History of Life on Earth – Comprehensive Study Notes

Major events in the history of the early Earth

• Big Bang ≈ 14 billion years ago.
• Our Sun begins fusing; planets (including Earth) form ≈ 4.6 billion years ago (Ga) during the Hadean Eon.
• First life (Prokaryotes) appear ≈ 3.5 Ga (Archaean Eon).
• Cyanobacteria begin photosynthesizing ≈ 2.7 Ga (Archaean/Proterozoic transition).
• First Eukaryotes appear ≈ 1.8 Ga (Proterozoic Eon).
• Multicellular organisms ≈ 1.0 Ga (Proterozoic Eon).
• Snowball Earth catastrophe ≈ 650 Ma (Proterozoic Eon).
• First animals appear ≈ 630 Ma (Ediacaran Period).
• Cambrian Explosion ≈ 550 Ma (Paleozoic Era).
• Age of reptiles ≈ 250–65 Ma (Mesozoic Era).
• Age of mammals ≈ 65 Ma to present (Cenozoic Era).
• Ice Age glaciations ≈ 2.6 Ma (Pleistocene Epoch).
• First human ancestors ≈ 2.1 Ma (Pleistocene Epoch).
• The Age of Humans ≈ 100,000 years ago to present (Anthropocene/Holocene Epoch).

The change of Earth’s continents and climate over time

• Continental drift and plate tectonics: Wegener proposed in 1912; gained acceptance in the 1960s after WWII submarine mapping.
• Mechanisms driving plate tectonics:
  • Convection Zone: Hot mantle rises and pushes apart ocean floors.
  • Ridge Zone: Area where seafloor expansion occurs.
  • Subduction Zone: Old crust is forced back into the mantle and melts.
  • Uplift Zone: Extra material is pushed upward, associated with volcanoes and earthquakes.
• Consequences for climate and continents:
  • Movement of continents alters ocean currents and climate.
  • Sea level changes accompany continental movements.
  • Reconfigurations of landmasses influence weathering, carbon cycling, and habitat distribution.

The major events in the development of life on Earth

• Macroevolution spans deep time; observable macroevolutionary patterns require geological context.
• Key transitions requiring geology:
  • Appearance of new traits and lineages.
  • Evolutionary branching and speciation across millions to billions of years.
• Microevolution occurs on short timescales (hours-days in lab) and does not by itself document long-term diversification; macroevolution requires deep-time records.

Extinction events and speciation

• Major extinction and adaptive radiation events shape the history of life:
  • Great Oxygenation Event (GOE)
  • Snowball Earth events
  • Ordovician–Silurian extinction
  • Devonian–Carboniferous extinction events
  • Permian–Triassic (PT) boundary extinction
  • Triassic–Jurassic extinction
  • Cretaceous–Paleogene (KT) extinction
• These events open ecological niches and drive subsequent adaptive radiations.

STUDYING EARTH’S HISTORY

• Microevolution vs macroevolution:
  • Microevolution can be observed in hours to days in lab.
  • Macroevolution involves longer timescales and large-scale patterning (appearance of new species, radiations).
• Macroevolution requires deep time and a solid geology background to interpret preserved records.

GEOLOGY

• The history of the Earth is recorded in rocks.
• Three kinds of rocks:
  • Igneous: Formed from magma/lava.
  • Metamorphic: Igneous rock altered by high pressure and temperature.
  • Sedimentary: Eroded material deposited and reformed into rock in the presence of water.
• Relative dating concepts:
  • Sedimentary rocks accumulate with oldest layers at the bottom (law of superposition).
  • Each depositional layer represents a stratum (pl. strata).
• Fossils are typically contained in sedimentary rock deposits.
• Fossil age is relative to the stratum in which it is found.

FOSSILS

• Fossils are preserved remains of ancient organisms.
• Inform us about the organism’s morphology (body form).
• Tell us when and for how long a species roamed the Earth.
• Provide a direct record of evolution.
• Fossils are rare; requires specific conditions to fossilize.

RELATIVE DATING

• Stratigraphy: Fossils can be dated relative to the stratum of rock in which they are deposited.
• Similar fossils are found in widely separated locations.
• Some fossils are consistently found in younger strata; others in older strata.
• Younger strata fossils are more similar to modern organisms.

