Seeking humanity's roots

Who were our earliest ancestors? How and when did they evolve into modern humans? And how do we define "human," anyway? Was it when some long-ago ancestor stood and walked; grew a brain of a certain size; or figured out how to make stone tools, control fire, plant crops or brew beer? The possible answers to such questions are themselves evolving, as anthropologists and archaeologists continually discover new fossils and artifacts that upset old theories and push the known dates of evolutionary m