The Hadean Period (4.5-3.8 bybp)

Geologic time is divided into the following time frames:

• The Cambrian (Phanerozoic) Eon represents the time during which the first complex life forms arose (the Cambrian Explosion). During the Precambrian Eon it was once thought that only simple, one-celled organisms (or loose colonial forms) existed. We now know that the first animal life arose before the Cambrian during the Vendian period (650-540 mybp). These first animal forms are known as the Vendian animals or Ediacarans (short version to study).
• During the Hadian era life was probably not possible because of extreme temperatures an massive bombardment by asteroids and comets. One Mars-sized chunk of rock hit the Earth during this era and is responsible for the formation of the moon (look here).
• By the Archean era we start seeing the first evidence of life starting with chemical fossils, followed by microfossils of cyanobacteria (including stromatolytes). The first of the bacterial forms were in the archaea domain and probably came to life near volcanic vents (see the Archaean era here). The cyanobacteria were responsible for changing the atmosphere from a reducing to oxidizing.
• The Proterozoic era is characterized by the first eukaryotes (probably an adaptation to increasing oxygen levels. During this time the ediacarans evolved.
• Who cares about the Phanerozoic era?

Hadean Links (These are all optional)

1. Hadean Movie and Plate Tectonics
2. The Hadean Eon- This is the main lecture material.
3. More on the Hadean is here
4. Oldest known terrestrial material
5. Wikipedia Reading

Louis Pasteur was responsible for ground-breaking research in medicine and chemistry, including germ-theory, immunization, and, of importance here, research on fermentation and spontaneous generation (more on Louie is here). Before this research even educated people believed that complex life forms arose spontaneously from non-living objects. Horse hair worms (nematomorphs) sprung from horse manes that had fallen into a watering trough and rats or mice developed directly from the grain in open bags in a barn. A flask of the type used to disprove spontaneous generation is depicted at the upper right. Broth was boiled in two flasks (Pasteurization). The trap of one flask was sealed to ensure that germs in the air could not gain entrance while the second flask remained open. After several weeks the open flask became cloudy with bacteria while the sealed flask remained clear, proving that spontaneous generation did not take place.

Present evolutionary biologists suggest that simple life forms were spontaneously generated about 3800 mybp (Abiogenisis) or arrived from space (Panspermia). In either case, there still would be a spontaneous generation of life (panspermia only pushes the event off-world). The explanation is that today's conditions are very different from those that existed during the Archean era. An oxygen atmosphere would destroy many of the biotic compounds that might spontaneously form today. Others would be gobbled up by existing creatures and, if any primitive life forms were to spontaneously evolve, they would not be able to compete with more advanced life forms.

In 1924 Operon hypothesized that the early earth was enveloped in a reducing atmosphere (methane (CH₄), ammonia (NH₃), hydrogen (H₂), and water (H₂O)).

In the early atmosphere energy from lightning and other sources reacted to form an organic ooze Haldane called the primordial soup.

The Miller-Urey spark gap apparatus. Simulated lightning was passed through a reducing atmosphere to form amino acids and other organic building blocks. Other than the reducing atmosphere, conditions for the evolution of early life was thought to be similar to today's (the Garden of Eden theory). Stanley Miller simulation is here!