Jack (webcomic)/WMG

The world presented is a post-apocalyptic version of Earth
Quite obvious, indeed. But wait, there's more:
 * The furries were artifically created. This one is also obvious, as it was showed in a flashback.
 * Jack, Jill and Central -- among others -- are part of the "first batch" of furries. As showed in the same flashback, they had no reproductive organs, for obvious reasons.
 * Somewhen after the obvious question was raised: What Measure Is a Non-Human?? This leads to many -- or several -- lawsuits, discussion and general outcry. Apparently, though, the furries lost in the whole process, and, morally speaking, things remained as they were.
 * Kane was the one that, secretly, started the creation of furries with sexual organs. His motivations quite probably led him to be stablished as the representative of "Envy", and this might explain why the heck he is in Furry Hell.
 * It was revealed Kane had created, while alive,, with a side of karma.
 * And somewhen after all that Jack went on a killing spree for some reason -- probably involving Jill' death, or may I say, execution -- and killed all humans. All of them. Whether he killed himself -- and maybe Central -- afterwards, User:Pro-Mole is not very sure.
 * I think it is going along the lines of
 * Also, all humans,, went to Heaven. That's why we didn't see them anymore, we don't get to see Heaven that often do we?
 * Or alternatively, there's a different part of each afterlife just for humans.
 * Or there are several billion humans in hell, but by now so much time has elapsed since the creation of furries that their souls outnumber the humans so hevily humanity is a tiny, invisable minority in the afterlife with the one exeption of Kane.
 * According to the comic, humans destroyed the world, and God had to do a hard reset on the universe.
 * Or rather, a hard reset of history, making sure things are more or less the same in certain aspects up until humanity's chronological high score is broken.

Jack has two or more parallel universes
One of them is where Jack, Jill, Central and possibly Plato come from, and one where human history is reentacted with furries. Seriously: furry Samson, the medieval ages, Vietnam, WTC? All of this couldn't possibly happen again just by chance. Whether arc XXIII takes place in yet another parallel dimention or is the future of one the two is debatable.
 * The "hard reset" of the universe by god also ensured that history would take the same basic course.

It's all the same universe, but Jack, Jill and Central were created a long, long time ago...
Human history had proceeded completely differently up to that point, with none of the above-mentioned historical events. After the furrocalypse, this universe's history unfolded, except with furries instead of humans.

Both this theory and the above one help to explain the obvious holes in Hopkins' universe caused by an attempt to hammer together a plot from a bunch of arcs written with no preplanning. Hopefully, he'll throw us some explanation, someday, but for now it seems we have a long time to wait...

There are no humans because they all reincarnated.
Simply put. The real reason is the artist says he can't draw humans well. In story, they reincarnated as furies. Then died again. All save 2.

The series takes place after the rise of the Great Old Ones
And in an alternate universe. When the Great Old Ones awoke and began to annihilate everything, one of the Elder Gods (God) created a hidden universe of their own. After spending thousands of years in darkness, either from mourning or fear of discovery, God creates the angels, and then gets the suggestion of creating a world. But after creation, God discovers that one of C'thulhu's Star Spawn ("Friggin Clithu Elder God") entered the universe at its creation, and is attempting to alert the mighty Old Ones. In a panic, God arranges the rebellion of angels to justify the creation of Hell, which is actually harvesting suffering to imprison the Star Spawn and keep its memories suppressed.
 * Humans were created first, and followed their own path as God let them. When they created the furries and then set the earth back to the stone age, God manipulated the furries into following the paths of the humans from the original universe, just for old time's sake.
 * God must have exaggerated the damage Her presence would do to mortals. Elder Gods don't tend to kill you by being near them.
 * Explained away in "sever the anger" with whats either a Hand Wave or Fridge Brilliance

There's still a lot of humans in the afterlife, but they're segregated from furries by both God and Satan.
God because of the potential for trauma (although those who seek shall likely find, to paraphrase God's number one bestseller), Satan because, well, why waste a perfectly good mindfuck all in one go? Rather than mash the revelation of furrykind's origins in with the early horrors of a soul's Hell experience, at which point it'd just seem bizarre and confusing and probably be distracting ("What the hell are all these bald monkeys?"), it's better to make souls work for it and reveal it in a super-traumatizing way. That way, it'll have an epically mind-shattering impact.

Apple Crater Lake was created in the Human-Furry War.
A moment of consideration for the poor furry geologists trying to figure out how it was formed. And a moment of silence for those who actually did figure it out and were secretly assassinated by the US government for coming dangerously close to the most dangerous secret on the planet.

Jack takes place in the same universe as Redwall.
Artie works at Saint Ninian's Hospital, and reads Redwall to the kids - possibly in their world it's a history book. There are hints in the Redwall series that there used to be humans in Mossflower and aren't anymore, and Farrago claimed to have been a female soldier "in the days before guns and artillery".

On that note, Farrago is a reincarnation of Ferahgo.
Possible going by the laws of the Jackverse, especially if he was reincarnated multiple times, as the reincarnation keeps some obvious aspects of the original self. If Satan was telling the truth to Drip, it is quite possible to change gender during reincarnations - we do know it's possible to change sexual orientation, but the concept of soulmates seems to be true, so this would make sense. The complete 180 in her personality from his turns this into Nightmare Fuel, though.

Jack wiped out humanity by causing Instrumentality.
First of all, let's consider just how unlikely it is that Jack alone could have killed 7 billion people. So he joined forces with SEELE, who managed to get Rei to merge with Lillith, kicking off instrumentality, reducing humanity to Tang. God saw what was done, and caused the furries to repeat human history, but also noticed that one human boy survived. Shinji Ikari, floating in EVA-01 in space. Out of mercy, God opened up a wormhole to another Earth that existed in that universe, and granted Shinji immortality, and formidable divine and psychic powers. Upon landing on that Earth, Shinji noticed how primitive the humans living there were, and begins to teach them - after recovering from shock, of course - and eventually he also becomes more masculine. Millennia pass, and humanity spreads to the stars, before collapsing into a new dark age. Shinji plans to fight this by creating genetically-engineered super-soldiers, the Space Marines, and their Primarchs, after crowning himself Emperor. The Horus Heresy happens, Shinji is wired up to the Golden Throne, and the general setting for Warhammer 40,000 is created. And Drip becomes a Daemon Prince of Slaanesh.

Some humans survived.
They somehow got sent to a duplicate of Earth in the distant past, and eventually evolved into another species, humens.

Humans weren't first.
They were, in turn, created by another race, Earth's original masters and (perhaps) the direct children of God. Unlike their creation, humanity, they'd developed extraterrestrial travel by the time their creation rebelled. Sensing that victory was too likely, they left the rebels stranded on their mudball, assuming they'd never have to deal with their vengeful children again. They were right. Their grandchildren, on the other hand...

Once upon a time, the wings of angels weren't feathered, but insectoid.
 * Despite what you might think, God never forgot her original creation. She changed her form to match the newest incarnation of her progeny, yes... but she never did change her eyes.