When anthropologists painstakingly identified the taxon of the skeleton that later became known as "Lucy's child,"

There was no eureka. There was no grand turning point. The evidence kept dribbling in, and through hard labor and some dogged thinking we did solve the puzzle, not through revelation but through a sort of absorption, just below the level of explicit consciousness. It was as if the truth had slowly seeped through our pores, until we had come know it without knowing that we did. So when the final, indisputable confirmation came, we hardly noticed the event. What had once been a mystery had become—in hindsight, mind you—obvious from the start (Johanson and James Shreeve 1989, p. 203).

Instead of there being a clear point at which the anthropologists knew that the specimen was Homo habilis, there was stratification: The researchers began from obvious ignorance, inched...