Some possible answers, gleaned from the Internet:

They came from everywhere. It's a conspiracy! How else could there be so many of them?!
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theyre sent from hell to predict the oncoming armeggeddon...i would know, as i am SATAN
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skwerls were an super-advanced interlectual race that lived on one of jupiters moons millions of years ago, along with another race knwn as the slrewks. the slrewks were good , but the skwerls are evil and eat all the food on the moon- and then the slrewks.

They then travelled to earth on sun rays, but on reaching earth it was too hot and so they lost all there intelligence and were bound to spend the rest of eternity surviving on there evil ways of seducing mindless humans with their brain frying chitters and nipping on their nuts...
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They're from another universe in which they took over, now they want our's!!!!
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squirrells come from satan, they are satan's helpers.DIE SQUIRRELS
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The came from chickens.
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They have a super underground civilization spanning across the globe...their sole purpose is to take over the world by acting as stupid rats
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In the early times,when the earth was first being mapped,a rival country sent squirrels to destroty every one who was not a part of the great squirrel empire but the plan back fired and now squirrels want to destroy every one
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They were an experiment gone awry after a disturbed scientist tried to cross a rat with a chinchilla in order to capitalize on the fast breeding habits of the rat but the looks of the chinchilla so so could make tons of expensive coats cheaply.
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They came a curious hick farmer's misguided attempt to recreate life by bathing various small animals in diesel fuel while injecting urine into their bloodstreams.
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It has absolutely no connection with disguised rats, or any other such mumbo-jumbo. They are simply a more advanced life form than we mere humans. They want us to believe that we're superior to lure us into a false sense of security. But note that the red squirrels are on our side. Grey squirrels mutated several millenia ago due to a pigment-related genetic fault.

Since then, to my knowledge, they have taken over the chemical engineering department at Leeds University (UK), and are led by professor P.A. Dowd. He uses the ingenious guise of a grey suit (created by a holographic projector planted within his torso which, as it happens, is made up of 4 grey squirrels)
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They are the metaphysical beings brought here from the underworld to taunt our dogs.
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skwerls are the devils work
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SOME ONE PLANTED THEM IN THE DIRT
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The squirrels are the new dominate species of the known Earth. I would like to tell them ahead of time-before the enslave us and make us pick nuts for them while beating the weak and useless to death with their tails-that i for one welcome our new leaders with open arms. Hail squirrles!
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Skwerls, like all superpreternatural creatures, except for the occasional ellifunt (which are, by all accounts, totally unaccounted for by lesser human species and subspecies which inhabit this {and for that matter, any other} planet, and, by the way, elementarily subsequent to their original specializational origins [although this goes without saying, I'm sure you'd agree if you'd just take the time to sit down, forget all of those other unoriginal and subjugative tasks at hand, and give it the light of true scientific method}, are metamorphic interpositional anomolies. Thank you
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That they are spawns of the world's most annoying adversary...BRITNEY SPEARS~~!!!
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that they are aliens come from the planet prekoriz3zetploorpiskitchie and were sent to prepare the earth for its greatest enemy... THE BACKSTREET BOYS!
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Skwerls come from piles of excess dryer lint. When these piles reach a certain weight they attain critical mass, and thus a skwerl is born.
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Skwerls are, of course, fungal. Originally in the classification Deuteromycota (meaning we don't know how they reproduce), they have been changed by the vast taxonomic "lumpers" into a mammalian catagory. The absurdity of this placement is obvious to anyone with a backyard. We only see adult animals- never the young!! Adults form sporangia under the cute bushy tails and leave the spore everywhere. When weather conditions are favorible (which is most of the time) the spore buds, and develops into skwerls - usually at night, often producing a full sized animal in a 24 hour period.

This is also the reason they are so difficult to eradicate. As anyone who has experienced a fungal infection can tell you, the cure is often elusive, and the infestation persistant.
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My theory is: squirrels were put on this earth to drive birdfeeders nnnnnnnnUUUUUUUTTTTTTTTSSSSS!!!
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that squirrels where created by a massive necleur explosion back in the 1950's and ever since then they have terrerized our world and attempted to destroy all of us in an attempt to take over the world.
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that the squirel's were created by the one and only Ozzy Osborne, because he felt sorry about taking the life of a single dove and felt he had to put another species back into the world.
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My theory is that Squirrels evolved from the squirrelissimo latino, as did their close relatives, the chipmunks. Yes, I AM A SQUIRREL AND PROUD OF IT!!!
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There is no theory. They were creted by God on the sixth day. Genesis 1:24,25:

And God said, Let the earth bring forth the living creature after his kind, cattle, and creeping thing, and beast of the earth after his kind: and it was so.25 And God made the beast of the earth after his kind, and cattle after their kind, and every thing that creepeth upon the earth after his kind: and God saw that it was good.God made man accountable to him.
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Skwerls come from Kibo.
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There are too many of them to be of and from any single source. Proof again, that along with us humans, other bad ideas abound in the universe. I know it's of little comfort, but I get some small smug satisfaction from them being nearly as bad an idea as we are.
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Squirrels, just like Barney and Power Rangers, is one of those mystical horrors that was released apun the opening of Pandora's Box. They may seem cute at first, but watch out.
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Skwerls are a natural adaptation of the more primitive life form - bellybutton lint.
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JON CALLS THEM IN BY NAILING NEW BOARDS TO HIS ROOF.....THE SQUIRREL KING!
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they are spawned by satan him self and they all have big orgie parties after each batch of new skwerls
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They grew from a wild science experiment that was trying to mesh the genetic structure of an amoeba with that of sharp cheddar chese. The genetic coding went haywire, and *poof!*, you have a squirrel
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