Page 52 - The Restless Earth Fossils

Marking turning points in evolution 51

The geological timeline can be illustrated using the armspan of an
average student.

origin and the middle finger of your right hand represent the
present moment. One arm span now equals 4,600 million years.
If that arm span is, say, 66 inches, then 1 inch = 69.7 million
years. The first fossil animals big enough to see with the naked
eye show up in sediments 540 million years old—about to the
wrist of your right hand. The Paleozoic Era, or “age of old life,”
extends from your right wrist to the base of your right fingers.
The basal and middle finger joints of the right middle finger rep-
resent the entire “age of middle life” (the Mesozoic Era, which
most people associate with dinosaurs) and the last finger joint,
including the fingernail, represents the “age of new life” (domi-
nated by mammals), or the Cenozoic Era.
Filing that fingernail down would erase all traces of our spe-
cies, Homo sapiens.
The divisions between major periods and eras reflect changes
in the fossil record. The most drastic changes happened at the

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