Earth’s History:Fossils and Absolute Age

The Big Question:

• How do we know long life has existed on Earth?

Fossils

Source: J. C. Carton/Carto/Bruce Coleman, Inc. New York

• Fossils are the remains or traces of prehistoric life.

• Fossils show the history of life on Earth and how different groups of organisms have changed over time

Fossil Formation

• What does a fossil need to form?

1. Organism needs hard parts

2. Rapid burial

Types of Fossilization

Precambrian Stromatolites

www.geology.sdsu.ed

Earliest Fossils…Calcium Carbonate deposits from algae

Cambrian Tribolites

Source: Breck P. Kent

Paleozoic Brachiopods

Source: Breck P. Kent

Paleozoic Plants

Mesozoic Ammonite

Source: Ken Lucas/Biological Photo Service

Natural Cast

Mesozoic Dinosaur

www.fossils.com

Cenozoic Hominid

et

“LUCY”

With 40 percent of her skeleton intact, Lucy remains the oldest and most complete adult human ancestor fully retrieved from African soil. (c) The Houston Museum of Natural Science

Australopithecus afarensis

Relative vs. Absolute Dating

Law of Superposition: in an undeformed sequence of sedimentary rocks, each bed is older than the one above it, and younger than the one below it.

Relative Dating• Can determine relative

age of rock layer and fossils

• Performed by estimating rock/fossil age compared with that of other rocks/fossils

• Drawbacks – provides no info about age in years

Absolute dating• Can determine the absolute

age in numbers

• Is performed by radiometric dating

• Drawback - part of the fossil is destroyed during the test

Fossils as a time indicator

• Index fossils are widespread geographically, are limited to a short span of geologic time, and occur in large numbers

Unstable Atomic Nuclei Decay

Strong Force?

Radiometric Dating

• Radioactive Uranium breaks down into more stable Lead.

• Can be used to date materials billions of years old.

• U-238 has a “half life “ of 4.5 billion years.

Figure 8-18 Graph -- Radioactive Decay

Meteorites Date Earth

• Radiometric age of meteorites date the formation of the Solar System and Earth (4.55 billion years old)

http://upload.wikimedia.org/wikipedia/commons/0/00/Crab_Nebula.jpg

Crab Nebula

Carbon-14 Dating

Carbon-14 Dating

• Radioactive Carbon breaks down into more stable Nitrogen.

• Can be used to date materials only up to 75,000 years old.

• C-14 has a “half life “ of 5,730 years.

Geologic Time Scale on p. 365

GEOLOGIC TIME SCALE on page 365