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Today I was wondering this question whether it is possible for two people to repopulate Earth.

What if everyone else disappeared, except for two people (man & woman)? Can they repopulate? In that case, what are the odds against and in favour of that ever happening?

The Artist
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    Possible?: of course. Probable?: hmm... . Would it generate a healthy human population?: No. – James Oct 18 '15 at 23:59
  • Possible duplicate of How many people are required to maintain genetic diversity? – March Ho Oct 19 '15 at 02:53
  • You cannot predict that. They may or may not lead to an unhealthy population of descendants but surely they'll face the risk of extinction because of lack of diversity. Extinction is dependent on the environment and it may or may not be adverse to this population. Since there are so many unknowns here, the answers would most likely be based on guesses. Hence I am closing this question as opinion-based. – WYSIWYG Oct 19 '15 at 06:00

2 Answers2

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Well, mathematically it might be possible. But, I would rather worry about the low amount of genetic diversity that the growing population would have, which would make it extremely sensitive to external stresses.

To repopulate earth from such a small pool of genetic material, one would need an organism that is much more sturdy as compared to humans in extreme conditions, multiplies fast and is much more prone to mutations that actually convert to a phenotypic significance so as to increase the genetic diversity of the total population.

Kunal24
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Update (after receiving many comments)

Before you read answer, note that this is hypothetical question and I am trying to answer it by creating hypothetical situations with some reasonable numbers

Short answer: Yes, hypothetically it is possible for two people to repopulate earth.

Quoting from Popsci website:

Maybe, but they'd be very busy

Hypothetically it is possible but practically it has very less probability. Let us calculate probability of repopulating earth to it's 10% population.

  • Let us assume one female can give birth to 50 children throughout her reproductive life spam and assume there is no death/disease/natural crises and other factors which might restrict this 'ideal' growth.
  • To get 'maximum' number of births in next generation, assume she gives birth to 1 male child and 49 female children. Probability of having that is $\left(\frac{1}{2}\right)^{50}$.
  • Now again assume all 49 females gives birth to again same ratio of children (too much work for male :P). Probability of happening this will be $\left(\frac{1}{2}\right)^{50+49}$.
  • If you go by same logic, you need at least 5 generations to make 10% of earth's population. That gives probability of $\left(\frac{1}{2}\right)^{50+49+49+49+49+49} = \left(\frac{1}{2}\right)^{295} = 1.57 \times 10^{-89}$

It is much lower than probability of we finding electron in nucleus :P

Now if you want to consider little bit realistic model where each couple will have two more couples and so on, It will need at least $2^{30}$ generations to repopulate to 10% of today's population. Considering average lifespan of 60 years, it will take $2^{30}\times60 = 6.4 \times 10^{10}$ years to do that. It is higher than entire biological evolution time scale.

Point of these calculations are to show that, hypothetically repopulating earth with one couple is possible but very less probable.

Dexter
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    5 generations for all those people seems very reasonable space of time and isn't the real issue of "is it possible". I think the answer here needs to approach genetic bottlenecking, survival scores of individuals over extended periods of time, and optimum conditions required for repopulation. – James Oct 18 '15 at 12:10
  • I doubt in this hypothetical situation genetic bottlenecking will matter and I don't know how will you implement survival score here. – Dexter Oct 18 '15 at 12:17
  • But if we are here in these numbers within some few million years when homo appeared - how would you explain that? And that probability calculation for 1:49 male:female is not to the point here. – AliceD Oct 18 '15 at 12:47
  • Down vote ? It is hypothetical question. I have also mentioned in answer that "Point of these calculations are to show that, hypothetically repopulating earth with one couple is possible but very less probable" – Dexter Oct 18 '15 at 12:51
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    The probability statements in this answer are not very precise. Let X(t) be the population t years after the human population is reduced to two people. What are the probabilities you are interested in? Pr[X(1000) > 0]? Or maybe the limit of that probability as t goes to infinity? Or the probability of reaching some threshold population in a certain amount of time? – Adrian Oct 18 '15 at 13:23
  • When you write "Let us calculate probability of repopulating earth to it's 10% population", for example... in what time frame? – Adrian Oct 18 '15 at 14:09
  • @Adrain Well, in first calculations I don't consider time period because biologically that kind of dynamics is not possible. In second calculation I have mentioned biologically plausible dynamics with time period. – Dexter Oct 18 '15 at 14:35
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    Correct me if I am wrong: in your first example, haven't you merely calculated the likelihood of 1 male descendent per female (while having 49 female offspring per female) over 5 generations? Yes, this chance is minuscule, but what has this to do with repopulation - apart from that this might be the "ideal" growth curve? Also - from a genetic point of view, a 50:50 male/female ratio would be a lot healthier. – Gerhard Oct 18 '15 at 22:49