6.3 Fossils

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Learning Objectives

• Discuss the significance of the fossil record as evidence for evolution.

Introduction

What's on this rock?

This sedimentary rock contains a fossilized stalked crinoid. Scientists study fossils of plants, animals, and other organisms in order to better understand what life was like on Earth many years ago and how it has changed over time. Fossils are important evidence for the theory of evolution.

Guided Learning

The Fossil Record

Fossils are preserved remains of animals, plants, and other organisms from the distant past. Examples of fossils include bones, teeth, and impressions. By studying fossils, evidence for evolution is revealed. Paleontologists are scientists who study fossils to learn about life in the past. Paleontologists compare the features of species from different periods in history. With this information, they try to understand how species have evolved over millions of years (Figure below).

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Evolution of the horse. Fossil evidence, depicted by the skeletal fragments, demonstrates evolutionary milestones in this process. Notice the 57 million year evolution of the horse leg bones and teeth. Especially obvious is the transformation of the leg bones from having four distinct digits to that of today's horse.

Until recently, fossils were the main source of evidence for evolution (Figure below). Through studying fossils, we now know that today's organisms look much different in many cases than those that were alive in the past. Scientists have also shown that organisms were spread out differently across the planet. Earthquakes, volcanoes, shifting seas, and other movements of the continents have all affected where organisms live and how they adapted to their changing environments.

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About 40 to 60 million years ago these insects were trapped in a gooey substance, called resin, that comes from trees. The fossils in the movie Jurassic Park were trapped in resin.

Rock Layers and the Age of Fossils

There are many layers of rock in the Earth's surface. Newer layers form on top of the older layers. Therefore, you can tell how old a fossil is by observing in which layer of rock it was found. The fossils and the order in which fossils appear is called the fossil record. The fossil record provides evidence for when organisms lived on Earth, how species evolved, and how some species have gone extinct. Geologists use a method called radiometric dating to determine the exact age of rocks and fossils in each layer of rock. This technique measures how much of the radioactive materials in each rock layer have broken down (Figure below).

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This device, called a spectrophotometer, can be used to measure the level of radioactive decay of certain elements in rocks and fossils to determine their age.

Radiometric dating has been used to determine that the oldest known rocks on Earth are between four and five billion years old. The oldest fossils are between three and four billion years old. Remember that during Darwin's time, people believed the Earth was just about 6,000 years old. The fossil record proves that Earth is much older than people once thought.

Review

• Fossils, or preserved parts of organisms from the distant past, have shown that species change over time.
• Radiometric dating can be used to determine the age of fossils by measuring the how much of the radioactive materials in each rock layer have broken down.

Vocabulary

Fossil

Preserved part of animals, plants, and other organisms from the distant past.

Fossil Record

Complete set of fossils that has been discovered, and the order in which the fossils appear.

Paleontologist

Scientist who studies fossils to learn about life in the past.

Radiometric Dating

Procedure used to determine the age of rocks or fossils by measuring how much of the radioactive materials in each sample were broken down.

Sedimentary Rock

Types of rock that are formed by the deposition of material at the Earth's surface and within bodies of water.