Complex sentence: Historians have conjectured that the Chinese mythology began in the 12th century BC. Legends were passed down in oral form for over a thousand years, before being written sown in books such as "Shan Hai Jing".
Split into simpler sentences: Historians have conjectured that the Chinese mythology began in the 12th century BCE. The myths and legends were passed down in oral form for over a thousand years, before being written in books such as "Shan Hai Jing".