Lesson Notes By Weeks and Term v5 - Grade 10

Lesson Video

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Performance objectives

• Describe the major geological time periods and their dominant life forms.
• Explain how fossils provide evidence for the history of life.
• Explain the use of relative and absolute dating to determine a fossil's age.
• Outline the key events in the origin of life.
• Relate mass extinctions to the history of life.

Lesson summary

The history of life on Earth is a fascinating journey spanning billions of years. Understanding this history, and the evidence left behind in fossils, helps us comprehend the incredible diversity of life we see around us today, including the unique biodiversity of South Africa. It provides context for understanding evolution, adaptation, and the interconnectedness of all living things. This knowledge is crucial for informed decision-making about conservation, resource management, and our role in preserving life for future generations. Many paleontological discoveries have occurred in South Africa, giving us an exceptional position to understand early life.

Lesson notes

2.1 Geological Time Scale: The geological time scale is a system of chronological measurement that relates stratigraphy (the study of rock layers) to time. It is divided into eons, eras, periods, epochs, and ages. Each division represents a significant period in Earth's history, characterized by specific geological and biological events. The major divisions relevant to the history of life are: Precambrian Eon (4.5 billion - 541 million years ago): This vast eon encompasses the formation of the Earth, the origin of life, and the evolution of the first prokaryotic cells (bacteria and archaea). Photosynthesis evolved, leading to an increase in atmospheric oxygen. The end of the Precambrian saw the evolution of the first multicellular organisms. Phanerozoic Eon (541 million years ago - Present): This eon is characterized by the diversification of life.

It is divided into three eras: Paleozoic Era (541-252 million years ago): Known as the "age of invertebrates" and the "age of fishes." The Cambrian explosion occurred, resulting in a rapid increase in the diversity of multicellular organisms. Plants colonized land, followed by arthropods and amphibians. The end of the Paleozoic was marked by the Permian-Triassic extinction event, the largest mass extinction in Earth's history.

Mesozoic Era (252-66 million years ago): Known as the "age of reptiles," particularly the dinosaurs. The first mammals and birds evolved. Flowering plants (angiosperms) appeared and diversified. The end of the Mesozoic was marked by the Cretaceous-Paleogene extinction event, which wiped out the non-avian dinosaurs. Cenozoic Era (66 million years ago - Present): Known as the "age of mammals." Mammals diversified and became the dominant terrestrial vertebrates. The first hominids (human ancestors) evolved in Africa. This era includes the Quaternary period, which encompasses the ice ages and the evolution of modern humans. 2.2 Fossil Formation and Types: Fossils are the preserved remains or traces of ancient organisms. They provide direct evidence of past life and help us understand the evolutionary relationships between organisms. Fossilization is a rare process because most organisms decompose rapidly after death. The following conditions favor fossilization: rapid burial, hard body parts (bones, shells, teeth), and anaerobic (oxygen-poor) environments.

Types of Fossils: True Form Fossils: The actual body or body parts of an organism are preserved. This is rare, but can occur in amber (fossilized tree resin), ice, or tar pits. Examples include insects in amber or woolly mammoths frozen in ice.

Cast Fossils: Formed when an organism dies and is buried in sediment. The organism decomposes, leaving a mold in the sediment. The mold is then filled with minerals, creating a cast of the organism.

Mold Fossils: An impression of the organism in the rock. Similar to a cast fossil, but the mold is not filled with minerals.

Trace Fossils: Evidence of an organism's activity, such as footprints, burrows, or coprolites (fossilized feces). The Laetoli footprints in Tanzania, made by early hominids, are a famous example of trace fossils.

Petrified Fossils: Minerals replace the original organic material of the organism, turning it into stone. Petrified wood is a common example. 2.3 Dating Fossils: Determining the age of fossils is crucial for understanding the timeline of life on Earth. There are two main methods for dating fossils: Relative Dating: Determines the age of a fossil relative to other fossils or rock layers. This method relies on the principle of superposition, which states that in undisturbed rock layers, the oldest layers are at the bottom, and the youngest layers are at the top. Index fossils, which are fossils of organisms that lived for a short period and were geographically widespread, can be used to correlate rock layers in different locations.

Example: If fossil A is found in a rock layer below fossil B, then fossil A is older than fossil

B. Absolute Dating (Radiometric Dating): Uses the decay of radioactive isotopes to determine the age of a fossil. Radioactive isotopes decay at a constant rate, known as their half-life (the time it takes for half of the radioactive atoms to decay). By measuring the amount of the original isotope and its decay product in a sample, scientists can calculate the age of the sample.

Carbon-14 Dating: Used to date organic materials up to about 50,000 years old. Carbon-14 is a radioactive isotope of carbon that is produced in the atmosphere and absorbed by living organisms. When an organism dies, it stops absorbing carbon-14, and the carbon-14 in its body begins to decay.

Potassium-Argon Dating: Used to date older rocks and fossils (millions or billions of years old). Potassium-40 decays into argon-40, which is trapped in volcanic rocks.