ABSOLUTE DATING

• Unstable radioactive isotopes (radioisotopes) decay into stable isotopes.
• Isotope decay occurs at a precise, universal rate.
• Example: $t_{1/2} = 5700\text{ years}$ for Carbon-14.
• Example dating relation: $\frac{N}{N<em>0} = \left(\frac{1}{2}\right)^{\frac{t}{t</em>{1/2}}}$
• Therefore, a 5,700-year half-life means a 5,700-year-old sample would have $\frac{N}{N_0} = \frac{1}{2}$.

ABSOLUTE DATING – WARNING!

• Carbon-14 dating is ONLY applicable to samples 100 to 60,000 years old.
• Potassium-40 dating is ONLY usable for samples 10 million to 4.5 billion years old.
• Uranium-235 dating is ONLY usable for samples 200,000 to 4.5 billion years old.
• Other isotopes are used in combination to date samples roughly between 50,000 and 10 million years old.

DATING GEOLOGIC EVENTS

• Dating events in geological time requires BOTH relative and absolute dating methods.
• Sedimentary rocks and depositional layers use relative dating.
• Igneous rocks can use absolute dating.
• To apply absolute dates to sediments, scientists find places where igneous and sedimentary rocks formed at the same time.
• Paleomagnetic dating: uses the orientation of iron-bearing minerals; Earth’s magnetic field reverses and sediments preserve that orientation; applicable to both sedimentary and igneous rocks.

EARTH HISTORY

• Time divisions:
  • Eon > Era > Period > Epoch > Millennium > Century > Decade > Year
• Divisions reflect dramatic global changes:
  • Fossil record: mass extinctions or species shifts.
  • Geological record: mass movements of crust and continents.
• Time divisions have evolved as new data emerges; major changes still occur but not recently.

STUDY HANDOUT

• Students should know: Eons, Eras, Periods, Epochs, ages of onset, and important events.
• The handout (CANVAS) includes instructor notes and rounded dates; study by using this resource.

A DYNAMIC EARTH

• Historically, Earth seemed stagnant and unchanging; lack of mechanism to explain geographic formations.
• 1912: Alfred Wegener proposed Continental Drift; faced initial resistance due to lack of mechanism.
• Wegener died during data collection in a North Pole expedition.
• Acceptance grew in the 1940s with submarine mapping during WWII.
• 1960s: Plate tectonics and Continental Drift Theory gained merit and acceptance.

CONTINENTAL DRIFT & PLATE TECTONICS

• Key concepts:
  • Convection Zone: hot mantle material rises and pushes apart ocean floors.
  • Ridge Zone: area of expansion at mid-ocean ridges.
  • Subduction Zone: old crust sinks back into mantle and melts.
  • Uplift Zone: excess material causes uplift; volcanism and earthquakes are common.

CONTINENTAL MOVEMENTS

• Visuals emphasize that plate movement has shaped landmasses and climates over time.
• Note: A video illustrating continental movement is available at the provided link; some language may be inappropriate for some audiences.

VOLCANISM AND BOMBARDMENT

• Massive volcanic eruptions have shaped Earth’s climate and biosphere.
• Meteorite impacts have dramatic climate and life implications.
• The Era of Heavy Bombardment occurred during the Hadean Eon.

EARTH’S CHANGING CLIMATE

• Climate is driven by: continental positions, volcanism, extraterrestrial impacts, and life itself.
• Global climate markers include:
  • Sea level changes
  • Temperature shifts
  • Humidity/precipitation patterns
  • Atmospheric O2 concentration
  • Atmospheric CO2 and N2 levels influence climate and life.

SEA LEVEL

• Sea level has experienced high and low phases corresponding to tectonics, glaciations, and climate change.
• The chart marks major eras with a schematic scale (e.g., Paleozoic to present) and variable levels.
• Notation indicates mass extinction events at certain intervals.

ATMOSPHERIC OXYGEN CONCENTRATION

• Timeline of oxygen in the atmosphere showing major steps:
  • First photosynthetic bacteria produce O2 as a byproduct.
  • GOE-associated rise in atmospheric O2 with significant ecological impacts.
  • First aerobic life emerges after sufficient O2 accumulation.
  • First multicellular eukaryotes appear as oxygen levels rise.
  • Later events include the appearance of chordates and giant insect lineages.
  • Invasion of land and later diversification of plants (including flowering plants).

