Lesson Notes By Weeks and Term v5 - Grade 10

Lesson Video

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Performance objectives

• Describe the major geological eras and their key biological events.
• Explain how fossils provide evidence for evolution.
• Outline the methods used to determine the age of fossils.
• Discuss the importance of South African fossil sites.
• Compare different types of fossils.

Lesson summary

The history of life on Earth is a fascinating journey, spanning billions of years, filled with incredible transformations and extinctions that have shaped the biodiversity we see today. Understanding this history is crucial not only for appreciating the complexity of life but also for understanding the environmental challenges we face in South Africa and globally. Studying fossils, the preserved remains of ancient organisms, provides tangible evidence of this evolutionary journey, allowing us to piece together the puzzle of life's past.

Lesson notes

2.1 The Geological Timescale The geological timescale is a system of chronological dating that relates geological strata (layers of rock) to time. It is used by geologists, paleontologists, and other Earth scientists to describe the timing and relationships of events that have occurred during Earth's history. The timescale is divided into eons, eras, periods, epochs, and ages, with eons being the largest division of time and ages the smallest. Here's a simplified overview relevant to our study of life's history: Precambrian Eon (4.5 billion years ago – 541 million years ago): This vast span of time saw the formation of Earth and the origin of life. Early life forms were primarily prokaryotic (lacking a nucleus), like bacteria and archaea. Photosynthesis evolved, leading to the accumulation of oxygen in the atmosphere. Towards the end, the first multicellular organisms appeared. Paleozoic Era (541 million years ago – 252 million years ago): Often called the "Age of Invertebrates" and "Age of Fishes." This era experienced the Cambrian explosion, a period of rapid diversification of life. Marine invertebrates flourished, followed by the evolution of fish, amphibians, and early reptiles. Plants colonized land. This era ended with the Permian-Triassic extinction event, the largest mass extinction in Earth's history. Mesozoic Era (252 million years ago – 66 million years ago): Known as the "Age of Reptiles." Dinosaurs dominated terrestrial ecosystems. The first mammals and birds appeared. Flowering plants (angiosperms) evolved. The era concluded with the Cretaceous-Paleogene extinction event, which wiped out the non-avian dinosaurs. Cenozoic Era (66 million years ago – present): The "Age of Mammals." Mammals diversified and became dominant land animals. Primates evolved, eventually leading to the emergence of humans. The Earth experienced cycles of warming and cooling, including ice ages. 2.2 Fossils: Windows to the Past Fossils are the preserved remains or traces of ancient organisms. They provide direct evidence of past life forms and ecosystems.

Fossils can be formed in several ways: Petrification (Permineralization): Minerals replace the original organic material, turning the organism into stone. This is common with bones and wood.

Molds and Casts: An organism decays in sediment, leaving a mold (an impression). If the mold is later filled with minerals, it forms a cast.

Impressions: Flat fossils, like leaves, can leave an impression in sedimentary rock. Preservation in Amber, Ice, or Tar: Organisms can be trapped and preserved in amber (fossilized tree resin), ice, or tar pits. This can preserve even soft tissues.

Trace Fossils: These are not remains of the organism itself but evidence of its activity, such as footprints, burrows, or fossilized dung (coprolites).

Example: Imagine a Glossopteris leaf falling into a swamp in the ancient Karoo region of South Africa. Over time, sediment covers the leaf. The organic material of the leaf slowly decays, but its shape is preserved as an impression in the rock. Millions of years later, geologists discover this fossil, providing evidence of the Glossopteris flora that once thrived in southern Africa during the Permian period. 2.3 Dating Fossils Determining the age of fossils is crucial for understanding the sequence of events in life's history.

Two main methods are used: Relative Dating: This method determines the age of a fossil relative to other fossils or rock layers.

Law of Superposition: In undisturbed sedimentary rock layers, the oldest layers are at the bottom, and the youngest layers are at the top. Fossils found in lower layers are generally older than those found in upper layers.

Index Fossils: Fossils of organisms that lived for a relatively short period and were geographically widespread. The presence of a specific index fossil in a rock layer indicates that the layer is of a particular age.

Absolute Dating: This method uses radiometric techniques to determine the numerical age of a fossil or rock sample.

Radiocarbon Dating (Carbon-14 Dating): Measures the decay of carbon-14 ( 14 C), a radioactive isotope of carbon, in organic material. It is useful for dating fossils up to about 50,000 years old.

Potassium-Argon Dating (K-Ar Dating): Measures the decay of potassium-40 ( 40 K) into argon-40 ( 40 Ar) in rocks and minerals. It is useful for dating older fossils and rock formations, often millions of years old.

Example (Radiocarbon Dating): A piece of wood from a cave in the Cradle of Humankind contains 25% of the original amount of carbon-

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4. The half-life of carbon-14 is 5,730 years. How old is the wood? After one half-life (5,730 years), 50% of the carbon-14 remains. After two half-lives (2 5,730 = 11,460 years), 25% of the carbon-14 remains.

Therefore, the wood is approximately 11,460 years old.

Example (Potassium-Argon Dating): A volcanic rock layer associated with a hominin fossil site contains 12.5% of the original potassium-40.