This primitive form of vegeculture may have been the economic tradition onto which the cultivation of summer rainfall cereal crops was graftedas it came into use south of the grassland areas on the Saharas southern borders.
as是翻译成由于还是当什么时间，use在into后面是名词词性吗？use后面接了一个south如何理解？
...as it came into use south of the grassland areas on the Saharas southern borders.
1. as 表示“当......时候”，由于它引导的从句跟主句没办法构成“因果关系”。
2. come into use 是个“习语”，use 是名词，不要拆开剖析。意为“开始用；投入用”。
3. south of 是“复合介词”。 south of = to the south of （在......南部。不接壤）
...as it came into use south of the grassland areas on the Saharas southern borders.
【直译】当它在撒哈拉南部边界草原区域以南的地方投入用时 （意思是：在草原区域以南的地方开始用。）
