher than most of us in the developed world are familiar with.

But if you can develop a culture where everyone is onboard with the "project", somewhere in the neighborhood of 300-350 years would be adequate.

If you went at a slower rate, of course it would take longer, but would be less arduous and would lead to a lifestyle more similar to the one we have now. For example, if you're willing to take 1,000 years to repopulate, somewhere in the neighborhood of 3.4 children per woman would do it.


With only two men, the population would not remain healthy and viable long enough to repopulate the entire Earth.

Just look at the genetic problems in Fundamentalist Mormon enclaves in the United States. They tried out your "two guys and a bunch of women" experiment. Ain't good.

A starter population of 160 is theoretically enough, perhaps more than enough, for a minimum viable population of humans that could be indefinitely healthy and sustainable. But about half of those 160 people would need to be men.


If 2 men and 158 women are the last people on earth, how long would it take to repopulate the world?

Instantly.

The world is repopulated with 160 people.

You did not specify a population number.


Interesting questions. If everybody is committed to repopulating, every woman could be pregnant within 3 months. After one year, you would have around 160 babies and within 5 years, you could triple the number.

20 years later, the 500 kids could start having babies (250 pregnant women could give birth to 750 new babies within 5 years) and so on. But it won't grow fast unless each woman has at least 4 kids who can reach 20.


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