JURASSIC & CRETACEOUS GLOBAL CONTEXT

• Global temperature trends are shown across deep time, with periods labeled Paleozoic, Mesozoic, and Cenozoic.
• General pattern: long warm periods with fluctuations; cooler intervals correspond to major climate events.
• Relative temperatures are depicted (e.g., COOL vs WARM phases) across eras.

ATMOSPHERIC CO2 & TEMPERATURE (DETAILED VIEW)

• Time scales span Precambrian to the present; temperature markers (in °C) and CO2 concentration are shown.
• Example axis values and historical markers appear as: ${CO_2}\text{ (ppm)}$ and $A\,(^{\circ}C)$ across times such as 4,600 Ma to today.
• The data illustrate long-term CO2 and temperature fluctuations tied to geological processes and life.

ADDRESSING MODERN CLIMATE CHANGE

• Earth’s climate has changed dramatically over 4.6 billion-year history.
• Historical changes in temperature, sea level, and CO2 occur over hundreds of thousands of years and were often linked to volcanic activity or bombardment events.
• When changes are abrupt, they can precede major extinctions.
• Modern climate change is at least partially anthropogenic (man-made).
• Notable driver: dramatic CO2 emissions rise over the last ~200 years, coinciding with fossil fuel use.
• Fossil fuels are formed by ancient plant matter during the Carboniferous Period; this connection is widely supported in science and ecology literature—note: some dissent exists among individuals funded by corporate interests.

QUATERNARY PERIOD CO2 & TEMPERATURE CHANGE

• A simple visualization shows CO2 and temperature changes over thousands of years (thousands of years before present).
• The CO2 axis shows values in the hundreds of ppm range; temperature is shown as a separate axis, reflecting corresponding climate responses.

STUDY HANDOUT

• Reminder to know: Eons, Eras, Periods, Epochs, ages of onset, and important events.
• Use CANVAS handout (with instructor’s notes) to study; focus on rounded dates and instructor-added notes.

MAJOR EVENTS IN EARTH HISTORY

• Big Bang ≈ 14 billion years ago.
• Sun forms and planets form ≈ 4.6 billion years ago (Earth in the Hadean Eon).
• First life (Prokaryotes) ≈ 3.5 billion years ago (Archaean Eon).
• Cyanobacteria begin photosynthesis ≈ 2.7 Ga.
• First Eukaryotes ≈ 1.8 Ga.
• Multicellular organisms ≈ 1.0 Ga.
• Snowball Earth ≈ 650 Ma (Proterozoic).
• First animals ≈ 630 Ma (Ediacaran Period).
• Cambrian Explosion ≈ 550 Ma (Paleozoic).
• Age of reptiles ≈ 250–65 Ma (Mesozoic).
• Age of mammals ≈ 65 Ma to present (Cenozoic).
• Ice Age glaciations ≈ 2.6 Ma (Pleistocene).
• First human ancestors ≈ 2.1 Ma (Pleistocene).
• Age of Humans ≈ 100,000 years ago to present (Anthropocene/Holocene).

BIG BANG (14 BILLION YEARS AGO)

• First subatomic particles formed: protons, neutrons, electrons.
• Nuclei formed within first minutes; neutral atoms formed over thousands of years.
• Primordial composition: hydrogen, helium, and traces of lithium dominate early universe.
• Heavier elements synthesized in stars and supernovae; later incorporated into stars, planets, and life-sustaining chemistry.

SOLAR SYSTEM FORMS (4.6 BYA; HADEAN EON)

• Early Earth was molten and unsuitable for life.
• Radioactivity kept the Earth hot for about 1 billion years after formation.

FIRST LIFE (PROKARYOTES) APPEARS (3.5 BYA; ARCHAEAN EON)

• Earth cools and oceans form as water vapor condenses.
• After cooling, simple prokaryotic cells appear: Domain Bacteria and Domain Archaea.
• Origin of life has several hypotheses; no single consensus, but all life descends from a common ancestor with cellular organization and nucleic acids/proteins/lipids/carbohydrates.

BACTERIAL PHOTOSYNTHESIS (2.7 BYA; ARCHAEAN EON)

• Great Oxygenation Event/Catastrophe occurs as cyanobacteria perform photosynthesis.
• Oxygen is released as a byproduct; atmosphere becomes more oxygen-rich and toxic to many anaerobes, causing mass extinctions of anaerobic life.

