I think the problem eventually comes down to finding enough food and clean water for so many people. As a population grows it must always be seeking out new lands and sources of water which eventually you run out of.
LIfespan only makes a very small difference in the time it takes to grow to a certain population (assuming all the women live long enough to have the average number of youngsters). I actually assumed people died as soon as they gave birth to the average number of children in my model above, but if you assume they live three times as long, it only makes a difference in time of one generation.
Measure for Measure. I’m having a little trouble understanding your post. But note that in the formula I gave the answer is in number of generations to reach the current population, not the number of years. Perhaps that’s the source of your confusion?
Even if your assumptions about the past were true (and I don’t think they are), women are effectively intertile once they are in their 40s and early 50s (but more genearlly in their late 30s), so a lifespan of 100 years is irrelevant.
Are all other life forms unaffected? No atastrophic climate change?
There would be countless examples of abandoned technology literally everywhere you look. It would greatly help if the first people knew more than one language and passed it on. If you taught people to read, you could jump start on the back of current technology, and things would be exponentially quicker. If you’re talking about removing or completely destroying all technology and starting completely from scratch it’s a crapshoot. We’re still starting from further ahead than the cavemen did though.
You’re going to run into nutrition and disease problems really quickly if you don’t have access to technology. That’s going to decimate your population and not just slow your growth, but imperil your survival. Where you choose to start from will also make a huge difference.
And are you strictly defining repopulate as “bring the population up to the exact same level it is now”? Because there’s a hell of a lot of people in this world today.
I’d say even with a fairly good starting scenario (first couple are a highly intelligent scientist and a doctor, say, and have time to build up a massive library and have lots of kids), it would still take close to a thousand years, maybe more. It’s going to take a lot of humping to get China and the US back to the norm in particular.
If you are also starting from scratch technologically I’d say anywhere from 10,000 to 100,000 years. It’s going to take thousands of years just to get to the more remote parts of the world.
I’ve run computer simulations on this type of problem in the past. One unexpected outcome is that, in any given generation, there’s a non-zero chance that every single baby will be of the same gender, and then it’s game over. IIRC, this happened in roughly 10% of the simulation runs. This problem goes away if you assume that the gender ratio is always 50-50.
Assume every single person lives to age 60, no accidents, no murders, no suicides.
Assume everyone begins breeding at puberty, no sterility, no abstinence.
Assume all pregnancies result in a live human for the next generation, no miscarriages, no infant mortality, no mothers dying in child birth.
Assume every young adult female gets pregnant once per year for 24 years (total of 24 babies per mother).
Assume the gender ratio is always 50-50.
Assume food, water, shelter, etc. are no problem.
Assume you start with two young adults.
Ignore the problems of inbreeding.
Under those assumptions, I estimate 38 years to reach 1,000 and then doubling every 5 years after that. Time to reach 7 billion is about 150 years.
I think it goes without saying that these are ridiculously unrealistic assumptions. The point is to have a comparison and ask “Theoretically it could be done in only 150 years but obviously it didn’t, so what factors slowed it down?”. The only way I can see these assumptions working is in some kind of zoo run by extremely powerful aliens with great medical knowledge and plenty of patience.
Duggar or non Duggar? Would make a LOT of difference 
Not to mention (OK, I’ll mention it) how nomadic they would become. If the current population density of 2.1 hectares per person is maintained then that last generation would have to be moving along at about 1.5 kilometers per day. That’s chalked up to the fact that population growth is exponential but area acquired is a squared function of spreading-out speed.
I may have slipped a decimal point but I don’t think so.
I could kick it off to a good start. I Could Love a Million Girls
With such little people it would take many centuries.
Same thing if 95% of people on earth died tomorrow :eek::eek::eek::eek:it would take centuries and much of science and technology would be lost.It would be worse than the middle ages.
Yes it was: thanks.
Actually no. We’d have a pre-industrial revolution population but with all the knowledge - because it’s been written down - of the modern era.
I don’t understand this.
If any given generation is all females, then it makes no difference at all, since males can readily reproduce into their 50s and we have many documented accounts of men reproducing into their 70s. In order for production of all females to have any effect at all, it would need to occur over at least 3 generations. To result in extinction probably 5 or 6 generations. The odds of that is much, much less than 10% for even a single individual, much less entire populations.
In fact, producing all females for a few generations will actually increase the population growth rate. this is why farms always have far more breeding females than males.
