When photographs were just beginning to emerge in the 1800s, it was considered improper to show off your big goofy grin. So, camera operators asked participants to say "prunes" in order to pucker their mouths and make them look taut. This predated the duck-lip selfie by roughly 200 years.
How Could This Possibly Be?
Oxford University is significantly older than the Aztec Empire. Oxford University was founded in 1096, and the alliance of the three city-states bordering the Aztec Empire occurred in 1428.