10,000 BC/WMG

All Roland Emmerich films occur in the same continuity.
There was a rebellion against aliens in 10,000 BC, UniSol escapades in 1992, a secret mission through the Stargate in 1994, an alien invasion in 1996 (no doubt a result of the Abydos expedition's actions), a monster attack on New York in 1998, and the start of a new ice age in 2004. It's just... a very unfortunate Earth.)
 * Furthermore, it's the Earth of Battlestar Galactica.
 * While this would be an appropriately crappy Earth for BSG to end up finding, there's a problem. New York was blown to hell by the aliens in Independence Day. So they rebuilt New York in two years, just in time for not-Godzilla to show up? The guessing isn't wild enough... yet.
 * Theories for that:
 * Godzilla takes place in the same continuity as the, *ahem* real Godzilla, not the 10,000 BC <-> Day After Tomorrow continuity.
 * The movies aren't necessarily all in chronological order. The Chrysler Building wasn't blown up in Independence Day, so Godzilla happened first. A shame, that.
 * Or New York was rebuilt very sloppily. Look at the skyscrapers as Zilla runs past them - Special Effect Failure, or cheap reconstruction?
 * Excluding Godzilla (since it's a different continuity), here is the tentative timeline for the Emerich-verse:
 * 10,000 BC: Slave revolt against an alien overlord, Ra, a "Roswell Grey" in his true form, who escapes to Abydos with human slaves (10,000 BC, Stargate)
 * 1992 AD: UniSol endeavors to create the perfect soldier to fight the alien menace (Universal Soldier)
 * 1994 AD: The Stargate project invades alien space and puts soldiers on an alien world, pissing off Ra and his buddies (Stargate- seems these people have a tendency to think they're the last of their kind when they're not), leading to:
 * 1996 AD: Ra's buddies- more of the Roswell Grey types- show up with a gigantic mothership, a few dozen destroyers, and thousands of fighters. The White House gets blown up, people die, President Awesomeness leads the survivors of the human race to victory, and an alcoholic crop duster saves the world (Independence Day). Remember how those aliens were Greys under their bio-suits? Remember how their fighters resembled scarabs?
 * 1990's, early 2000s AD: Though not Emmerich, we may as well fit the events of the Mad Max films here by presuming that Australia never recovered from the Alien Invasion (Mad Max, The Road Warrior, Thunderdome).
 * 2004 AD: Weather happens (The Day After Tomorrow). This especially sucks, since the human race had to spend eight years rebuilding.
 * In conclusion: Yes, it's a very disaster-prone Earth. Expect the Robot War to happen any day. It is possible that large governments in the Emmerich-Verse are drugging their citizens to erase the memories of so much tragedy. If true, by the late 21st century, the Emmerich-Verse may grow to encompass the timeline of Vonnegut's "Harrisson Bergeron."
 * Emmerich's new movie is going to end this parallel universe then?
 * 2012? At least the dates line up.
 * Think of it as a mercy killing.
 * Then again, maybe Roland Emmerich is related to HAL Emmerich, meaning that the MGS storyline could be added in somewhere. A nuclear war caused in the confusion after the defeat of the Patriots could cause The Day After Tomorrow nuclear winter, setting the stage for Battlestar Galactica.
 * Where does The Patriot fit in, then? It's pretty normal...

The entire movie was an Edutainment-style film for a Far Future museum about the Ancient Past.
This would explain all the random bits of information that made absolutely no sense. It's happened that scientists and archaeologists find pieces of information from the past and put it together in the wrong way, sort of a 2+2=5 deal - In this far future, things have degraded even further, and trying to make a coherent timeline results in semi-Egyptians using mammoths and dinosaurs to build their Obelisks until they're toppled by a Caucasian caveman with a sabertooth soul and an army of Magical Negroes.

10,000 BC isn't 10,000 years before Christ, but...
10,000 years Before Computers. Roughly 2000 more years to the equation would make the cavemen progress more plausible.
 * That MIGHT cover one of this movies' inconsistencies, but we're still at a loss for everything else.

'10,000 BC is actually the film version of Sid Meier's "Civilization."
Random cultures clashing, along with seemingly random wilderness encounters? Yeah, Civilization all the way.
 * Would also explain the wacky geography. (Must be a small map...)
 * One (human or computer?) player was just about to complete the Pyramids wonder, but although he was able to produce Horsemen and Triremes before everyone else, he was defeated by the other players via Zerg Rush using a lot of Warrior units.

"BC" stands for "Big Coverup"
It takes place in the present, in an obscure third-world country that no-one has heard of. The "god" is an American puppet-dictator, and the feds want to keep the whole thing under-wraps. By disguising his findings as fiction, Emmirich can publish them on a wide-scale so that intelligent people (like me) can figure it out.

10,000 BC is just an excuse for...
Archaeologists to play '10,000 BC: the drinking game', you don't get a detail wrong by eleven and a half thousand years without it being intentional, and if they'd done something comparable like give them guns then everyone would get to play.

10,000 B.C. takes place on Pangaea
Just think about it. In one scene, the characters are trudging through a snowy mountain pass, and in the very next scene, they are hacking through a steamy jungle. A few more scenes later, and they're in an African desert. How is this possible? The continents had not moved apart yet in this movie.
 * Thing is, the continents were already in their current position before modern humans evolved; in fact, only the very beginning of the age of the dinosaurs had the continents still as Pangaea. Most likely this is a parallel universe where the continents just collided or never changed positions and Pleistocene species happen to live on it.
 * Even if it was Pangaea, that wouldn't justify having arctic wastelands and tropical jungles so close to one another.
 * The Other Wiki says it's set in South America. The icy mountains are the Andes; the jungle is on their rainy side, and the desert is on their dry side. The only "African" thing in the film are the African-American actors playing one of the tribes, and those folks probably represent "Luzia woman"'s people, same as the hero is from "Kennewick man"'s culture.

10,000 B.C. takes place in the world of Land of the Lost.
Random elements from widely varied time periods and even alternate timelines are jumbled together into a single world.

10,000 B.C. takes place on another planet.
No where does the film explicitly say its earth; it could be any number of illy-defined near-earth sci-fi planets. Now take your pick: convergent evolution, alien gene-seeding, etc.