History of Life and Geologic Time Scale

The History of Life

• Earth formed approximately 4.6 billion years ago.
• The Hadean Eon (4600-3850 Ma) was a period from the origin of the solar system to approximately 3850 Ma.
• The Hadean Eon had almost no geologic record.
• Key events of the Hadean Eon include: differentiation of earth into crust, mantle & core; origin of the atmosphere; condensation of water; & origin of continental crust.

Geologic Time Scale

• The Phanerozoic Eon is broken down into the Cenozoic, Mesozoic, and Paleozoic Eras, each with periods and epochs.
• The Cenozoic Era includes the Quaternary Period (Holocene & Pleistocene Epochs), Neogene Period (Miocene & Oligocene Epochs), and Paleogene Period (Eocene & Paleocene Epochs).
• The Mesozoic Era includes the Cretaceous, Jurassic, and Triassic Periods.
• The Paleozoic Era includes the Carboniferous, Devonian, Silurian, Ordovician, and Cambrian Periods.
• The Precambrian includes the Proterozoic and Archean Eons and Hadean(earlier).

The Archean (3850-2500 Ma)

• Origin of life (approximately 3800 Ma).
• Early organisms used methane, sulfate, and hydrogen sulfide based metabolisms, producing CO2 and alcohol as byproducts.
• Photosynthetic organisms appeared approximately 3500 Ma.
• An oxygen environment developed between 3,500 and 2,800 Ma.

What is "Life"?

• Attributes of life include autonomous replication (mitosis, meiosis); critical levels of complexity (simpler subunits into multiple complex combinations); and the ability to evolve via natural selection.
• Requirements of life include an energy source, basic chemicals (nucleic acids, proteins, minerals), and an external environment that sustains life.

The Proterozoic (2500-542 Ma)

• Origin of eukaryotes (1500 Ma).
• Rapid diversification of soft-bodied multicellular animals and green algae (1500-600 Ma).
• Eukaryotic cells have a nucleus, DNA storage, RNA transcription, and membrane-bound organelles.

Serial Endosymbiosis Theory (SET)

• Eukaryotic cells evolved when aerobic bacteria infected or were engulfed by a larger host cell and later established a symbiotic relationship.
• Mitochondria are thought to be derived from purple bacteria.
• Chloroplasts are thought to be derived from cyanobacteria.

Cambrian Explosion

• The Cambrian explosion (approximately 542-530 million years ago) saw a relatively sudden appearance of diverse animal forms in the fossil record.
• Explanations include intrinsic changes in animal development and extrinsic changes in the environment.

Challenges of Multicellularity

• Increasing size necessitates greater support, rigidity, and more complex reproduction.
• Surface to volume ratio (S/V) decreases with size leading to exchange limitations (gasses, nutrients). Multicellular organisms develop ways to deal with these limitations using larger surface areas such as in the circulatory system and lungs, or by adjusting the ratio utilizing shape.