What has a small brain cavity but human like jaw and teeth?

The creature you're describing could be a species of primate, specifically a small-bodied hominin like Australopithecus. These early ancestors of modern humans had a relatively small brain cavity compared to modern humans but exhibited a jaw structure and teeth that were more similar to ours, indicating a diet that likely included both plant and animal matter. This combination reflects an evolutionary adaptation towards bipedalism and complex social behaviors.