09 History of Earth and Life on Earth 2021 (2)

Lecture 9: History of Life on Earth

Formation of Earth

• Timeframe: 4.56 billion years ago (bya)

• Key geological events:

  • Core Formation

  • Moon Formation

• Key locations for early evidence:

  • Jack Hills Zircon (oldest known material)

  • Isua: Evidence of early sediment and isotopes indicating life (3.7-3.5 bya)

  • Acasta Gneiss (4.0 billion years old)

  • Apex Chert fossils (3.5 billion years ago)

Geological Time Scale

• Eons:

  • Hadean (4.6 - 4.0 bya)

  • Archean (4.0 - 2.5 bya)

  • Proterozoic (2.5 - 0.54 bya)

  • Phanerozoic (0.54 bya - present lasting over 500 million years)

Evolutionary Milestones

• Rise in Atmospheric Oxygen: 2.5 billion years ago—Great Oxygenation Event where phototrophic bacteria released oxygen.

• Development of Hard-shelled Animals: Mesozoic era marks the emergence of complex land structures.

Expansion of the Universe

• Current Expansion Rate: 73.5 ±1.4 kilometers/second/Mpc.

• Inflation of Space: Started at 13.8 billion years ago till present, showing a consistent expansion of galaxies and clusters.

The Formation and Evolution of Stars and Planets

Cosmic Timeline

• Big Bang: Approx. 13.7 billion years ago.

• Key Events:

  • Formation of elementary particles (quarks, protons, neutrons)

  • Formation of atoms and the first stars

  • Creation of galaxies and clusters

Nebulae: Birthplaces of Stars

• Characteristics:

  • Composed of clouds of gas and dust

  • The process of star formation involves clumps forming, leading to:

    1. Dense cores that are precursors to stars.

    2. Young stars surrounded by dusty disks leading to planet formation.

Accretion and Planet Formation

• The process by which planets form through gravitational attraction of dust and debris around young stars.

• Example: Formation of our Sun (4.6 bya) and Earth (4.5 bya).

Early Earth History

• Hadean Era: Cooling process of Earth, formation of liquid water by 4.1 bya allowing support for anaerobic life.

• Archean Eon: (4.0-2.5 bya) characterized by volcanic activity, continent formation, and emergence of first prokaryotes.

Major Eons in Earth's History

Archean (4,600-2,500 mya)

• Features:

  • Continents begin to form

  • Ocean formation

  • Formation of methane-rich atmosphere

  • First life, prokaryotes, emerge as chemoheterotrophs.

Proterozoic (2,500-543 mya)

• Significant events:

  • Rise of first photoautotrophs and eukaryotic cells

  • Multicellular organisms appear towards the end of this eon.

Phanerozoic (543 mya-present)

• Periods:

  • Paleozoic (543-251 mya): Dominance of marine life, rise of terrestrial plants.

  • Mesozoic (251-65 mya): The age of reptiles and flowering plants.

  • Cenozoic (65 mya-present): Diversification of mammals and birds; evolution of diverse life forms.

Key Events in Evolution

• First Complex Animals: Emergence around 550 mya.

• Land Plants: Flora appears around 430 mya.

• Tetrapods (360 mya): Transition from aquatic to terrestrial life.