Chapter 18 Fossils and Geologic Time

426 CHAPTER 18: FOSSILS AND GEOLOGIC TIME
cause later rocks have covered most of the rocks from this
time, or the rocks were recycled by weathering and erosion,
or altered by metamorphism.
Most of the earliest forms of life inhabited the oceans,
which provided them with nutrients and protected them from
harmful solar radiation. Carbon dioxide and nitrogen probably
dominated Earth’s early atmosphere, making it similar in
composition to the present atmosphere of Venus and Mars.
Photosynthesis eventually began in single-celled Precambrian
organisms. They used carbon dioxide to store energy,
releasing oxygen as a waste product. The addition of oxygen
to the atmosphere had two very important results. Oxygen in
the form of ozone absorbs harmful radiation, such as ultraviolet
rays, from outside Earth. Oxygen also is necessary for
air-breathing animals. Both of these factors made possible
the later development of life outside the oceans.
Paleozoic Era: The Origin of Complex Life-forms
The name Paleozoic can be translated from its word origin as
“the time of early life.” This era began a little more than half
a billion years ago. The abundance of fossils that marks the
beginning of the Paleozoic was a result of rapid evolution.
Hard parts, such as skeletons and shells, allowed some organisms
to move rapidly in search of food and others to protect
themselves from becoming food. Many of the specialized
features of today’s most complex life-forms, including our
sense organs, can be traced back to structures that appeared
early in the Paleozoic Era.
The first fish as well as the first plants and animals to inhabit
the land appeared in the Paleozoic along with the earliest
insects and amphibians. Amphibians lay their eggs in
water; but as adults, many move onto the land. Reptiles lay
eggs with protective shells, and live and reproduce entirely
on land. Trilobites appeared in the oceans early in the Paleozoic.
They evolved many variations in shape before they became
extinct at the end of the Paleozoic.
The end of the Paleozoic Era is marked by the extinction of
as much as 95 percent of living species. The cause of this ex-