what kinds of materials were available then that could have been involved in the creation
of life. Scientists believe the early earth contained no oxygen gas, but did contain other
gases, including nitrogen, carbon dioxide, carbon monoxide, water vapor, hydrogen sulfide
and probably a few others.
Life from Random Reactions
Today, we have evidence that life on Earth came from random reactions between chemi-
cal compounds that formed molecules; in a series of random steps, these molecules created
proteins and nucleic acids (RNA or DNA), and then cells. We know that the ingredients
for life (the building blocks of life), were present at the beginning of Earth’s history. Some
chemicals were in water and volcanic gases. Other chemicals would have come from mete-
orites in space. Energy to drive chemical reactions was provided by volcanic eruptions and
lightening. Keep in mind that this process may have taken as much as 1 billion years. Our
understanding of how life originated on Earth is developing gradually (Figure7.33).
Figure 7.33: Some clues to the origins of life on Earth come from studying the early life forms
that developed in hot springs, such as the Grand Prismatic Spring at Yellowstone National
Park. This spring is approximately 250 feet by 300 feet wide. ( 38 )
Geologic Time Scale
Geologistsandotherearthscientistsusegeologictimescalestodescribewheneventsoccurred
throughout the history of Earth. The time scales can be used to illustrate when both
geologic events and events affecting plant and animal life occurred. All of the earth events
we see happening today, such as earthquakes, volcanic eruptions, and erosion, have happened