The Origin and History of Life on Earth
\begin{definition} \textbf{The origin and history of life on Earth} refers to the processes and events that led to the emergence, evolution, and diversification of living organisms over billions of years. It encompasses the formation of the first simple molecules, the development of self-replicating systems, and the progression of life from single-celled organisms to complex multicellular forms. \end{definition}
The Geologic Record of Life's History
How Can the Order in Which Geological Events Occurred Be Determined?
- Uniformitarianism: The principle that the processes shaping Earth today, such as erosion and sedimentation, have operated in the same way throughout its history.
- Relative Age: Determining the sequence of events without exact dates.
- Absolute Age: Assigning specific dates using radioactive decay.
Determining Relative Age
- Law of Original Horizontality: Sediments are deposited in horizontal layers.
- Principle of Superposition: In undisturbed layers, the oldest is at the bottom.
\begin{callout} Students often assume that rock layers are always horizontal. Remember, tectonic forces can tilt, fold, or overturn layers, so always check for disturbances before applying the principle of superposition. \end{callout}
Disturbances in Rock Layers
- Folding and Faulting: Layers can be tilted or broken by tectonic forces.
- Igneous Intrusions and Extrusions:
- Intrusions: Molten rock cuts through existing layers, making it younger than the layers it penetrates.
- Extrusions: Lava flows on the surface, forming a layer younger than those below it.
\begin{callout}
- Intrusion: A dike of basalt cutting through sedimentary rock.
- Extrusion: A lava flow covering older sedimentary layers. \end{callout}
Unconformities
- Angular Unconformity: Tilted layers are eroded, then covered by horizontal layers.
- Disconformity: Erosion between parallel layers.
- Nonconformity: Sedimentary layers overlie eroded igneous or metamorphic rock.
\begin{callout} When identifying unconformities, look for evidence of erosion, such as missing layers or irregular surfaces between rock strata. \end{callout}
Correlation of Rock Layers
- Walking the Outcrop: Physically tracing layers across locations.
- Matching Physical Characteristics: Using features like composition or color.
- Index Fossils: Fossils of organisms that were widespread but existed for a short time, allowing precise dating.
\begin{callout}
- Index Fossils: Trilobites and ammonoids are classic examples.
- Key Beds: The iridium-rich layer marking the Cretaceous-Paleogene extinction event. \end{callout}
How Can Earth's Geologic History Be Sequenced from the Fossil and Rock Record?
The Geologic Column
- A global sequence of rock layers pieced together through correlation.
- Forms the basis of the geologic time scale, which divides Earth's history into:
- Eons: Largest units (e.g., Archean, Proterozoic, Phanerozoic).
- Eras: Subdivisions of eons (e.g., Paleozoic, Mesozoic, Cenozoic).
- Periods: Subdivisions of eras (e.g., Jurassic, Cretaceous).
- Epochs: Subdivisions of periods (e.g., Holocene, Pleistocene).
\begin{callout} The Phanerozoic Eon is the most recent eon and includes the Paleozoic, Mesozoic, and Cenozoic eras, which are characterized by the evolution of visible life forms. \end{callout}
\begin{callout} Think of the geologic time scale as a library of Earth's history. Each eon is a section, each era is a shelf, and each period is a book detailing the events and life forms of that time. \end{callout}
How Can the Absolute Age of a Rock Be Determined?
Radioactive Decay
- Isotopes: Variants of elements with different numbers of neutrons.
- Radioactive Decay: Unstable isotopes transform into stable decay products at a constant rate.
\begin{callout} The half-life of an isotope is unaffected by environmental factors such as temperature or pressure, making it a reliable tool for dating rocks. \end{callout}
Key Isotopes Used in Dating
- Uranium-238 to Lead-206: Half-life of 4.5 billion years, used for ancient rocks.
- Potassium-40 to Argon-40: Half-life of 1.3 billion years, used for rocks 100,000 to 4.6 billion years old.
- Carbon-14 to Nitrogen-14: Half-life of 5,730 years, used for dating organic remains up to 50,000 years old.
\begin{callout}
- A rock contains 50 grams of potassium-40 and 50 grams of argon-40.
- This means one half-life (1.3 billion years) has passed, so the rock is 1.3 billion years old. \end{callout}
\begin{callout} When using carbon-14 dating, remember that it is only effective for dating organic materials less than 50,000 years old. For older samples, isotopes like potassium-40 or uranium-238 are more appropriate. \end{callout}
How Can the Rock Record and Fossil Evidence Reveal Changes in Past Life and Ancient Environments?
Fossils as Evidence of Evolution
- Fossil Record: Shows gradual changes in life forms over time.
- Evolution: The process by which species change through natural selection.
\begin{callout}
- Trilobites: Early marine arthropods that evolved over millions of years.
- Ammonoids: Cephalopods with distinct shell patterns used as index fossils. \end{callout}
Fossils as Indicators of Ancient Environments
- Coral Fossils: Indicate shallow, tropical seas.
- Coal Layers: Formed from tropical rainforests.
- Pollen Grains: Reveal past climates and vegetation.
\begin{callout} Fossil coral found in rocks in Canada suggests that the region was once covered by a warm, shallow sea, even though it is now far from the equator. \end{callout}
\begin{callout}
- How do geologists use the principles of superposition and cross-cutting relationships to determine the relative ages of rock layers?
- What is the difference between an angular unconformity and a disconformity?
- Why are index fossils so important for correlating rock layers across different regions? \end{callout}
