History of the Earth (Basic Earth Science: Prerequisites)
Hello! In this chapter, we will explore the epic history of the planet we call home, from its birth right up to the present day—the History of the Earth. You might be thinking, "This looks like a lot of memorization..." but don't worry! Earth's history unfolded through the deeply intertwined drama between "environmental changes in the atmosphere and oceans" and "the story of prehistoric life" that lived through them. Once you grasp the overarching narrative, this topic will become a reliable source of points on your Common Test. Let's organize the key ideas step by step together!
*Note: "The Dynamic Earth (Plate Tectonics, Volcanoes, and Earthquakes)" and "Atmospheric and Oceanic Circulation" will be covered in detail in separate chapters.
---1. Strata and Methods for Investigating the Past
Past events on Earth are recorded within strata (rock layers), rocks, and fossils. First, let's master the basic rules for decoding past eras and environments.
(1) Law of Superposition
As long as strata have not been overturned, this fundamental principle states: "Lower strata are older, and upper strata are younger."
(2) The Two Roles of Fossils: Facies Fossils and Index Fossils
Fossils serve two distinct purposes: some tell us about past "environments," while others reveal past "geological periods." This distinction is an absolute must-know topic that frequently appears on the Common Test!
① Facies Fossils (Clues to the Environment)
・These fossils indicate the specific environment (water depth, water temperature, climate, etc.) at the time the sediment was deposited.
・Requirement: Organisms that can only survive in a specific, restricted environment (a narrow habitat range).
・Examples: Corals (warm, shallow, clear marine waters), brackish-water clams (Corbicula / Shijimi) (brackish waters such as estuaries), short-neck clams (Asari) (shallow sea).
② Index Fossils (Clues to the Geological Age)
・These fossils are used to determine the geological age in which the stratum was deposited.
・Requirement: Organisms that were widely distributed geographically, evolved rapidly, and existed for a short period of time.
・Examples: Trilobites (Paleozoic), Ammonites (Mesozoic), Vicarya (Cenozoic).
[Common Mistake]
Mixing up the requirements for facies fossils and index fossils is an extremely common error!
・To determine the environment (facies): The organism must have had a narrow habitat range (if it could live anywhere, we couldn't pinpoint the environment).
・To determine the age (index): The organism must have had a wide geographic distribution and a short evolutionary lifespan.
(3) Radiometric Dating (Absolute Age)
Specific numerical values (ages) indicating when rocks or minerals formed are measured using the radioactive decay of radioactive isotopes contained within the rock.
The time required for half of the radioactive isotopes to decay is called the half-life. Because the number of parent isotopes decreases regularly by half, then by half again, we can calculate the time elapsed since formation by determining the ratio between the remaining parent isotopes and the daughter isotopes produced.
[Key Point]
If the half-life is denoted by \(T\) and the initial quantity of parent isotopes is \(N_0\), the remaining amount of parent isotopes \(N\) after an elapsed time \(t\) is expressed as follows:
\(N = N_0 \left(\frac{1}{2}\right)^{\frac{t}{T}}\)
For example, after two half-lives, the parent isotope decreases to \(\frac{1}{4}\) of its initial amount; after three half-lives, it becomes \(\frac{1}{8}\).
2. The Birth of Earth and the Precambrian Era
The vast expanse of time from the formation of Earth approximately 4.6 billion years ago to the beginning of the Paleozoic Era around 540 million years ago is called the Precambrian. It accounts for nearly 90% of Earth's entire history.
(1) The Primitive Earth and the Formation of the Atmosphere and Oceans
① Planetesimal Impacts: Earth grew through the accretion and collision of planetesimals. Impact energy, combined with the greenhouse effect of water vapor, heated the surface to extreme temperatures, forming a magma ocean of molten rock.
② Differentiation into Core and Mantle: Heavy elements like iron and nickel sank to the center to form the core, while lighter silicate materials formed the outer mantle.
③ Birth of the Oceans: As planetesimal collisions subsided, the surface cooled. Vast quantities of water vapor in the primitive atmosphere condensed into torrential rain, forming the primitive oceans. Carbon dioxide (\(CO_2\)), the main component of the primitive atmosphere, dissolved into the oceans and was locked away as limestone, causing atmospheric carbon dioxide levels to drop drastically.
(2) The Origin of Life and the Rise of Oxygen
Life is believed to have originated in the primitive oceans around 4.0 to 3.8 billion years ago.
・Appearance of Cyanobacteria: Photosynthetic cyanobacteria emerged and began releasing oxygen (\(O_2\)). The layered sedimentary rock structures formed by the metabolic activity of cyanobacteria are known as stromatolites.
・Formation of Banded Iron Formations (BIFs): Oxygen released by cyanobacteria reacted with abundant dissolved ferrous ions (\(Fe^{2+}\)) in seawater to form iron oxides, which settled on the seabed. These became the banded iron formations seen around the world today.
