Let 'em hump. What else are they gonna do?
Well, there would be lots of Real Doll’s available as alternatives. 
If the question is whether or not the two should hump, well, that’s up to them (if we are simply talking about recreational purposes). I’d say that they SHOULD (for procreation reasons), since even a remote chance for species survival is better than no chance at all.
How many kids can two people take care of, while struggling to scratch out a living from a nuclear wasteland?
Even if it’s not a nuclear wasteland, if everyone on earth just drops dead leaving everything intact, it’s not going to be easy.
How would they even know they are the last two people? they would grow old looking for others before they decided to start fucking.
I see the same question as a reason for them to start having kids. Even if you’re genetically doomed by having such a small gene pool, there’s always the chance that you (or your descendents) eventually discover some other pockets of survivors. At that point, you can all do a big wife swap and get diversity moving along.
They’re the last two people. Doesn’t that cut down disease opportunities somewhat?
Also, unlike the folks who suffered through the bubonic plague, they understand germ theory and the importance of washing hands. Doesn’t that tip the odds a bit?
Infectious disease would not be a problem for generations. When human beings are reduced to two individuals the vast majority of human infectious diseases will be extinct.
For things like chicken pox and colds and flu, the two individuals are likely already infected with chicken pox. Except their immune systems have suppressed the virus to such an extent that they don’t shed virus. So their children will never get infected with chicken pox because that requires exposure to someone who is shedding the virus during an active infection.
So good bye common cold until the population grows to the millions again. And even then viruses will have to start from zero. Colds persist because even as older strains are being wiped out by the human immune system new strains are mutating from the older strains. Wipe out the reservoir of colds and flu and there are no old strains for new strains to evolve from.
The only infectious diseases to worry about are those with an animal vector. So yeah, rabid skunks are still a problem, and malaria.
Well, there’s plenty to eat. Provided you don’t mind eating people.
But hey, you’re already dorking your sister so why start drawing lines now?
Just so we are clear here, this isn’t my area of specialty…in fact, I flunked 2 semesters of Chemistry in college and I skipped biology (I took physics instead). So, grain of salt.
However, these two people presumably still need to eat. That probably means that at some point they are going to have to do the whole animal husbandry thingy…and a lot of human diseases come from domesticated animals. Or they are going to go hunting for things like birds, which carry diseases that can jump between species. Unless they are in a cold war bunker that’s stocked with everything they need for decades, they are going to be exposed to disease.
I suppose if you want to project that they will be in an entire sterile environment, with all their needs taken care of (food, water, shelter, medicine, power, etc), and then ask ‘could the human species rebuild from just two people’ then it’s possible. But, the thing is, that without a lot of genetic variation the descendents are going to have issues when they come out of the bunker, because even with mutation it’s going to be a LONG time before enough diversity is incorporated into the gene pool to withstand a major outbreak, even if the new population disperses rapidly. At least that’s my take on it.
Does it really matter if the last two are brother and sister? Obviously, regardless of how the last two are related, everybody in the next generation are going to be brothers and sisters…
Animal diseases jump to humans because there are millions of chances of it happening every day - even then it happens not particularly often. With only two humans left that is no longer the case and as said above, they could probably not worry too much about most diseases for quite a while.
As far as the genetics go, there may be some problems with recessive genes for a while but I would imagine (also not an expert) that genetic drift would eventually get us to a decent place. After all, hasn’t it been established that about 120 000 years ago there is one genetic “Eve” and from about that same period one genetic “Adam” that we all are related to?
Having already created “Gangnam Style”, humanity has fulfilled its purpose and can add no more of significance to the universe.
Y-chromosome adam and mitochondrial eve are for those alone, not the species as a whole - there were other people alive at the same time and some of their gene line may survive, just not their y-chromosomes/mitochondria. But still, look at some populations with very small bases today like the Amish - they have greater than their share of genetic problems, but they don’t just up and die off wholesale. I suspect disease (but not person-person disease - infected wounds, etc), starvation, or death in childbirth before we reach a few hundred survivors are going to account for most of the chance of us going extinct with only two people left. IMO genetic issues will be almost meaningless unless we get really unlucky with the genomes of the two people alive.
This situation offers an option of Do, or Don’t. Don’t insures failure. Do offers only probable failure, perhaps partial success for a time.
So you Do.
It is really that simple.
Humans are not emotionally designed to look failure in the eye and then roll over and die. At least I hope not, yet.
Well, yes. However the initial pair are or are not related, each member of the next generation will share very nearly 50% of their somatic DNA with their siblings. Very nearly 50%.
There are, on average, about 65 new mutations in each human offspring. Many do not affecting coding DNA. The odds that the first generation of offspring from this initial pairing get even one of the same 65 new mutations is remote.
So genetic diversity will rebound over time if other stuff doesn’t kill the survivors off first.
Right. Suppose the last two human beings on earth were an unrelated man and woman. They decide to repopulate the earth, and have 12 kids. Every one of those kids will have to mate with their brother or sister.
By making the last two humans brother and sister, all you’ve done is add one generation of brother-sister mating. For the long-term survival of the species that one generation is irrelevant.
If it was me and my sister I’d let humanity die off. In fact, I’d probably convince my sister to commit suicide with me.
Not actually true. Look at “exotic” domestic cat breeding for a current example. There’s lots of matching parent-to-suitable-offspring going on, and once you get another generation, grandparent-to-suitable-grand-offspring.
If Adam and Eve have 12 kids, and Johnny happens to take after either of Adam’s parents particularly strongly, then it might be better for him to sleep with Mommy instead of a sister, to get another layer in there. Same if Jane is very similar to Eve’s parents - Adam gets to take care of the daughter as well, to spread the variances out a little.
Even without that, the idea of permanently pairing off to produce multiple children isn’t a given - the best outcome would be to continually mix things up until each person has produced offspring from each available person of the opposite gender. Then do it all over again, with all of the generations mixing for as long as the fertility of the older generations holds out (use sperm banks and frozen eggs to extend that range) and as soon as each younger generation becomes fertile.
If you’re really aiming for diversity quickly, even once there were enough people to pair or trio or group up into “married” people, you’d still want them to use sperm and egg banks, or just to sleep around for the purposes of fertilization.
Of course, with cats, you’re working on a very specific set of visual and behavioral cues to determine which mating pairs work best to enhance the goals you have for the breed, and with this magic pair of humans in a presumed high-tech wasteland, they can look at each others’ genetic code and make multiple and sequential matches based on the best genetic pairings available, regardless of the relationship statuses of the pairs in question, for as long as is needed to establish a “healthy” variance in people’s genetic makeup.
I reject that I have any duty to have children. I don’t care how few people there are.
I can never understand these scenarios. I just don’t think it would be worth trying. I care about people, not “humanity.” If everyone else is already dead, everything I care about is already gone.
That’s the way I look at it.
Also the op seems to be implying they can artificially inseminate each other so no worries about humping your sibling, it’s purely about genetics.