Identify starting materials for the Earth and planets.
Learn about the divisions of geologic time.
Discover when major geologic events occurred.
Explore the evolution of life and its reasons.
The Big Bang
Occurred approximately 13.7 billion years ago.
Marked the instantaneous creation of all matter in the Universe, primarily Hydrogen (H) and Helium (He).
Followed by an expansion of space-time itself.
A few hundred million years post-Big Bang, giant hydrogen clouds formed massive stars, many of which exploded in supernovae, producing new elements such as Iron (Fe), Silicon (Si), and Oxygen (O).
The remnants of stars formed clusters, leading to the creation of the first galaxies, including our own Milky Way.
Before the Solar System - Gas and Dust
Our Solar System originated from a huge cloud of cold, thinly dispersed interstellar gas and dust in a quiet sector of the Milky Way.
The Active Star Formation region in the Large Magellanic Cloud (LMC) is identified as a significant site for star formation, showcasing swirling clouds of hydrogen and oxygen mixed with dust (photo credit: NASA's Hubble Space Telescope).
Composition of Dust
Dust comprises solid microscopic particles made from:
Minerals: Olivine, Pyroxene.
Metal Oxides and Sulfides: Iron, Aluminum, Nickel.
Carbon: In organic forms and as graphite.
Materials were synthesized in supernovae and dispersed throughout the galaxy.
A scanning electron microscope image of interplanetary dust shows fine grains containing carbon, primitive silicates, and sulfides. The measured scale bar indicates 1 micrometer.
Star Formation "Today"
Observations by Hubble of the Great Orion Nebula reveal protoplanetary disks where stars (and subsequently planets) are forming.
Formation of the Sun
After approximately 10 million years, nuclear fusion initiated at the Sun's center, allowing it to emit significant energy.
The Sun exerts a solar wind, which expels any remaining gases from its vicinity.
Planet Formation: Accretion
Accretion occurs when particles collide and adhere to each other via electrostatic attraction or gravity, forming progressively larger bodies.
A protoplanetary disk surrounding young star HL Tauri exhibits such gaps where planets have aggregated dust and rock.
The Birth of Earth
Earth began forming around 4.6 billion years ago, achieving stability after finishing accreting materials from the surrounding protoplanetary disk.
This event marks the beginning of Geologic Time.
Eons of Geologic Time
Phanerozoic: "Visible life" (542 Ma - present).
Notable for biodiversity and diversification.
Proterozoic: "Before life" (2.5 to 0.542 Ga).
Characterized by tectonic activity and increasing atmospheric O2.
Archean: "Ancient" (3.8 to 2.5 Ga).
Includes the formation of the first continents.
Hadean: "Hell" (4.6 to 3.8 Ga).
Noted for differentiation and a primitive atmosphere.
Earth’s History: Hadean Eon
The Earth, immediately post-formation, was too hot for any significant amounts of surface water.
Its atmosphere was predominantly composed of Hydrogen and Helium from space, with no geological records to reflect specific environmental conditions from this period.
Life likely could not exist during this eon, exacerbated by intense volcanic activity.
Hadean – Post-Formation Earth
Volcanic eruptions released Water (H2O), Carbon Dioxide (CO2), and Methane (CH4) into the atmosphere.
Water accumulated on the surface, leading to the formation of the first oceans.
Contrary to prior beliefs, it is possible that Earth had a solid, cooler surface and a hydrosphere that developed sooner than assumed, potentially hastening the emergence of life.
Differentiation of Earth
Continuous impacts and radioactive decay generated enough heat to melt the Earth's surface.
This led to:
Molten Iron and nickel sinking to form the core.
Formation of the mantle and crust primarily from less dense silicate rocks.
The internal structure of Earth stabilized into layers based on density:
Core: Metal.
Mantle: Dense rock.
Crust: Light rock.
Archean Eon: Atmosphere Formation
Atmospheric composition evolved approximately 4 billion years ago.
Initial atmospheres were dominated by Hydrogen and Helium, which escaped into space.
Volcanism released gases forming an atmosphere enriched with Methane, CO2, Water vapor, and Ammonia.
Evidence of Archean Life
Noted fossils:
Algal filament fossils from prokaryotes, approximately 3.4–3.5 billion years old, found in regions of Arctic and Western Australia.
Identification of simple cells with potential cell division dating back to 3.0–3.1 billion years ago.
Proterozoic Eon and Early Life
Began around 2.5 billion years ago, often referred to as the period of "early life."
Microorganisms evolved near volcanic vents in oceans, utilizing chemicals and heat for energy.
Emergence of chemosynthesizers and thermophiles.
Evolution shifted towards photosynthesis, which significantly increased atmospheric oxygen.
Proterozoic: Oxygen in the Atmosphere
Marine organisms developed photosynthesis capabilities; Blue-green algae created stromatolites, contributing to atmospheric oxygen in shallow waters.
