Check Your Understanding
- What are fossils?
- How does the fossil record contribute to the evidence of evolution?
Introduction
It is no surprise that people have wondered about the age of the earth, how it was formed,
and how life began on Earth for hundreds, even thousands, of years. Try to imagine how
ancient philosophers tried to explain the history of the earth and life. Many people used
mythology or cultural beliefs to explain elaborate stories about how and when the earth
formed.
The past two to three hundred years has been an exciting time for geologists, paleontologists
and other scientists who are trying to trace the history of the earth. What was once a hobby,
studying land forms and fossils has become a science that is revealing the history of the earth
and life on Earth.
Age of Earth
During the 1800s, geologists, paleontologists and naturalists found several forms of physical
evidence that confirmed that the earth is very old, far older than the 6,000 years that some
leaders had claimed. Their evidence included:
- Fossils of ancient sea life on dry land far from oceans: This supported the idea that the
earth changed over time and that some dry land today was once covered by oceans. - The many layers of rock: When people realized that rock layers represent the order in
which rocks and fossils appeared, they were able to start to trace the history of the
earth and life on Earth. - Indications that volcanic eruptions, earthquakes and erosion that happened long ago
shaped much of the earth’s surface. This supported the idea of an older Earth.
Radiometric Dating
During the past one hundred years, geologists and paleontologists have been able to delve
even deeper into the earth’s history with new tools of science. The most convincing method,
calledradiometric dating, was developed more than one hundred years ago. Rocks are
made up of minerals. Scientists found that they could measure the age of rocks by mea-
suring the radioactivity of certain minerals in rocks. Geologists and paleontologists still use
variations of radiometric dating to determine the age of fossils and rocks today (Figure
7.31).