The early history of the human race is most
clearly and readily studied by observing the
stages of human technological progress. That
is why it was a long-standing convention to
divide early history into broad periods
according to the hard materials — stone,
copper, bronze, and iron, in that order -—
from which cutting tools and weapons were
made. Then, as knowledge of the past
increased, the broad periods were further
subdivided. The Stone Age was divided into
the Old (Paleolithic) period and the New
(Neolithic) period when it was noticed that
the technique of sharpening flint and other
stones changed from flaking to grinding.