3.1G Earth history and the Anthropocene (HL) Notes

Evidence of Evolution

1. Earth is approximately 4.5 billion years old, and life has evolved gradually through multiple physical, chemical, and biological processes.
2. Over this time, geological, climatic, and biological changes have continuously shaped the planet’s surface and influenced the evolution of species.
3. These changes occurred on geological timescales, meaning they took millions to billions of years, far beyond human lifespans.

Fossil Record

1. The fossil record provides chronological evidence for the evolution of life on Earth.
2. Fossils are preserved remains or traces of organisms from past geological ages found in sedimentary rocks.
3. By examining fossil layers, scientists can trace changes in species over time, demonstrating how life has evolved.

Mechanisms of Change

1. Plate tectonics reshaped continents and oceans, isolating populations and driving evolution.
2. Volcanic activity caused global climate disruptions, influencing which organisms survived.
3. Atmospheric evolution, such as oxygen accumulation during the Great Oxidation Event, allowed aerobic organisms to thrive.
4. These long-term processes created the conditions necessary for complex life to evolve.

Role of Carbon Dating and Ice Core Data

1. Carbon dating helps determine fossil age by measuring radioactive decay of carbon-14.
2. Ice core samples provide evidence of ancient atmospheric gas concentrations, linking climate trends to biodiversity shifts.

The Geological Time Scale

1. Earth’s 4.6-billion-year history is divided into hierarchical time intervals: Eons → Eras → Periods → Epochs → Ages
2. This division helps scientists interpret biological and environmental changes over immense time scales.

Major Eons

1. Hadean (4.5–4.0 billion years ago): Formation of Earth; no life existed.
2. Archean (4.0–2.5 billion years ago): First simple life (bacteria).
3. Proterozoic (2.5 billion–541 million years ago): Rise of oxygen and multicellular organisms.
4. Phanerozoic (541 million years ago–present): Explosion of visible life; rapid diversification of species.

Major Eras of Biological History

Precambrian (4.6 billion – 541 million years ago)

1. Formation of Earth and earliest life.
2. Cyanobacteria began photosynthesis, releasing oxygen into the atmosphere (the Great Oxidation Event).
3. Emergence of simple multicellular life near the end.

Paleozoic Era (541 – 252 million years ago)

1. Cambrian Explosion: Rapid diversification of marine life.
2. First vertebrates, land plants, and amphibians appeared.
3. Ended with the Permian mass extinction, the largest in Earth’s history.

Mesozoic Era (252 – 66 million years ago)

1. Known as the Age of Reptiles.
2. Dominated by dinosaurs, marine reptiles, and early birds and mammals.
3. Ended with the Cretaceous–Paleogene (K–Pg) extinction event, wiping out ~75% of species.

Cenozoic Era (66 million years ago – Present)

1. Known as the Age of Mammals.
2. Mammals and birds diversified to fill ecological niches left by dinosaurs.
3. Humans (Homo sapiens) evolved in the late Quaternary period.

Epochs as Biological and Environmental Markers

1. Epoch transitions reflect major environmental changes that caused extinction or evolution of dominant life forms.
2. These boundaries are identified through fossil evidence, sediment analysis, and radiometric dating.

Fossil Record and Epoch Division

1. Fossils act as “biostratigraphic markers”, indicating evolutionary milestones.
2. Changes in fossil assemblages mark transitions between epochs, revealing how species adapted after global events.
3. The Pentadactyl limb (five-limbed structure in mammals, reptiles, and amphibians) demonstrates divergent evolution from a common ancestor, highlighting adaptive modification over time.

Mass Extinction

1. A mass extinction is a global, rapid decrease in biodiversity, where a large number of species go extinct in a relatively short geological period.
2. Earth has experienced five major mass extinctions, each reshaping ecosystems and paving the way for new dominant species.

Causes of Mass Extinction

1. Tectonic plate movements: Alter habitats and climate through continent shifting.
2. Super-volcanic eruptions: Cause global cooling or acid rain, devastating ecosystems.
3. Climate changes: Long-term ice ages or global warming events disrupt food webs.
4. Sea-level fluctuations: Submerge or expose habitats.
5. Meteorite impacts: Cause dust clouds, blocking sunlight and collapsing food chains.

The Five Major Mass Extinctions

1. Ordovician-Silurian (≈440 mya): Climate cooling and sea-level fall killed 85% of marine life.
2. Late Devonian (≈375 mya): Widespread ocean anoxia (oxygen loss).
3. Permian-Triassic (≈252 mya): Known as “The Great Dying,” wiped out 96% of marine species.
4. Triassic-Jurassic (≈201 mya): Volcanic activity linked to the breakup of Pangaea.
5. Cretaceous-Paleogene (≈66 mya): Asteroid impact; extinction of non-avian dinosaurs.

The Sixth Mass Extinction (Ongoing)

1. Many scientists argue that human activity is causing a sixth mass extinction, occurring faster than previous ones.
2. Key drivers include:
   1. Habitat destruction and deforestation
   2. Climate change
   3. Overexploitation of resources
   4. Pollution
   5. Introduction of invasive species
3. Current extinction rates are estimated to be 100-1,000 times higher than natural background rates.

Speciation After Mass Extinction

1. Extinction removes dominant competitors, freeing ecological niches.
2. Surviving species undergo adaptive radiation, evolving rapidly to exploit new habitats.
3. This leads to increased biodiversity over millions of years.

The Anthropocene Epoch

1. The Anthropocene is a proposed geological epoch in which human activity is the dominant influence on Earth’s environment, climate, and geology.
2. It marks a shift from natural to anthropogenic (human-driven) processes.
3. Some scientists argue it should be officially recognized as a new epoch following the Holocene (11,700 years ago).
4. Debate centers around when it began:
   1. 1610: Drop in atmospheric CO₂ after European colonization of the Americas (“Orbis spike”).
   2. 1950s: Nuclear fallout and “Great Acceleration” in industrialization.
   3. 1964: Carbon-14 spikes from nuclear testing.

Characteristics of the Anthropocene

1. Rapid species extinction (sixth mass extinction).
2. Climate change caused by fossil fuel combustion.
3. Deforestation and land-use change altering ecosystems.
4. Ocean acidification from increased CO₂.
5. Chemical and plastic pollution preserved in sediments.
6. Global mixing of species through trade and transport.

Human Activities as Geological Forces

1. Humans now move more sediment, rock, and biomass than all natural processes combined.
2. Activities such as mining, construction, and damming rivers have reshaped landscapes.
3. Urbanization creates anthropogenic rock layers distinct in structure and composition.

Human Impacts and Geological Evidence

1. Human activities are leaving planetary-scale signatures that will be visible in geological strata:
   1. Chemical pollution: Persistent organic pollutants, fertilizers, and microplastics accumulating in sediments.
   2. Invasive species mixing: Fossil evidence of species occurring far from their natural ranges.
   3. Nuclear residues: Fallout particles (e.g., plutonium, cesium-137) embedded in sediments.
   4. Sediment modification: River damming, deforestation, and soil erosion altering deposition patterns.
   5. Human-made minerals: Concrete, aluminum alloys, plastics, and synthetic compounds forming permanent strata.

Planetary Boundary Effects

1. Human impacts are pushing several planetary boundaries (e.g., biodiversity loss, climate regulation, nitrogen cycle).
2. These pressures are likely to cause permanent changes in atmospheric, oceanic, and sedimentary records.