get to 到达
这句话后半段就是个方式状语从句.
……for they were /as near as early man could get to writing/.
你提问的部分是情态动词+动词原形的形式,could do sth翻译为“能做……”
整个翻译：……因此他们就如同早期人类的书写方式相似.