"They" did not all write it down...I doubt that a farmer in 1500 BC would have written or read anything at all...let alone bothered his tired head with stories about ditch-digging deities...etc.
To suggest that everyone believed this story is as preposterous as suggesting that everyone in 18th century France was a deist because a copy of Voltaire's Candide has been found.
Most people in 18th century France would never have read - let alone understood - Voltaire's philosophy and, I'm guessing the average French farmer couldn't have cared less about the philosophical or religious ideas of some fancy-pants fop in a wig even if he knew of it...
I see no reason to assume that the fanciful musings of a bronze age Mesopotamian priest would have featured prominently on the reading lists of the Babylonian hoi polloi...we have absolutely no idea what the average Joe in the middle east 3500 years ago believed because "they" didn’t write anything down at all.
No one said "They all wrote it down" .. most people could not write in 1500 BC .. but we have various cultures that did write it down .. and we can read their stories .. Canaanites and surrounding peoples (moabites, elamites, amalekites, Hittites and so on ) , Assyrians, Babylonians all had the same creation story .... there slight variations in the stories .. but it is the same story that each culture has .. same basic plot ... the names of some of the Gods change over time .. this was the same story believed for thousands of years .. longer than the period folks have believed in the story of Jesus.
Your claim that we have no writings from 1500 BC is simply false .. we have plenty of cunaiform tablets from that period .. the gilgamesh epic for example is much earlier .. all kinds of myths and stories .. as well as historical and other writing.
In the Sumerian tale of Inanna and the Huluppu Tree (c. 2900 BCE), for example, Gilgamesh appears as her loyal brother who comes to her aid. Inanna (the Sumerian goddess of love and war) plants a tree in her garden with the hope of one day making a chair and bed from it. The tree becomes infested, however, by a snake at its roots, a female demon (lilitu) in its center, and an Anzu bird in its branches. No matter what, Inanna cannot rid herself of the pests and so appeals to her brother, Utu-Shamash, god of the sun, for help.
Gilgamesh
Gilgamesh is the semi-mythic King of Uruk best known as the hero of The Epic of Gilgamesh (c. 2150-1400 BCE) the great Babylonian poem that predates Homer's Iliad and Odyssey by 1500 years and, therefore...
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