Although Thanksgiving is a North American holiday and a recent invention in the grand scheme of things, the tradition of breaking the wishbone comes from Europe, and is thousands of years older.
A bird’s wishbone is technically known as the furcula. It’s formed by the fusion of two clavicles, and is important to flight because of its elasticity and the tendons that attach to it. Clavicles, fused or not, aren’t unique to birds. You and I have unfused clavicles, also known as collarbones, and wishbones have been found in most branches of the dinosaur family tree.
The custom of snapping these bones in two after dinner came to us from the English, who got it from the Romans, who got it from the Etruscans, an ancient Italian civilization. As far as historians and archaeologists can tell, the Etruscans were really into their chickens, and believed that the birds were oracles and could predict the future. They exploited the chickens' supposed gifts by turning them into walking ouija boards with a bizarre ritual known as alectryomancy or “rooster divination.” They would draw a circle on the ground and divide it into wedges representing the letters of the Etruscan alphabet (which played a role in the formation of our own). Bits of food were scattered on each wedge and a chicken was placed in the center of the circle. As the bird snacked, scribes would note the sequence of letters that it pecked at, and the local priests would use the resulting messages to divine the future and answer the city’s most pressing questions.
When a chicken was killed, the furcula was laid out in the sun to dry so that it could be preserved and so that people would still have access to the oracle's power even after eating it. (Why the wishbone, specifically—and not, say, the femur or the ulna—is a detail that seems to be lost to history.) People would pick up the bone, stroke it, and make wishes on it, hence its modern name.
As the Romans crossed paths with the Etruscans, they adopted some of their customs, including alectryomancy and making wishes on the furcula. According to legend, the Romans went from merely petting the bones to breaking them because of supply and demand. There weren't enough bones to go around for everyone to wish on, so two people would wish on the same bone and then break it to see who got the bigger piece and their wish. This doesn’t make a whole lot of sense to me—Were there really that few chickens being slaughtered in Rome? If a resource is already scarce, why would you break what supply you do have into pieces?—but I can’t find much more than this about the bone-breaking aspect of the tradition.
Anyway, as the Romans traipsed around Europe, they left their cultural mark in many different places, including the British Isles. People living in England at the time adopted the wishbone custom, and it eventually came to the New World with English settlers, who began using the turkey’s wishbone as well as the chicken’s.
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