圣人都会犯错,所以您老师理解错误并不奇怪.originated 修饰的不可能是Rome,它就是the Latins 或 the Latins who founded Rome 的谓语动词.如果把句子重组一下,可能更容易明白：
The Latins,who founded Rome,probably originated from the fertile plain of Latium that created the habits and skills of landed settlement,landed property,landed economy,landed administration,and a land-based society.