This resource from the PBS Evolution Library discusses how early stone tool making marks an important juncture in evolution. Compared to chimpanzees, early humans had longer thumbs with better dexterity in them, due to the presence of three muscles missing in chimp thumbs, as well as brains that gave them the ability to make effective stone tools. Once humans could manufacture and use tools, they could obtain more and better food. This enabled them to successfully raise more children, who in turn were likely to inherit their parents' hand morphology, leading to the increased manipulative abilities seen in modern humans.
Students could use this tool to discuss how something as simple as a thumb can drive evolution. Have them use the Chimps, Humans, Thumbs, and Tools resource listed below for activities related to thumb use in humans.