Geologic Time Scale: Eons

• The Geologic time scale represents life forms that have dominated Earth on a specific time scale.
• Geologic combines the words "geologic" and "chronologic", referring to rocks and time.
• The time scale is developed by studying rock layers and fossils worldwide.
• The divisions of the geologic time scale are Eon, Era, Period, and Epoch.

Eons

• There are four Eons: Hadean, Archean, Proterozoic, and Phanerozoic.

Hadean Eon

• The Hadean Eon spanned from 4.6 to 4.0 billion years ago.
• It is characterized by extreme conditions, making life unlikely.
• It represents Earth's earliest formation, with intense heat, volcanic activity, and asteroid/comet bombardment.

Archean Eon

• The Archean Eon spanned from 4.0 to 2.5 billion years ago.
• This was a pivotal period that marked the emergence of life and significant geological changes.

Major Groups of Organisms in the Archean Eon

• Bacteria: Early forms that metabolized sulfur and other elements without oxygen.
• Archaea: Another domain of prokaryotic life that coexisted with bacteria.
• Cyanobacteria: It emerged mid-to-late Archean and importantly produced free oxygen through photosynthesis.

Proterozoic Eon

• The Proterozoic Eon is a geological period spanning from 2.5 billion to 541 million years ago.
• It translates to "earlier life" and marks the longest eon in Earth's history.

Major Groups of Organisms in the Proterozoic Eon

• Eukaryotes: Organisms with complex cells containing nuclei, foundational for multicellular life.
• Prokaryotes: Single-celled organisms lacking a nucleus and membrane-bound organelles, were the dominant life forms for much of the Proterozoic.

Phanerozoic Eon

• The Phanerozoic Eon spans from approximately 541 million years ago to the present.
• It is characterized by an explosion of visible life forms.
• Subdivided into three eras: Paleozoic, Mesozoic, and Cenozoic.

Eras

• The three major eras are Paleozoic, Mesozoic, and Cenozoic.

Paleozoic Era

• The Paleozoic Era, also known as the "Age of Invertebrates" or "ancient life", lasted from 538.8 to 252 million years ago.

Major Groups of Organisms in the Paleozoic Era

• Trilobites: Diverse, ancient marine arthropods from the Cambrian to Permian Periods with hard, segmented exoskeletons and were among the first complex animals on Earth.
• Anomalocaris: Ancient, large predatory marine arthropods from the Cambrian Period. They had segmented bodies, grasping appendages, and frontal appendages resembling swimming paddles who preyed on other marine animals.
• Hallucigenia: This was a worm-like marine animal with a slender body, spines/spikes along its back, and appendages pointing in different directions whose strange appearance led to confusion about its orientation in early reconstructions.
• Brachiopods: Shell-bearing marine animals resembling clams, with two asymmetrical shells (valves) enclosing their body and a fleshy stalk anchored to the seafloor filtered plankton for food in ancient oceans worldwide.
• Early fish: Appeared during the Silurian Period and included some of the earliest jawless fish with simple, bony plates covering their bodies for protection.

Periods in the Paleozoic Era

• Cambrian Period (541-485.4 million years ago): Rapid diversification of life, known as the Cambrian Explosion. Trilobites, brachiopods, and early vertebrates evolved.
• Ordovician Period (485-444 million years ago): Marine life continued to diversify.
• Silurian Period (443-419 million years ago): The first jawed fish evolved.
• Devonian Period (419-358 million years ago): The "Age of Fishes," characterized by a wide variety of fish species, and the first amphibians and insects evolved.
• Carboniferous Period (358-298 million years ago): Vast forests of ferns and other primitive plants formed, and the first reptiles evolved.
• Permian Period (298-251 million years ago): Reptiles diversified, and the Permian-Triassic extinction event, the largest mass extinction in Earth's history occurred.

Mesozoic Era

• Known as the "Age of Reptiles" or "middle life," lasted from 252 to 66 million years ago.

Major Groups of Organisms in the Mesozoic Era

• Dinosaurs: Dominant terrestrial vertebrates displaying incredible diversity in size, shape, and diet, with adaptations for bipedalism, quadrupedalism, and flight (in the case of birds).
• Pterosaurs: Flying reptiles that dominated the Mesozoic skies, possessing large wings formed by skin stretched between the body and elongated fourth finger.
• Ferns: First plants to evolve on land, having frond-like leaves, and reproduce using spore-producing structures.
• Gymnosperms: Plants with a central stem. Some of the earliest plants to evolve produced cone-like structures on their branches and contained seeds.
• Cycads: These are tree-like plants with a single trunk and a whorl of leaves at the top. These were abundant during the Mesozoic era and are now rare.

Periods in the Mesozoic Era

• Triassic Period (252 to 201 million years ago): The first period of the Mesozoic Era: marked by the rise of dinosaurs.
• Jurassic Period (201 to 145 million years ago): Dominated by large dinosaurs like Brachiosaurus and Stegosaurus.
• Cretaceous Period (145 to 66 million years ago): Diversification of dinosaurs, the appearance of the first birds, and a mass extinction event that wiped out most dinosaurs at the end of the era.

Cenozoic Era

• The current geological era began 65 million years ago and continues to the present day.
• It's known as the "Age of Mammals" or "recent life."

Major Groups of Organisms in the Cenozoic Era

• Mammals: Dominant terrestrial vertebrates characterized by mammary glands (milk production), hair or fur, and three middle ear bones.
• Angiosperms: Dominant plant group characterized by producing flowers and seeds enclosed within fruits, enhancing seed dispersal and reproduction.
• Humans: The Cenozoic Era is also marked by the evolution of humans (genus Homo). The emergence of Homo sapiens and the development of human civilization are significant events in the later part.

Periods in the Cenozoic Era

• Paleogene Period (66 to 23 million years ago): Marked by the rapid diversification of mammals.
• Neogene Period (23 to 2.6 million years ago): Characterized by the evolution of grasslands and the rise of grazing mammals.
• Quaternary Period (2.6 million years ago to present): The current period is known for the Pleistocene ice ages and the appearance of modern humans.

Epochs

• Paleogene Period: Paleocene, Eocene, Oligocene.
• Neogene Period: Miocene, Pliocene.
• Quaternary Period: Pleistocene, Holocene.