The Story of Life on Earth

Oct 8, 2024

Geologic Time and the History of Life on Earth

Introduction

  • Life on Earth has been evolving for about four billion years.
  • Humans are a very recent development in this history.
  • The concept of time in billions of years is difficult to comprehend.
  • Geologists use rock layers to understand life's history, including evolutionary bursts and extinctions.
  • The Geologic Time Scale (GTS) helps organize these events.

Development of Geologic Time Understanding

  • Nicholas Steno (1669): Developed the first laws of stratigraphy, suggesting layers closer to the surface are younger.
  • Giovanni Arduino (1760s): Named rock layers: Primary (metamorphic/volcanic), Secondary (sedimentary), Tertiary, and Quaternary.
  • William Smith (1819): Used fossils to match the ages of rocks across different locations.

Geologic Time Scale (GTS)

  • Organized into subgroups: Eons, Eras, Periods, Epochs, and Ages.
  • Allows for asking different scale questions about Earth's history.

Eons and Eras Overview

Hadean Eon (4.6 - 4 billion years ago)

  • Earth formation period, characterized by extreme conditions.
  • No fossils due to inhospitable environment; possible early life signs through organic carbon.

Archean Eon (4 - 2.5 billion years ago)

  • First flourishing of life; microbial mats and stromatolites developed.
  • Atmosphere mostly CO2; cyanobacteria emerge.

Proterozoic Eon (2.5 billion - 541 million years ago)

  • Oxygen appears in the atmosphere due to photosynthetic bacteria.
  • Development of eukaryotes and complex life forms.

Phanerozoic Eon (541 million years ago - present)

  • Known for "visible life"; contains three major eras.

Paleozoic Era (541 - 252 million years ago)

  • Cambrian Explosion: Diversity in oceans; development of hard parts in animals.
  • First land ecosystems with plants and arthropods; early vertebrates evolve.
  • Ends with the "Great Dying," a massive extinction event.

Mesozoic Era (252 - 66 million years ago)

  • "Age of Reptiles," development of dinosaurs and other reptiles.
  • K-Pg Extinction Event ends this era, likely due to an asteroid impact.

Cenozoic Era (66 million years ago - present)

  • Diversity of mammals and modern flora.
  • Development of grasslands and new species like primates.
  • Human evolution and the shaping of the Earth.

Conclusion

  • Humans are significant contributors to Earth's geological story.
  • Our era may be defined by the impact we've had on the Earth.

Final Thoughts

  • The story of life on Earth is vast and complex.
  • We play an ongoing role in this story and have the opportunity to shape the future.
  • Encourage curiosity and continued learning about Earth's history and geology.
  • Engage with educational content on platforms like PBS Digital Studios.