Lesson Notes By Weeks and Term v5 - Grade 10

Lesson Video

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Performance objectives

• Define a fossil and explain fossilisation processes.
• Describe relative and absolute dating methods for fossils.
• Outline major events in the history of life on Earth.
• Explain how fossils support the theory of evolution.
• Discuss the significance of the Cradle of Humankind.

Lesson summary

The history of life on Earth is a fascinating journey spanning billions of years, revealing how simple organisms evolved into the complex biodiversity we see today. Fossils are crucial pieces of evidence that provide insights into past life forms and environments. Understanding this history allows us to appreciate the interconnectedness of all living things, understand the processes of evolution, and make informed decisions about conservation and environmental management in South Africa and globally.

Lesson notes

What is a Fossil? A fossil is any preserved remains, impression, or trace of a once-living organism from a past geological age. Fossils can include bones, shells, teeth, footprints, burrows, or even fossilized dung (coprolites). They provide invaluable evidence about the organisms that lived in the past, their environments, and how life has changed over time.

Fossilisation Processes: Several processes can lead to fossil formation.

The most common include: Permineralization: This occurs when minerals carried by water fill the empty spaces within the remains of an organism, like bone or wood. Over time, these minerals solidify, creating a rock-like fossil. For example, petrified wood is a result of permineralization.

Casts and Molds: When an organism decays after being buried in sediment, it leaves behind a mold, an empty space in the shape of the organism. If this mold is later filled with sediment that hardens, it creates a cast, a replica of the original organism. Think of it like making a jelly mold - the jelly is the cast, the mold is... well, the mold!

Impressions: Organisms can leave impressions in soft sediment that later hardens into rock. Footprints, leaf prints, and skin impressions are examples of impression fossils.

True Form Preservation: In rare cases, entire organisms can be preserved intact, usually in amber (fossilized tree resin), ice, or tar pits. These provide the most complete picture of the organism.

Dating Fossils: Determining the age of fossils is crucial for understanding the history of life.

There are two main methods: Relative Dating: This method determines the age of a fossil relative to other fossils or rock layers. It relies on the principle of superposition, which states that in undisturbed rock layers, the oldest layers are at the bottom, and the youngest are at the top.

Example: If fossil A is found in a rock layer below fossil B, we can conclude that fossil A is older than fossil B. Index fossils, which are fossils of organisms that lived for a short period of time and were widely distributed, can also be used for relative dating.

Limitations: Relative dating cannot provide an exact age in years.

Absolute Dating: This method uses radioactive decay to determine the age of a fossil in years. Radioactive isotopes decay at a constant rate, which is measured by their half-life (the time it takes for half of the atoms in a sample to decay).

Radiocarbon Dating: This method uses the decay of carbon-14 to date organic materials up to about 50,000 years old.

Example: A bone sample contains 25% of its original carbon-

1

4. Since carbon-14 has a half-life of 5,730 years, and 2 half-lives have passed (100% -> 50% -> 25%), the bone is approximately 2 5,730 = 11,460 years old.

Potassium-Argon Dating: This method uses the decay of potassium-40 to argon-40 to date rocks and minerals millions or billions of years old. This is useful for dating much older fossils than radiocarbon dating can handle.

How it works: Volcanic rock contains Potassium-

4

0. When it erupts and cools, the Potassium-40 starts to decay into Argon-40, which gets trapped within the rock. By measuring the ratio of Potassium-40 to Argon-40, scientists can calculate the age of the rock.

Major Events in the History of Life: 4.5 billion years ago (bya): Formation of Earth. 3.8 bya: First evidence of life (prokaryotes - bacteria and archaea). 2.5 bya: First photosynthetic organisms (cyanobacteria) begin to release oxygen into the atmosphere, leading to the Great Oxidation Event. 1.8 bya: First eukaryotes (cells with a nucleus). 540 million years ago (mya): Cambrian explosion – a rapid diversification of multicellular life. 488 mya: First vertebrates (fish). 360 mya: First amphibians move onto land. 300 mya: First reptiles. 252 mya: Permian-Triassic extinction event (the "Great Dying"), the largest mass extinction in Earth's history. 230 mya: First dinosaurs. 200 mya: First mammals. 66 mya: Cretaceous-Paleogene extinction event (killed off the non-avian dinosaurs). 6 mya: First hominins (human ancestors) in Africa. This is especially relevant to South Africa and the Cradle of Humankind. 200,000 years ago: Appearance of Homo sapiens (modern humans).

Mass Extinctions: These are periods in Earth's history when a large percentage of species died out in a relatively short period of time. Mass extinctions are often caused by catastrophic events such as volcanic eruptions, asteroid impacts, or climate change. They create opportunities for new species to evolve and diversify.

Fossil Evidence and Evolution: Fossils provide direct evidence of evolutionary change.

Transitional Fossils: These fossils show intermediate stages in the evolution of one group of organisms into another. Archaeopteryx, for example, is a transitional fossil between reptiles and birds. It had reptilian features like teeth and a bony tail, but also bird-like features like feathers and wings.