Chapter 5: The Earth’s Atmosphere, Rocks, and Continents

Earth’s second atmosphere, which formed between 4.2 and 3.8 billion years ago (Bya), was initially composed mainly of water vapor and carbon dioxide. Early single-celled organisms called methanogens, whose signs appear around 3.7 Bya, introduced methane, a greenhouse gas that likely helped keep the early Earth warm. However, the atmosphere fundamentally changed when aquatic cyanobacteria—dubbed the “architects of the atmosphere”—began photosynthesis more than 3.5 Bya, releasing free oxygen. Significant oxygen accumulation, which was disastrous for existing anaerobic life, started around 2.3 Bya, transforming the planet into its current state where oxygen makes up approximately 21% of the volume. Earth itself consists of concentric layers, including a core of solid and molten iron and nickel, surrounded by a thick, ductile mantle, and finally blanketed by a thin crust. The outer structure, composed of the crust and uppermost mantle, is known as the lithosphere. The crust contains three types of rocks—igneous (cooled magma), sedimentary (compressed weathered particles, important for finding fossils), and metamorphic (changed by heat and pressure)—all recycled through a continuous rock cycle. Stratified sedimentary rocks allow relative dating of geological history using the law of superposition, which states that in undisturbed layers, the oldest strata lie at the bottom. The geological timeline is categorized into the Precambrian Eon and the Phanerozoic Eon (starting 545 million years ago, My), which includes the Paleozoic (Age of Fishes), Mesozoic (Age of Reptiles), and Cenozoic (Age of Mammals) Eras, defined by changes in their fossil assemblages. These geological periods are tied directly to plate tectonics, which explains the historical movement and reshaping of continents. Alfred Wegener proposed the continental drift hypothesis, suggesting that all continents were once a single landmass, Pangaea, which later fragmented into Laurasia and Gondwana. Support for continental drift comes from the geographic fit of continental profiles, the correlation of fossil and rock systems across different continents (like the Gondwana formations), paleomagnetism (fossilized magnetic orientation in rocks), and sea-floor spreading at mid-oceanic ridges. Plate boundaries involve separation, sliding past one another (faults), or convergence where oceanic crust often plunges beneath continental crust via subduction, triggering volcanism and mountain building. These continental movements have drastic biological consequences; the isolation of landmasses, such as Australia, leads to unique evolutionary paths for groups like monotremes and marsupials, while the joining of continents, like the formation of the Panama Isthmus in the Pliocene, facilitates massive invasions and competition among previously separated mammals, resulting in significant evolutionary change and extinction.