Analysis of a clutch of turkey eggs in Mexico suggests humans domesticated turkeys as early as 1,500 years ago -- the earliest evidence of turkey domestication yet discovered.

The ancient eggs were unearthed by archaeologists at a dig site in Oaxaca, a state in Southwestern Mexico. Researchers believe the eggs were used as a ritual offering.

"Our research tells us that turkeys had been domesticated by 400 to 500 A.D.," Gary Feinman, an archaeologist with Chicago's Field Museum, said in a news release. "People have made guesses about turkey domestication based on the presence or absence of bones at archaeological sites, but now we are bringing in classes of information that were not available before. We're providing strong evidence to confirm prior hypotheses."

The Oaxaca dig site has offered a variety of artifacts left behind by the Zapotec people, members of the predominant indigenous pre-Columbian civilization that flourished during the 1st century in the Oaxaca Valley.

Scientists knew right away their find was of archaeological significance.

"The fact that we see a full clutch of unhatched turkey eggs, along with other juvenile and adult turkey bones nearby, tells us that these birds were domesticated," said Feinman. "It helps to confirm historical information about the use of turkeys in the area."

Researchers say the turkeys of ancient Zapotec civilization were not unlike those involved in America's first Thanksgiving.

But whereas Eurasians had access to a wider array of domesticated meat sources, Zapotec were limited to turkeys and dogs.

Today, the people of Oaxaca eat beef, chicken and pork, meat sources introduced by the Spanish. Yet, the turkey's deep historical significance remains.

"Turkeys have much greater antiquity in the region and still have great ritual as well as economic significance today," Feinman said. "Turkeys are raised to eat, given as gifts, and used in rituals. The turkeys are used in the preparation of food for birthdays, baptisms, weddings, and religious festivals."

Researchers described their egg discovery in the Journal of Archaeological Science.