FIRST EUKARYOTES APPEAR (1.8 BYA; PROTEROZOIC EON)

• Eukaryotes evolved from prokaryotes.
• Key features:
  • Larger size; complex cytoskeleton; ability to change form.
  • Endocytosis/exocytosis capabilities.
  • True membranous organelles and nucleus.
  • Endosymbiosis gave rise to mitochondria and chloroplasts.

MULTICELLULAR ORGANISMS (1.0 BYA; PROTEROZOIC EON)

• Eukaryotes form colonies with cell specialization.
• Emergence of primitive tissues, organs, and organ systems.
• Led to true multicellularity.

SNOWBALL EARTH CATASTROPHE (650 MA; PROTEROZOIC EON)

• Continental shifts and massive volcanism drive a global ice age with equator freezing as cold as Antarctica.
• Most life dies; survivors are highly adapted organisms.
• Upon thaw, an adaptive radiation occurs, seeding the Cambrian explosion.

FIRST ANIMALS APPEAR (630 MA; EDIACARAN PERIOD)

• Earliest true animals appear; typical organisms include sponges, jellyfish, corals.
• Marine life dominated oceans; most organisms were simple and filter-feeders.

THE CAMBRIAN EXPLOSION (550 MA; PALEOZOIC ERA)

• Rapid diversification of animal life; many modern animal lineages begin.
• Predator–prey interactions escalate; oceans become more dynamic and dangerous.

COLONIZATION OF LAND (480 MA; ORDOVICIAN PERIOD)

• Primitive plants, fungi, and some animals move onto land to escape marine predation.
• Land animals remain minimal; life on land still dependent on water; soil and terrestrial habitats begin to form.

AGE OF FISHES (420 MA; DEVONIAN PERIOD)

• Ocean abundant with sharks, skates, rays; advanced ray-finned fishes; predatory mollusks.
• Tetrapods begin to leave water for land; Tiktaalik as a key transitional fossil example.

AGE OF AMPHIBIANS (350 MA; CARBONIFEROUS PERIOD)

• Tetrapods diversify on land; giant insects; fungi function as decomposers.
• Decomposition declines due to fungal declines leading to the accumulation of plant matter and fossil fuels formation.

AGE OF REPTILES (250–65 MA; MESOZOIC ERA)

• Permian extinction opens the age of reptiles; Triassic, Jurassic, and Cretaceous periods.
• Dinosaurs and other reptiles dominate; gymnosperms are the dominant plants; flowering plants evolve late in the Cretaceous.
• KT extinction at the end of the Cretaceous ends the age of the dinosaurs.

AGE OF MAMMALS (65 MA TO PRESENT; CENOZOIC ERA)

• Post-KT, mammals diversify and radiate; birds diversify with flowering plants.
• Mammals and flowering plants form mutual dependencies with ecological networks.

ICE AGE GLACIATIONS (2.6 MA; PLEISTOCENE EPOCH)

• Ice ages reshape habitats and drive adaptations in mammals like woolly mammoths and sabretooth tigers.
• Glaciers extend into mid-latitude regions including areas like Pennsylvania.

FIRST HUMAN ANCESTORS (2.1 MA; PLEISTOCENE EPOCH)

• Australopithecus and Ardipithecus lineages evolve in central Africa; tool use and complex cognition emerge.
• Homo genus emerges and disperses from Africa.

THE AGE OF HUMANS (100,000 YEARS AGO TO PRESENT; ANTHROPOCENE/HOLocene)

• Homo sapiens emerge and expand globally; cranial capacity increases; jaw size and muscle strength decrease with technology and cooking.
• Development of culture, tools, and fire accelerates societal complexity.

SUMMARY OF MAJOR EXTINCTIONS & ADAPTIVE RADIATIONS

• Key events:
  • Great Oxygenation Event
  • Snowball Earth Event
  • Ordovician/Silurian extinction
  • Devonian/Carboniferous extinctions
  • Permian/Triassic extinction
  • Triassic/Jurassic extinction
  • Cretaceous/Tertiary (KT) extinction
• These events drive major changes in biodiversity and pave the way for ensuing radiations.

That’s all Folks!