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EX LIBRO LAPIDUM HISTORIA MUNDI 29

Then we need a way to appreciate how far back in Earth history

that date lies. Here we are helped by a remarkable coincidence.

Earth history is, by chance, about one million times as

long as the written record of human history. Writing was invented

about 5,000 years ago, and the Earth was formed about

5,000 million-years ago.1 So we can recognize the recentness or

antiquity of a date in Earth history given in million years by

comparing it to a date in human history in years. The Quaternary

ice ages began 2 million years ago, which in the sweep of

Earth history is as recent as a human event just 2 years ago. The

Cretaceous-Tertiary boundary, 65 million years ago, is the Earth

equivalent to a human event 65 years ago, within the memory of

many people. Just as human history seems really remote when

we get to events many hundreds of years ago, so Earth events

many hundreds of million years ago belong to the really distant

past.

These are the ways of thinking that young geologists absorb

when they begin to study Earth history—they learn the names,

change their units from years to million years, and come to appreciate

which dates