United States History Student Edition

BIOGRAPHY THOMAS JEFFERSON (1743–1826) While Thomas Jefferson is best

In 1946, an American scientist named Willard Frank Libby discovered that all living things contain an element called carbon-14. After plants, animals, and humans die, the carbon-14 gradually disappears. By measuring how much carbon-14 remains, scientists can estimate how old an object is. This method is called radiocarbon dating. Another method—thermoluminescence (THUHR•moh•LOO•muh•NEH•suhns) dating— helps scientists make more precise measurements back to 200,000 years. This method dates an object by measuring the light given off by particles trapped in the soil surrounding it. Newer methods for analyzing remains, such as blood, hair, and plant tissues left on rocks, tools, and weapons, give archaeologists still more information. Scientists have discovered that blood molecules can survive millions of years. The discovery can tell us how some tools were used and the types of animals that hunters killed. DNA is also providing new data. By analyzing the remains of plants on stone tools, for example, scientists can find out more about early farming. 7 CHECK FOR UNDERSTANDING 1. Making Connections How do scientists help the study of history? 2. Explaining Why is it important to identify the dates of artifacts? Measuring and Organizing Time GUIDING QUESTION What tools do we use in measuring time? To study the past, historians must have a way to identify and describe when events happened. They do this by measuring and labeling time in different ways. Periods of History Historians divide history into blocks of time made up of a number of years. A period of 10 years, for example, is called a decade . A period of 100 years is known as a century . Centuries are grouped into even longer time periods, which are

known as a patriot and a president, he may have been the first American to conduct an organized

archaeological study. Sometime, probably in the early 1780s, Jefferson undertook a careful archaeological

examination of a Native American mound near his home in Virginia. In his book, Notes on the State of Virginia, Jefferson describes using archaeological techniques adopted by professional archaeologists many years later.

Paleontologists may study long-extinct animals, such as dinosaurs. They might even study the remains of early humans. Anthropology (AN•thruh•PAH•luh•jee) is the study of human cultures and how they develop over time. Anthropologists study artifacts and fossils too. They also look for clues about what people valued and believed, such as laws or religions. Dating Artifacts One of the most important and difficult jobs for paleontologists and archaeologists is dating the artifacts that they find. How do they do this? Early scientists correctly assumed that artifacts buried more deeply in the ground are older than those closer to the surface. In most cases, that observation is still true. Another way that archaeologists date artifacts is by using trees. Each year, trees form a new growth ring. Scientists count the number of rings in a wooden object, such as a house beam, and compare the pattern with the rings of a tree whose age they know. In that way, they can identify dates as far back as 3,000 years. anthropology the study of human cultures and how they develop over time HT8

decade a period of 10 years century a period of 100 years

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