Probably fire (how to start, contain and control it). That led to several actual physical evolutions for people, mainly your teeth, jaw muscles and how long infants needed to be nursed. Softer foods meant people could eat things at an earlier age, freeing mothers from nursing to do other tasks or have more children sooner than when children had teeth strong enough to chew raw food. Also there's how much energy your body saved from fighting off disease and such from eating uncooked foods. That energy went to the brain to think about the world, plans for growth, etc. Not to mention lighting dark areas andthe night for more exploration and travel as well as providing security against the wildlife out to eat them and provide warmth for survival in colder climate. Civilization all started in part from fire free up time and open up opportunities to discover the world around them~