The fall of mankind.

And cooking food destroyed all the Biophotons that fed our 6th sense (our sense of connection), it was only by cooking food that we started to consume foods that we weren't biologically adaptable to consume. This resulted in us all becoming sick and weaker with each passing generation. The sickness also brought out a dark side to our behaviour that made man turn on its own kind.
And that kids was the fall of mankind.

In summery.
Brains grew bigger, we mastered fire and then "the fall of mankind" we started cooking our foods.. we started to consume the dead.