Google Doodle today for the 41st anniversary of the discovery of Lucy the Australopithecus, the oldest hominid we know.
Doctor Who started with troublesome cave people, and Primeval did the obvious adventure of a time-travelling villain targeting humanity’s earliest common ancestors, but I wouldn’t let that stop me. And not just because I want to see mammoths and smilodons.
A bit further forward, you could use some of the prehistoric adventure hooks from this RPGnet thread. The secret of fire gets a mention in the sixth hook.
An Australopithecus on the TARDIS might be trouble...