・Formation of the Ozone Layer: Once the dissolved iron in the oceans was completely oxidized, oxygen escaped into the atmosphere, creating the ozone layer in the upper atmosphere. This blocked harmful ultraviolet radiation, establishing the conditions needed for organisms to later venture onto land.
・Ediacaran Biota: Near the end of the Precambrian (late Proterozoic), the Ediacaran biota—a group of large, soft-bodied multicellular organisms lacking hard skeletons—flourished.
[Fun Fact]
Much of the iron ore we rely on today is mined from banded iron formations diligently produced by Precambrian cyanobacteria pumping out oxygen! Earth's environmental history is directly linked to modern industry.
3. The Phanerozoic Eon (Paleozoic, Mesozoic, and Cenozoic Eras)
The span of time characterized by an abundance of macroscopic fossils visible to the naked eye is called the Phanerozoic Eon. From oldest to youngest, it is divided into the Paleozoic, Mesozoic, and Cenozoic eras.
(1) Paleozoic Era (approx. 540 to 250 million years ago): Transition from Sea to Land
The Paleozoic was an era when life exploded with diversity in the oceans and eventually expanded its living domain onto land.
・Cambrian Period: A wide variety of animal phyla with hard shells and skeletons appeared abruptly (e.g., the Burgess Shale fauna). The representative index fossil is the trilobite.
・Ordovician Period: The ozone layer continued to develop, and jawless fishes (primitive vertebrates) appeared.
・Silurian Period: Plants (such as Cooksonia) colonized land, followed shortly by arthropods.
・Devonian Period: Known as the "Age of Fishes"; early amphibians (such as Ichthyostega) ventured onto land.
・Carboniferous Period: Vast forests of giant spore-bearing tree ferns (such as Lepidodendron and Calamites) flourished, forming the coal beds we use today. Reptiles also appeared.
・Permian Period: Landmasses coalesced to form the supercontinent Pangaea. At the end of this period, the greatest mass extinction in Earth's history occurred (wiping out more than 90% of marine species), causing the extinction of trilobites and fusulinids.
(2) Mesozoic Era (approx. 250 to 66 million years ago): Age of Reptiles and Gymnosperms
A warm climate prevailed; reptiles such as dinosaurs dominated the land, while ammonites flourished in the oceans.
・Flora: Gymnosperms, including cycads and ginkgoes, flourished. Angiosperms (flowering plants) also emerged during the Cretaceous Period.
・Fauna: Dinosaurs ruled the land, while ammonites thrived in the seas. During the Jurassic Period, birds (such as Archaeopteryx) and primitive mammals appeared.
・End-Cretaceous Mass Extinction: Around 66 million years ago, dinosaurs and ammonites went completely extinct, primarily driven by a massive asteroid impact.
(3) Cenozoic Era (approx. 66 million years ago to Present): Age of Mammals and Angiosperms
Filling the ecological niches left vacant by the extinction of the dinosaurs, mammals and birds underwent rapid diversification. This era is subdivided into the Paleogene, Neogene, and Quaternary periods.
・Paleogene & Neogene Periods: Angiosperms and mammals flourished. Famous index fossils include the Neogene gastropod Vicarya, Desmostylus, and Nummulites.
・Quaternary Period: An ice age characterized by alternating cold glacial periods and warmer interglacial periods. Mammoths and Naumann's elephants roamed, and humans evolved and spread.
[Summary for Memorization: Representative Index Fossils by Era]
・Paleozoic: Trilobites, Fusulinids, Graptolites
・Mesozoic: Ammonites, Dinosaurs, Trigonia
・Cenozoic: Vicarya, Nummulites, Naumann's elephant, Metasequoia
4. Chapter Summary and Review Checklist
[Key Points]
1. Facies fossils reveal "environment" (narrow habitat range); Index fossils reveal "geological age" (wide geographic distribution, short evolutionary lifespan).
2. Primitive atmosphere and oceans: Magma ocean cools \(\rightarrow\) Rain falls, creating primitive oceans \(\rightarrow\) \(CO_2\) dissolves into oceans to form limestone, reducing atmospheric concentrations.
3. History of oxygen: Photosynthesis by cyanobacteria \(\rightarrow\) Formation of banded iron formations (oceanic iron precipitates) \(\rightarrow\) Oxygen escapes into the atmosphere \(\rightarrow\) Ozone layer forms \(\rightarrow\) Terrestrial colonization by living organisms.
4. Evolutionary timeline of life: Plants (Cooksonia) colonize land (Silurian) \(\rightarrow\) Amphibians move onto land (Devonian) \(\rightarrow\) Giant spore-bearing plants thrive (Carboniferous) \(\rightarrow\) Dinosaurs and gymnosperms flourish (Mesozoic) \(\rightarrow\) Mammals, angiosperms, and humans flourish (Cenozoic).
Keep this chronological flow and these key concepts connected in your mind, and you will be fully prepared to ace any questions on Earth's history on the Common Test!