Fossil stromatolites serve as evidence for ancient algae formations. They thrived due to high salt concentrations deterring other organisms.
Banded Iron Formations (BIFs)
Early Proterozoic oceans were acidic with high dissolved iron content.
With increasing oxygen levels, iron was oxidized, forming minerals like magnetite and hematite, which settled to the ocean floors as Banded Iron Formations.
Banded Iron Formations are significant sources of iron ore today.
Oxygenation of the Atmosphere
The interplay of biological photosynthesis and geological processes led to a wave of free oxygen flooding the atmosphere.
This marked the first significant accumulation of oxygen in Earth's history, markedly transforming the planet.
Evolution of Life Due to Oxygen
Increased oxygen levels catalyzed the evolution of new organisms capable of thriving in a less acidic environment.
By the close of the Proterozoic, modern atmospheric oxygen levels and the formation of the ozone layer began to emerge.
Formation of the Ozone Layer
Ozone, a product of atmospheric oxygen interacting with UV rays, developed to shield Earth's surface from harmful radiation.
Supercontinent Rodinia
Toward the end of the Proterozoic, continents amalgamated into the supercontinent Rodinia, leading to geological formations like the Adirondack Mountains.
Rodinia existed as a unified landmass until approximately 650 million years ago, when its breakup resulted in notable carbon reduction in the atmosphere, prompting global cooling.
Snowball Earth
An era of extreme cold, termed Snowball Earth, led to widespread ice coverage across the planet.
Ice spread significantly from the poles towards the equator, with thinner ice nearer the equator, allowing some life to persist.
Modern Life: Phanerozoic Eon
The Phanerozoic Eon is regarded as the era of "visible life" and consists of three principal eras:
Cenozoic: "Recent life."
Mesozoic: "Middle life."
Paleozoic: "Ancient life."
The Cambrian Period signals the onset of the Paleozoic, forming the foundational rock types.
Timeline of the Phanerozoic Eon
The Phanerozoic Eon spans 542 million years to present, constituting 16% of Earth's total age of 4600 million years.
Crossing to Land: Flora and Fauna
Paleozoic Land Plants
Plant and insect colonization of the land initiated around 430 million years ago during the Silurian Period.
By 360 million years ago in the Carboniferous Period, significant swamp and jungle ecosystems evolved, contributing plant materials leading to modern coal deposits.
Paleozoic Fish
Fish, derived from eel-like organisms in the Devonian Period, saw sharks emerging as ancient vertebrates. They later adapted to terrestrial habitats.
Transition from Fish to Amphibians
Increased terrestrial food sources prompted fish to evolve into amphibians with similar bone structures, marking the advent of land-inhabiting animals during the Permian Period.
Evolution of Reptiles from Amphibians
The lineage of tetrapods as four-limbed animals commenced with amphibians, giving rise to amniotes, the ancestors of modern reptiles, birds, and mammals. Reptiles first appeared towards the close of the Paleozoic, dominating throughout the Mesozoic.
The Mesozoic Era
A generally drier climate benefitted reptiles, allowing them to reproduce without requiring aquatic environments.
Characteristics such as hard-shelled eggs and rough skin facilitated survival amidst scarce water.
Dominance of dinosaurs characterized the Mesozoic, with reptiles becoming the primary terrestrial life forms.
Rise of Dinosaurs
Dinosaurs evolved to become the chief terrestrial species during the Triassic and proliferated by the Jurassic Period, even as mammals and birds developed by the Cretaceous era.
The Cretaceous-Paleogene Event
A significant asteroid impact generated a dust cloud resulting in a K-Pg boundary, responsible for the extinction of 70% of species, including all dinosaurs.
This catastrophic event served as a major reset for Earth's biological history.
The Cenozoic Era: Mammals and Birds Rise
Following the downfall of dinosaurs, early mammals and birds began flourishing due to lesser predation.
Earth cooled significantly during the Paleogene Period, adapting fur-bearing mammals to diverse habitats.
Prominent human ancestors, termed Primates, appeared around 80 million years ago.
The human lineage diverged and evolved about 25-40 million years ago.
Emergence of Earliest Humans
Hominids, originating from a common ancestor with chimpanzees, emerged approximately 2.3 million years ago, in mid-to-southern Africa.
Multiple species competed for survival but most became extinct, leaving modern humans as the sole surviving lineage over the last 30,000 years.
Timeline of Earliest Life and Milestones
Origin of Life: Roughly 3.8 billion years ago, single-celled organisms emerged.
Cambrian Explosion: Around 542 million years ago, complex life forms appeared.
700 million years ago, atmospheric oxygen levels rose significantly.
The Age of Dinosaurs and subsequently the Age of Mammals illustrate key transitions in Earth's biological history.
In conclusion, Hominids represented only about 0.05% of Earth's history, calculated by: $$ rac{2.3 ext{ million years}}{4600 ext{ million years}} imes 100 = 0.05 ext{%}.