The Fossil Record and the Origin of Life

Geologic Time

Earth has a rich and extensive history filled with change. The geologic time scale holds secrets to the life that has existed on Earth since the beginning of time. In this interactivity, select the different areas of the geologic time scale to view more information about their characteristics and the types of organisms that lived in each time. Make sure to view each segment. Click the player button to begin.

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Early Earth's atmosphere was composed of carbon dioxide, methane, ammonia, hydrogen and water. In 1952, a scientist named Stanley Miller conducted an experiment whereby he showed that a mixture of these substances could form organic molecules when exposed to UV light similar to the Sun, and a spark similar to lightning. These organic molecules were amino acids, which are the basic components of life. Scientists theorize that these molecules came together to form more complex molecules. These complex molecules were the building blocks for future generations of molecules that eventually began to take on the qualities of life.

Mass Extinctions

Of all the plants and animals that have lived on Earth, almost 90% are extinct. The term "extinct" means that the organism and its species no longer exist. The most well-known extinction event occurred at the end of the Cretaceous period. Dinosaurs roamed the Earth for 186 million years and they are now extinct. There are no dinosaur fossils in any rocks younger than 65 million years old. Scientists believe that a large meteor impacted the Earth near the Yucatan Peninsula in the Gulf of Mexico. This massive impact caused tremendous amounts of dirt, rock, and debris to enter the atmosphere and led to the extinction of all the dinosaurs. In addition to dinosaurs, many other species present on the surface also went extinct. Events when massive natural disasters cause numerous species to become extinct are called mass extinctions. There have been four major mass extinction events discovered by paleontologists. These extinctions cause one division of geologic time to end, thus beginning another division.

Fossils

A fossil is the preserved remains or the imprint of a living organism, either plant or animal, in the Earth's crust. Organisms can form fossils in a variety of ways, including freezing, petrifying, or getting caught in a substance that preserves the organism, like tar or amber. Trace fossils are indirect preserved remains of life. Instead of preserving parts of the animal, evidence of the organism's life is preserved. Examples of trace fossils include footprints, nests, coprolites (preserved excrement), egg shells, and burrows.

Of all the organisms that live and have lived on Earth in the past 3.5 billion years, only a very small percentage become fossilized and a part of the fossil record. The soft tissues of organisms break down easily. Sometimes, the hard parts of organisms are also destroyed by forces such as weathering and erosion. When fossils are found, they are almost always a harder part of the organism. Bones, teeth, and claws are all examples of harder parts found on organisms. It is these parts that are likely to survive for millions of years.

Fossil Clues

In order to discover what happened millions, even billions of years ago, biologists rely on the clues held in rocks and the information they can discover from fossils. In this activity, learn how the many clues regarding Earth’s past are derived from the study of fossils. Click the player button to begin.

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Relative and Radiocarbon Dating

Relative dating is a process of determining the ages of rocks and geologic events in a given sequence. Relative dating involves assigning ages to fossils without using numbers or other finite values. By comparing fossils from older rock layers with fossils from younger rock layers, scientists can make hypotheses about how life on Earth has changed over time.

Radiocarbon dating is used to find the age of once-living materials between 100 and 50,000 years old. This range is especially useful for determining the ages of human fossils and habitation sites. An organism naturally collects carbon-14 throughout its lifetime. Once the organism dies, it stops collecting carbon-14. The carbon-14 has a half-life of 5,730 years. Every 5,730 years, only half of the original carbon-14 will remain. The daughter product for carbon-14 is nitrogen-14.

After the organism’s death, carbon-14 decays to stable nitrogen-14 by releasing a beta particle. The nitrogen atoms are lost to the atmosphere, but the amount of carbon-14 that has decayed can be estimated by measuring the proportion of radioactive carbon-14 to stable carbon-12. As shown by the graph, the amount of carbon-14 decreases relative to the amount of nitrogen-14. Scientists can use this ratio to find an accurate age of the organism.

 

The Fossil Record Review

Now that you have learned about how scientists use the fossil record to study the history life on Earth, check your knowledge in this non-graded activity. Read the directions provided with each question and select the correct answer. Then, click SUBMIT to check your response. Click the player button to get started.