Even without the threat of inbreeding, I’d let humanity die out. It would be cruel to bring a child into that world.
Agreed.
Let’s say they do produce a child–what are they going to feed it? There’s no formula, no rice cereal, no jars of baby food. So the baby breast feeds for a while, at least 18 months by my experience. So, mom’s ovulation is effectively eliminated for those 18 months, then she gets pregnant right away again, leading to approximately 27 months per birth, on average. Now, let’s say Mom starts out at age 14 and won’t reach menopause until 55. In later years it will become more difficult for her to get pregnant, so perhaps we’ll round the pregnancy frequency up to every 3 years. She can produce in that time about 13 children.
(Granted, Michelle Duggar is averaging a child per year, but that is with the use of food outside of breast milk, which allows her to ovulate more quickly.)
Ok, so what are the chances that she die from each of those pregnancies? Historical maternal death rates are 1 in 100, but that’s with folks who know what they are doing helping out–midwives and such–in halfway decent conditions. In our post-apocalyptic wasteland with only her brother to help out I can’t imagine she will have any such luck. According to this Wikipedia article, at its worst maternal death rates were around 40%. So let’s split the difference and say her average chance of dying with each pregnancy is 20%. Statistically speaking we probably have a good chance of her having 10 kids before her maternal death number is up. Of course, if she dies birthing the last one that baby will probably die too, either in birth or due to lack of milk. So, 9 babies.
Cool. Now we have to be concerned with whether those 9 kids will live to breed themselves. This article says estimates of child death rates in medieval times was between 30 and 50%. Since we won’t have to deal with measles, rubella, and other basic contagious diseases we can assume the death rate to be on the lower end of the spectrum. Certainly not zero, since there are plenty of diseases that don’t need human vectors (tetanus, rabies, etc). So, 6 of those 9 babies perhaps (if everything goes very well in this wasteland) survive until the age of fertility.
Maybe we’ll get lucky and all 6 of those babies are girls and are equally fecund with their father. Or, maybe we’ll get unlucky and Mom will be unable to adequately feed her babies, watching them all die of starvation in front of her. Or maybe, they will have a few babies, who will survive into late childhood, only for their parents to die for whatever reason and them to be left alone in a wasteland.
I guess what I’m trying to say is, the brother and sister can fuck all they want, but they had better not get her pregnant.
For most of human history, we haven’t had formula, rice cereal, or jars of baby food. It’s possible to mince or mash or grate quite a lot of foods so that they are suitable for a baby. You can make a simple grater from a tin can lid, a nail, and a hammer…just punch holes into the lid with the hammer and nail. I’m sure that there will be plenty of all of these items after The Big One hits.
Formula usually isn’t necessary, but if the mother can digest dairy, then it’s a good idea for her to drink milk, eat cheese, and consume whatever other dairy she can. Let’s hope there’s a few cows and/or goats around. Goats are better, they can eat a wider variety. I’m pretty sure that they are smarter than cattle, too. At least some breeds of goats can be sheared for their wool, which is another point in their favor.
I used a food grinder sometimes, when my daughter was very young and just starting on foods, but for the most part just cutting stuff up pretty small worked quite well.
“Time to get biz-aaa, Sis!”
“Yeah, right, whatever…”
If ‘humanity’ is down to just you and one person of the opposite sex (with the OP’s implicit assumption that you’re both young and fertile), then it’s really your call. You don’t owe a thing to the billions who are gone; whether or not you and your companion decided to keep the human race alive, it won’t bring back the billions who are dead.
If you want to make the effort, then do, but have as many children as you can, just to make sure that as much of the genetic diversity in your two bodies continues on. If you don’t want to make the effort, then don’t. If you decide not to, nobody but the two of you will ever know.
Incest is purely a moral choice. One I happen to uphold, but not one I’d get upset about if anyone breaks it, assuming they are consenting adults (and preferably of the same generation).
The last two humans on Earth are brother and sister, but they are also the De Facto whole of Earth’s government and any other presiding authority (i.e. church, etc, if they happen to be members) - so they can decide for themselves what they want to do.
Probable extinction is better than certain extinction. Guinea pigs and cheetahs get by without much genetic diversity. So boink away.
I once read that Cleopatra was the descendant of 13 generations of brother-sister mating. May as well try.