Earth's History and Geological Timeline

• Humans exist in the most recent chapter of Earth's history.
• Earth formed 4.6 billion years ago when the sun's gravity pulled planets into orbit.

Geological Timeline

• The geological timeline mirrors the history of life on Earth, excluding the Hadean Eon.
• It encompasses geological events alongside the life forms present.
• The history is divided into eons, eras, periods, epochs, and ages, from largest to smallest time spans.
• Understanding the timeline requires knowing how scientists date rocks and fossils.

Dating Rocks and Fossils

• Scientists use geological and fossil records to determine the age of the planet and its life.
• Radioactive dating measures element decay rate for accurate dating of rocks and fossils.
• Carbon-14 is used for fossils because all living things are carbon-based.
• Rubidium-87 and Potassium-40 are often used for geological objects.
• Half-life is the time for half a substance to radioactively decay; it can be used to calculate an object's creation date.
• Relative dating determines age by rock and fossil depth; deeper layers are older.
• Geological events help determine what life existed at a specific time and place correlating the geological timeline and life's history.

Hadean Eon

• The Hadean eon, the first in the Precambrian Supereon, spanned from 4.6 to 4 billion years ago.
• There was no life on Earth during this time, so it is excluded from the history of life.
• Early Earth was volcanic, with lava flows and a toxic atmosphere of hydrogen, helium, ammonia, and methane.
• Earth collided with a sister planet, which sheared off one-third of the mass of both spheres, and formed Earth and the Moon.

Archean Eon

• The Archean Era followed the Hadean, from 4 to 2.5 billion years ago.
• The atmosphere was toxic, and Earth was hotter than today.
• Life began as water-dwelling prokaryotic organisms about 3.5 billion years ago.
• Prokaryotes lacked a distinct nucleus or organelles.
• Some prokaryotes could photosynthesize, forming stromatolites.
• Cyanobacteria, a type of mat-forming organism, produced oxygen, which changed Earth's atmosphere and allowed multicellular life to flourish.

Proterozoic Eon

• The Proterozoic eon lasted from 2.5 billion to 541 million years ago.
• The atmosphere became oxygen-rich, which led to eukaryotic life evolving from prokaryotes around 2 billion years ago.
• Eukaryotic cells have a distinct nucleus and organelles.
• Earth experienced two ice ages during this eon.
• A diverse group of organisms existed by the end of the Proterozoic eon.

Paleozoic Era

• The Paleozoic era spanned from 541 to 252 million years ago as the first era in the Phanerozoic Eon.
• It includes 6 periods: Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian.
• The Cambrian period began with the Cambrian Explosion.
• The oxygen-rich atmosphere allowed for the diversification of life and multicellular organisms dominated the oceans.
• The Ordovician period saw the evolution of land plants.
• Land vertebrates evolved in the Silurian period.
• Trilobites were common arthropods, used to determine the relative age of geographical features.
• Ancient ferns, mosses, early trees, vertebrates, and wingless insects covered the landscape during the Devonian period
• The Devonian period is also known as the age of fish due to fish evolution.
• The Carboniferous period saw tectonic plates form Pangea and ice caps at the poles, which created terrestrial land.
• Early coniferous trees, winged insects, and land reptiles existed in the Carboniferous period.
• The Permian period marked Pangaea's formation, which changed climates; deserts formed in the supercontinent's center.
• The Paleozoic era ended with a mass extinction event due to volcanic eruptions, meteor strikes, and methane release, resulting in 96% of life going extinct.

Mesozoic Era

• The Mesozoic era lasted from 252 to 65.5 million years ago.
• It includes the Triassic, Jurassic, and Cretaceous periods.
• Dinosaurs evolved, starting in the Triassic and diversifying in the Jurassic period.
• Plant life evolved from ancient fern-like plants to conifers and flowering plants (angiosperms).
• The second mass extinction, caused by a meteor impact 66 million years ago, wiped out 80% of life, with the exception of bird ancestors.

Cenozoic Era

• The Cenozoic era began 65.5 million years ago and continues to the present day.
• This is the era of mammals, but also other organisms saw much diversification, such as flowering plants, birds, insects, and fish.
• It includes the Paleogene, Neogene, and Quaternary periods.
• The Paleogene period saw plant and animal diversification, including penguins, horses, primates, palms, grasses, and other flowering plants
• Global warming and ice cap melting occurred, but grasslands developed towards the end of the period.
• The Neogene period involved mountain range formation due to tectonic plate movement as Pangea separated.
• Many modern plant and animal families appeared and a land bridge formed between Russia and Alaska, allowing animal migration.
• Early hominids evolved and separated into distinct groups in Africa during the Neogene period.
• The Quaternary period includes cycles of warming and cooling, with ice ages followed by interglacial periods.
• Modern homo sapiens evolved and migrated during this time.

The Anthropocene

• The Anthropocene is an unofficial epoch that marks the period when humans have had a lasting impact on the planet.
• Some scientists argue that humans are still in the Holocene epoch.
• The Anthropocene is supported by evidence of human impact, including atmospheric changes from greenhouse gas emissions, radioactive material from atomic bombs, and mass extinction.

Explore Earth's 4.6 billion-year history and the geological timeline. Learn about the eons, eras, and periods that mark significant events and life forms. Discover how scientists use radioactive dating methods like Carbon-14, Rubidium-87, and Potassium-40 to accurately date rocks and fossils.