That leaves us with the production of only males. But even then, this needs to occur over at least two generations to lead to “game over”. If there were no female babies born from now on, girls born today would continue to ensure a fresh supply of children for the next 40 years at least.
Even with only the first generation of 6 children, the odds of every single child being male would be 0.5 chance of producing a single male ^ 3 children each woman ^ 3 women. That’s orders of magnitude less than 10%. And as the generation number/population size increases, the probability shrinks exponentially.
I can’t image how any model could result in 10% of runs ending with all male children. There’s something seriously wrong with the probability there.
It pretty much goes away if you assume the gender ratio is anything less than 100-0. Even a single female child every two generations will result in the scenario rebooting, not extinction. Once your population hits around 200 people, you’ve probably got a better chance of them being wiped out by a meteor strike than due to production of only single sex children.
You’re right that, when such failures occur, it’s almost always before the population reaches 200
It’s not as simple as just rolling the dice to see if it comes up snake eyes. In any given generation, the gender ratio can get closer to 50-50 or further away from 50-50. If the latter occurs, it creates instability which makes the next generation more vulnerable.
It also depends greatly on your assumptions. If you assume that every female will give birth to the same number of babies, then the failure rate drops way down, much much lower than 10%. But if you make the number of babies per female random, the simulation fails a lot more often. Each woman has 24 years of fertility, during which she might give birth 5 times or 20 times, or maybe she’s sterile and she’ll have no babies at all, or maybe she’ll die before she has a chance to have any babies.
Consider this example. We have the first couple, call them Adam and Eve. On most simulation runs, Eve has 10-15 babies but, on this particular run, she has 6 babies. Five are male and the sixth is female – let’s call her Ruth. Fourteen years later, Ruth has her first baby, and it’s a male. Ruth dies during childbirth. Now there are nothing but males, except for Eve who is in menopause. Game Over. Granted, the chances of this happening are maybe 3%, but consider what happens if Ruth’s baby is female. Let’s call her Rachel. At that point, we’ve dodged the bullet but the gun is still shooting. There’s a chance that Rachel’s children will fail to produce any female grandchildren. Then it’s Game Over again.
Each individual scenario leading to extinction is highly unlikely but when you take them together cumulatively, they add up to a very real possibility. Depending how you program the variables, average number of babies per female, infant mortality rate, chances of dying from childbirth or some other accident, etc. you might see simulations which fail 1% of the time or you might see 10%. If you make optimistic assumptions, like every female produces 12 babies, you end up in the 1% range. If you make pessimistic assumptions, like a complete lack of medical care and high infant mortality rates, you end up in the 10% range.
Like I said before, it all depends on what assumptions you make. Many people who do this calculation might have assumed 50-50 gender ratios without realizing that said assumption is far from guaranteed.
I learned a new word today. C-o-n-s-a-n-g-u-i-n-i-t-y. Consanguinity.
I actually had to figure that one out from roots on an anthropology test once. That day I was glad I had played a video game that made it clear that “sanguine” was the color of blood.
Interesting, consanguinity both causes problems and fixes others. It’s not a complete negative. Any recessive lethal genes be carried will get weeded out because the probability of such genes “coming up” is enormously higher.
On the other hand, a couple of nasty things will happen over the long term :
1. If you really start with only 1 couple, that means just 4 total variants of the MHC alleles, and a very limited number of possible segments for certain genes that determine the epitopes that immune cells (such as B-cells) respond to.
This means a monoculture, meaning it is much more likely that an optimized pathogen could wipe everyone out.
However, it would take only modest levels of genetic engineering to solve this problem. Any number of tweaks, achievable with today’s tech, would fix it.
2. The early stages of growth, the population will be so small that genetic drift will permanently eliminate some alleles. This reduces genetic diversity further. You could reduce this problem, again, with technology. You could freeze eggs and sperm from the "original" couple and keep using those eggs and sperm for century, long after the original couple is no longer breeding, in order to retain what limited genetic diversity you have.
The typical sci-fi solution to this problem is to bring along a huge bank of frozen embryos or egg/sperm, donated from thousands or millions of people. You would unpack and implant these embryos in surrogate mothers or artificial wombs (assuming you can get one working, they are hugely beyond current tech) over time.
Of course, if you’re introducing artificial wombs, I should point out the real best case scenario is approximately 40 years.
You “just” need self replicating machinery and a large supply of raw materials and energy. Have your self-replicating factories copy themselves for about 10 years, doubling every month or so. Once you have 6 billion artificial wombs, start them growing 6 billion humans in parallel. 30 years later, you have about as many adults as we have today.