Page 15 - The Restless Earth Fossils

14 Fossils

been petrify’d and turn’d into the nature of stone, just as I even
now observ’d several sorts of wood to be.”
The insights of Steno and Hooke—that fossils represented the
remains of once-living creatures—began a revolutionary change
in the way people viewed the world. If the forces of nature could
transform ocean beds (and the creatures they contained) into
stone while piling them up into mountains, then the Earth must
have a history—a very long history. This concept ran contrary to
orthodox Christian convictions of the time: that God had created
nature all at once and pretty much “as is.”
Steno’s study of shark teeth led to many geological observa-
tions that he summarized in an essay for his patron, Grand Duke
Ferdinando. “In various places,” Steno wrote, “I have seen that
the earth is composed of layers superimposed on each other at an
angle to the horizon.” Steno realized that, like the layers of pearl
that form around a sand grain, those layers implied a history. The
oldest layers must be on the bottom of the pile and the younger
layers on top. This last statement summarizes what geologists
now call Steno’s principle of superposition.
“Water is the source of sediments,” said Steno, and when
water fills a container, whether that container is a glass or a vast
basin, gravity ensures that the surface of the water is parallel to
the horizon. As rocks and finer particles settle out, they will also
come to lie horizontally. Steno’s second principle, then, is the
principle of original horizontality. If rock layers are tilted,
that tilting must have happened after the sediments originally
formed.
These were simple ideas, but not obvious ones. They made
people realize that a fossil or any natural object contains clues
to its own history. Steno’s insights opened a vast new perspec-
tive on living things. The Earth transformed from a static stage
for human activities into a restless, dynamic planet that not only
changed the kinds of life it supported, but was changed by that
life in turn. Fossils, the relics of ancient life, became the key to
understanding Earth’s long and exciting story, although it took
time before everyone recognized their importance.

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