Stone tools changed over time because people kept finding new ways to use stone tools. These tools were altered according to the environment around them. If there were elk or caribou around, the early people would make stone arrowheads to hunt the animals, but they would need a completely different tool to transport things. These changes took place slowly because things changed slowly. You don't need to figure out how to harvest plants in the winter, and you don't need to figure out how to keep warm in the summer. Also, it is hard to invent things. You have to try many times to get things right, and it would take even longer then than it does now to figure things out because of the limited materials available.
