History of Life on Earth

Earth estimated to be approximately 4.6 billion years old.
Life on earth started 3.8 billion years ago with unicellular, prokaryotic cells.
Scientists relate the history of life to:
- Increase in oxygen levels
- Climate change (e.g., ice ages)
- Geological events (e.g., continental drift)
- Fossil evidence

Early earth had low oxygen levels.
First life forms (prokaryotes) didn't need oxygen, respired anaerobically.
Photosynthetic bacteria arose $3.5$ to $2.5$ billion years ago, increasing oxygen levels.
Aerobic organisms developed as oxygen levels increased.
Increased oxygen led to a greater variety of life forms.

Ice ages are long periods of decreased temperature.
Glaciation occurs due to cooling temperatures.
Evidence of at least four ice ages.
Ice ages caused:
- Extinction of species unable to adapt.
- Migration towards the equator.
- Drier climates leading to terrestrial species extinction.
- Lower sea levels decreasing aquatic habitats.

200 million years ago, continents were fused as Pangaea.
Pangaea split into Laurasia and Gondwanaland, then further into current continents.
Continental drift caused:
- Climate changes.
- Habitat changes/destruction.
- Extinctions and adaptations.
Biogeography supports the idea of connected continents.
Closely related species on different continents suggest common ancestry.
- Flightless birds (ostrich, emu, nandu, moa) as example separated when Gondwanaland broke apart.

Fossils: complete organisms, remains, imprints, or traces preserved in rock.
Palaeontology: the study of plant and animal fossils.
Fossils provide:
- Evidence of extinct organisms.
- Information about the history of life.
- Indications of past climate and environment.
Examples of fossils:
- Bivalves and ammonites (Makhatini plains).
- Trilobites (Karoo).
- Whale fossils (Sahara desert).

Divides earth's history into geological time units.
Represents a timeline of life.
Time units are based on the age of discovered fossils.
Each unit is characterized by specific fossils.
Largest unit: aeon, divided into eras (Palaeozoic, Mesozoic, Cenozoic), then periods (Quaternary, Tertiary, Cretaceous, etc.).
Precambrian: before Palaeozoic ( $4600 - 570$ mya).

Cenozoic Era
- Quaternary Period (Present): Modern Humans, Modern Mammals, Extinction of Large Mammals, First Hominins.
- Tertiary Period (65 mya): Birds, Mammals and Insects.
Mesozoic Era
- Cretaceous Period (140-65 mya): Extinction of Dinosaurs, Flowering Plants Increase, Gymnosperms Decrease.
- Jurassic Period (190-140 mya): Dinosaurs (Land, Sea, Air), First Birds.
- Triassic Period (250-190 mya): First Dinosaurs, First Mammals, Gymnosperms Increase.
Palaeozoic Era
- Permian Period (280-250 mya): Increase in Reptiles, Decrease in Amphibians, Gymnosperms.
- Carboniferous Period (345-280 mya): First Reptiles, Increase in Amphibians, First Insects, Ferns Dominate.
- Devonian Period (400-345 mya): First Amphibians, Primitive Vascular Plants.
- Silurian Period (435-400 mya): First Plants and Animals on Land, Mosses on Land.
- Ordovician Period (515-435 mya): Algae Dominant.
- Cambrian Period (570-515): "Explosion" of most animal groups, First Vertebrates (Fish), Invertebrates.
Precambrian (4600 - 570): First Invertebrates, Origin of Eukaryotes, Prokaryotes.

Lists all discovered fossils of different ages.
Incomplete and not indicative of all past organisms.
Gaps exist, especially during transitions between organisms.
Transitional fossils are rare.
Archaeopteryx: transitional fossil between reptiles and birds from the Jurassic period.
- Reptile characteristics: teeth in sockets, fingers with claws, long bony tail.
- Bird characteristics: feathers, wishbone.

Rapid appearance of early forms of most animal groups during the Cambrian period.

Many life forms changed over time (e.g., evolution of the horse and human).
Evolution of the Human
- Genus Homo appeared in Africa about 2.2 million years ago.
- Homo habilis lived in Africa 2.2 - 1.6 million years ago. Smaller and more ape-like than Australopithecus with a larger brain capacity.
- Homo erectus lived 1.8-0.3 million years ago. First hominins to migrate from Africa.
- Homo sapiens appeared about 200 000 years ago. Large brain capacity of about $1400 cm^3$ .
Living fossils: life forms that changed very little (e.g., coelacanth, cycads, turtles).

Many species disappear over a short period.
Five major mass extinctions occurred, all with physical causes.
End of Ordovician period (435 mya): Climatic changes, possible ice age.
Late Devonian period (345 mya): Possible ice age.
End of Permian period (250 mya): Biggest mass extinction, climatic changes, volcanic eruptions.
End of Triassic period (190 mya): Uncertain cause, possible ice age, volcanic eruptions.
End of Cretaceous period (65 mya): Extinction of dinosaurs, possible meteorite collision, volcanic eruptions. Mammals increased.

Volcanic eruptions in India.
- Dust blocked sunlight, cooling the atmosphere.
- Ice sheets formed, sea levels dropped.
- Less photosynthesis, decreased O₂ levels.
- Increased CO₂, intensifying the greenhouse effect.
Meteorite collisions with earth.
- Dust blocked sunlight, leading to temperature drops.
- Worldwide fires, tsunamis, and acid rain.
- Stopped Photosynthesis and plants became extinct.
- Loss of photosynthesis led to a drastic drop in the O₂-levels
Present extinction rate is potentially the sixth mass extinction and is caused by humans.

Remains, imprints, or traces of organisms preserved in rocks.
Most fossils are found in sedimentary rocks.
Also found in tree resin (amber), tar pits, ice, volcanic lava, anaerobic swamps.
Fossilisation: inorganic processes transforming dead organisms into fossils.
Ideal conditions for fossilisation
- Organism covered immediately after death for anaerobic conditions.
- Harder parts simplify fossilisation.
Formation in sedimentary rocks:
- Organism dies and is covered by sediment.
- Soft tissues decay.
- Hard parts remain and are replaced by minerals.
- More sediment layers cover it.
- Sediment solidifies into sedimentary rocks.
Body fossils: preserved remains (complete organisms, parts) - Mammoths in the Arctic ice in Siberia, An insect in amber
Moulds: imprint after decomposition.
Cast fossils: mould filled with minerals.
Trace fossils: Signs or marks of organisms that were once living; footprints and burrows.
Fossil Dating:
- Radiometric dating: uses radioactive isotopes (uranium, potassium, carbon).
- Relative dating.