(CNN)Ancient Judeans ate non-kosher fish at a time when it was thought to have been prohibited in the Bible, according to a new study.
Researchers analyzed ancient fish bones from 30 archeological sites in areas that are now part of Israel and Egypt, dating from the Late Bronze Age (1550-1130 B.C.) to the end of the Byzantine period in 640 A.D.
The main principles of Jewish dietary law are laid down in the Chumash, or written Torah -- thought by many scholars to have been compiled during the Persian era (539-332 B.C.) and based on long-held traditions.
Observant Jews still comply with kosher requirements as laid out in the book -- including the requirement that any fish consumed must have fins and easily detached scales.
After analyzing fish bones at many Judean sites dating to the Iron Age (1130-586 B.C.) -- including at the Judean capital city of Jerusalem -- researchers found a significant proportion of non-kosher fish remains, such as catfish and shark.
The team also found evidence of non-kosher fish consumption in Jerusalem during the Persian era -- the point at which the laws were thought to have been compiled.
Yonatan Adler, senior lecturer in archeology at Ariel University on the West Bank, told CNN that he and his colleague Omri Lernau, from University of Haifa in Israel, wanted to use archeology to pinpoint the exact time when ancient Judeans became aware of the Torah and started to observe it in everyday life.
"We have in two places in the Torah, (in) Leviticus and Deuteronomy, a prohibition against eating finless and scaleless fish," he explained.
"What we found was that throughout the Iron Age ... there's no evidence that Judeans or Israelites were abstaining from scaleless fish," he said.
As a result, say the study authors, their findings call for a rethink of the assumption that long-held traditions were the basis for the food laws as described in the Torah.
Adler said more research needs to be done to pinpoint exactly when Judeans began to abstain from scaleless fish, adding that there is a gap in his team's data for the Hellenistic period (332 B.C. to 63 B.C.) -- the time between the Persian and Roman periods.
"Afterwards, during the Roman period, when we find Judean assemblages of fisher remains, they are almost completely absent of prohibited fish," he said.
The study was published in the archeological journal Tel Aviv.