Celestia and Luna were sitting in their throne when in barged twilight sparkles, undoubtably trying to tell her idol about some discovery, What she did say was a shock to them.

"princesses have being called... 'Humans' ever existed"

they were both shocked by this. ANd for more reasons then they let on.
after reassurign twilight thagt "Humans" never existed and there just boogeymen invented to scare ponies twilight edited.

As soon as her student left celestied ran over to a hidden panel next to her throne and pushed it revealing a secret tunnel Which she promptly walked down. ANd Luna followed.

When they hit the bottom of the tunnel out walked a royal guard with a beaker cuty mark.

Nodding silently the three reached back for their cutie marks and pulled away, revealing their "cutie marks" to be holographic projectors. ANd they turned into humans

Celestia turned into a tall woman with flower golden hair, a white lab coat, and amber eyes. While luna turned into almost an exact clone of her sister but with a blue labcoat instead and dyed blue hair.

Walking into the room in the back of the tunnel they found their monitors and equipment all set up for surveying ponyville.

for ponyville was all that existed in this world, that and this lab they nicknamed "canterlot"

it was set up to be a stimuli response centeer where they tested animals who had genes injected into them to give them somewhat anthromorphic abilities only humans should have.

ANd so far their secret was safe and the experiment was going fine...