yet once language developed ,the possibilities for human kin

developed是形容词修饰yet once language ,increased同样也是后置修饰the possibilities for human kind's future attainment and culture growth 这个整体.
再问： 还是不太明白 那部分是主语啊？ 那部分是谓语啊？
再答： 你可以理解为这样： Yet once language had developed ,the possibilities for human kind's future attainment and cultural growth have increased .这样主语和谓语就分清了。