The person four generations back from you lived in the time of your great-great-grandparents, perhaps in the early twentieth century.

Ten generations back takes you to the seventeenth century.

A hundred generations back takes you past the Norman Conquest, past the fall of Rome.

Five hundred generations back takes you before the invention of writing.

And 1,240 generations back — a number that seems almost mythological — takes you to a child playing on the bank of the Yana River, in the Ice Age Arctic, who would eventually shed a milk tooth into the sediment of that river terrace.

The child does not know that we will find the tooth.