Justin Rowlatt goes right back to the origin of our species two million years ago to explore how the mastery of fire by early humans transformed our metabolism, helping us to evolve our uniquely energy-hungry brains. The physical evidence for early use of fire is frustratingly thin on the ground, according to archaeologist Carolina Mallol. But primatologist Jill Pruetz says she has learned a lot from observing chimpanzees interact with wildfires on the African savanna.