The lines A, B and C indicate where the Y axis was at sometime in the past. For this discussion we will primarily be looking at the accumulation of knowledge and its almost inseparable twin invention. If we go to the far left side of the graft we will be looking at the beginnings of the accumulation of knowledge. Even if we consider the knowledge base to have doubled from the discovery of fire to the development of the first stone clubs if it were over a long enough period our line will remain essentially flat and straight. From one account of world history we find these following entries.
Egyptian agriculture develops.
Western Europe is neolithic, without metals or written records. Copper used by Egyptians and Sumerians. And in another account of world history we find the following about the stone age. The dates of the Stone Age vary considerably for different parts of the world. In Europe, Asia, and Africa it began about 2 million years ago.