- Explain the big-bang theory and origin of the elements
- Explain the solar system’s origin and the consequences for Earth.
- Describe the turbulent beginning of Earth during the Hadean and Archean Eons
- Identify the transition to the modern atmosphere, plate tectonics, and evolution that occurred in the Proterozoic Eon
- Describe the Paleozoic evolution and extinction of invertebrates with hard parts, fish, amphibians, reptiles, tetrapods, and land plants; and tectonics and sedimentation associated with the supercontinent Pangea
- Describe the Mesozoic evolution and extinction of birds, dinosaurs, and mammals; and tectonics and sedimentation associated with the breakup of Pangea
- Describe the Cenozoic evolution of mammals and birds, paleoclimate, and tectonics that shaped the modern world
Entire courses and careers have been based on the wide-ranging topics covering Earth’s history. Throughout the long history of Earth, change has been the norm. Looking back in time, an untrained eye would see many unfamiliar life forms and terrains. The main topics studied in Earth history are paleogeography, paleontology, and paleoecology and paleoclimatology—respectively, past landscapes, past organisms, past ecosystems, and past environments. The changes that have occurred since the inception of Earth are vast and significant. From the oxygenation of the atmosphere, the progression of life forms, the assembly and deconstruction of several supercontinents, to the extinction of more life forms than exist today, having a general understanding of these changes can put present change into a more rounded perspective. This chapter will cover briefly the origin of the universe and the 4.6 billion year history of Earth. This Earth history will focus on the major physical and biological events in each eon and Era.
- 15.1: Origin of the Universe
- The mysterious details of events prior to and during the origin of the universe are subject to great scientific debate. The prevailing idea about how the universe was created is called the big-bang theory. Although the ideas behind the big-bang theory feel almost mystical, they are supported by Einstein’s theory of general relativity. Other scientific evidence, grounded in empirical observations, supports the big-bang theory.
- 15.2: Origin of the Solar System—The Nebular Hypothesis
- Our solar system formed as the same time as our Sun as described in the nebular hypothesis. The nebular hypothesis is the idea that a spinning cloud of dust made of mostly light elements, called a nebula, flattened into a protoplanetary disk, and became a solar system consisting of a star with orbiting planets. The spinning nebula collected the vast majority of material in its center, which is why the sun Accounts for over 99% of the mass in our solar system.
- 15.3: Hadean Eon
- Geoscientists use the geological time scale to assign relative age names to events and rocks, separating major events in Earth’s history based on significant changes as recorded in rocks and fossils. This section summarizes the most notable events of each major time interval. For a breakdown of how these time intervals are chosen and organized. The Hadean Eon, named after the Greek god and ruler of the underworld Hades, is the oldest eon and dates from 4.5–4.0 billion years ago.
- 15.4: Archean Eon
- Objects were chaotically flying around at the start of the solar system, building the planets and moons. There is evidence that after the planets formed, about 4.1–3.8 billion years ago, a second large spike of asteroid and comet impacted the Earth and Moon in an event called late heavy bombardment. Meteorites and comets in stable or semi-stable orbits became unstable and started impacting objects throughout the solar system.
- 15.5: Proterozoic Eon
- The Proterozoic Eon, meaning “earlier life,” is the eon of time after the Archean eon and ranges from 2.5 billion years old to 541 million years old. During this time, most of the central parts of the continents had formed and the plate tectonic process had started. Photosynthesis (in organisms like stromatolites) had already been adding oxygen slowly to the atmosphere, but it was quickly absorbed in minerals.
- 15.6: Paleozoic
- The Phanerozoic eon is the most recent eon and represents time in which fossils are common, 541 million years ago to today. The word Phanerozoic means “visible life.” Older rocks, collectively known as the Precambrian (sometimes referred to as the Cryptozoic, meaning “invisible life”), are less common and the fossils that exist represent soft-bodied life forms. The invention of hard parts like claws, scales, shells, and bones made fossils more easily preserved, and thus, easier to find.
- 15.7: Mesozoic
- Pangea started breaking up around 210 million years ago in the Late Triassic. Clear evidence for this includes the age of the sediments in the Newark Supergroup rift basins and the Palisades sill of the eastern part of North America and the age of the Atlantic ocean floor. Due to sea-floor spreading, the oldest rocks on the Atlantic’s floor are along the coast of northern Africa and the east coast of North America, while the youngest are along the mid-ocean ridge.
- 15.8: Cenozoic
- The Cenozoic, meaning “new life,” is known as the age of mammals because it is in this era that mammals came to be a dominant and large life form, including human ancestors. Birds, as well, flourished in the open niches left by the dinosaur’s demise. Most of the Cenozoic has been relatively warm, with the main exception being the ice age that started about 2.558 million years ago and (despite recent warming) continues today.
Thumbnail: Spider Rock, within Canyon de Chelly National Monument, not only has a long human history with the Diné tribe but also has a long geologic history. The rocks are Permian in age and formed in the desert conditions that dominated North America toward the end of the Paleozoic through the middle Mesozoic. Erosion of the canyon occurred in the Cenozoic.