Fossils
adapted from the National Park Service
Fossils are the remains or traces of organisms that were once alive. Fossils that are the remains of an actual organism, such as a shell, leaf, or bone, are known as body fossils. Those fossils that record the action of an organism, such as a footprint or burrow, are known as trace fossils. Paleontologists study fossils to help understand the evolution and the history of life on Earth.
There are two different methods to age-date fossils. Relative dating simply indicates which of two fossils is older, and which is younger. Fossils are most commonly found in sedimentary rock, which forms in layers. The layers at the bottom formed first, therefore these layers are older than those above them. Dinosaurs are found in layers that are below those containing the first humans, so it can be concluded that dinosaurs and humans did not live at the same time.
Absolute dating allows scientists to determine exactly how long ago an organism lived. Certain elements in rocks are radioactive. These elements are unstable and decay, or change, over time into a stable element. This decay occurs at a steady, constant rate; it's like a clock that starts running as soon as a rock forms. So, by first knowing the rate of decay and then measuring the amount of the parent element left compared to amount of the daughter element, scientists can determine the age of a rock. The age of fossils found in those rocks can then be determined.
Fossils help people understand the life in the past. They tell of the mighty dinosaurs, tiny three-toed horses, and thousands of other prehistoric forms that are long extinct. One of the ways people learn about the ancestors of today's species is by studying fossils. Fossils of walking whales, for example, indicate that whales evolved from four-footed mammals that lived on land, and fossils of ancient humans provide clues about the earliest civilizations in Africa.
10
The author explains how fossils help people understand the past by
A.
listing the different types of animals that have gone extinct over time.
B.
distinguishing between the different types of fossils based on shape.
C.
detailing the process used to determine the age of a fossil.
D.
indicating why paleontologists are passionate about studying fossils.