The development of human mental ability can be tracked through the progressive crafting of stone tools, Van der Leeuw explained. First we learned to shape an edge—a line—then the surface, then the whole volume of the tool, then the sophisticated sequence required to make a superb spear point. It took 2 million years. But by 300,000 years ago the human brain had developed a sufficiently complex short-term working memory to keep 7 (plus-or-minus 2) considerations in mind at once. We could handle problems of multi-dimensionality…