Chapter 18 Fossils and Geologic Time
Chapter 18 Fossils and Geologic Time
Chapter 18 Fossils and Geologic Time
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4<strong>18</strong> CHAPTER <strong>18</strong>: FOSSILS AND GEOLOGIC TIME<br />
HOW DID LIFE BEGIN ON EARTH?<br />
Figure <strong>18</strong>-3 These petrified<br />
logs in Utah help identify<br />
the rocks in which they are<br />
found as the remains of a<br />
l<strong>and</strong> environment.<br />
Scientists are not sure how life began on Earth. It is clear<br />
that chemical reactions in the environment of early Earth<br />
could have produced amino acids. Living organisms are made<br />
of these basic compounds. Still, how amino acids become organized<br />
to make even the simplest life-forms is not understood.<br />
Experiments to produce living material from nonliving<br />
processes have not been conclusive. However, millions of years<br />
of chemical changes on the primitive Earth might have produced<br />
results that cannot be duplicated in short-term laboratory<br />
experiments.<br />
Life could have started in places not usually considered<br />
good places for living organisms. Some of the most primitive<br />
forms of life have recently been discovered in nearly solid rock<br />
many kilometers below the surface. Life-forms have been<br />
found near hot water vents in the deep ocean bottom where<br />
sunlight cannot penetrate. Perhaps conditions in one of these<br />
places gave rise to the first life-forms.
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