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3 of the experience of the last few generations. The group of
happily unexperienced events includes large bolide impacts with the
Earth. The evidence for the occurrence of such impacts at intervals
of some tens of millions of years is quite convincing, and Lyell
stands admonished by Hamlet: "There are more things in heaven and
earth, Horatio, than are dreamt of in your philosophy. " The role
of bolide impacts on the history of life during other portions of
the Phanerozoic Eon is less clear (see Raup and Fischer, both this
volume), and catastrophic changes unrelated to extraterrestrial
processes may have been important (see Holser, this volume).
Changes in the later Precambrian biota are still difficult to
interpret, in part because the preservation of soft-bodied animals
from this period of Earth history is so unusual (see Seilacher,
this volume). During the past billion years or so, bolide impacts
have exerted a significant effect on the Earth's surface and its
inhabitants, but not on its interior. The 3800 Ma rocks at Isua in
West Greenland are the oldest terrestrial rocks that are currently
available for inspection (see Dymek, this volume). They contain
abundant evidence for the operation of chemical and physical
processes that are similar to those of the present day. This
situation could not have prevailed during the entire 700 Ma
preceding the formation of the Isua rocks.
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