Isn't the theory that agriculture meant some book-keeping was needed, to keep track of the seasons and when to plant. This book-keeping then caused the development of writing in general. And writing caused the birth of inter-generational memory and the ability to accumulate knowledge. Of course oral histories can be cross-generational as well, but writing is much more efficient storage medium than human brain alone. Writing allows human brains to co-operate over vast geographical distances and time-scales.