Evolutionary History of the Earth

This is a simple synopsis of the major events in the history of life, focusing on the history of animals. This assumes little prerequisite knowledge except for the Geologic Time Scale, around which it is organized.

Hadean Eon - 4.56 - 4.03 Ga (Note Ga = billion years ago).

  • Formation of Earth through accretion of planetesimals.
  • Accumulated heat of bombardment initially melts Earth's surface.
  • Differentiation of core, mantle, crust.
  • Formation of Moon from debris torn off of Earth by impact with Mars-sized impactor.
  • Early atmosphere mostly CO2, so oceans highly acidic.
  • No life.

    Archean Eon - 4.03 - 2.5 ga.

  • 3.8 Ga: Oldest clastic sedimentary rocks in a continental environment. Liquid water was flowing!
  • 3.7 Ga: Oldest strong evidence of life is fractionated carbon in deep sea sediments preserved in Greenland.
  • 3.4 Ga: Oldest strong body fossils - Fig Tree Cherts

  • Photosynthesis: For a while, organisms got away with eating the organic materials that were floating around in the ocean. As these started to get scarce, one group, the cyanobacteria, came up with a new method of capturing energy from the environment - Photosynthesis,

    6 CO2 + 6 H2O + energy (sunlight)---> C6H12O6+ 6 O2

    Note that oxygen is a product.

    We can't tell from looking at microscopic fossils which were photosynthesizers, but photosynthesis had momentous consequences for the rock record. As oxygen is released into ocean water, it quickly reacts with ions such as iron, that act as oxygen sinks. ~3.0 Ga: Banded Iron formations (BIFs), marine sedimentary rocks with alternation of gray and rust red bands of hematite (Fe2O3) begin to appear, indicating that photosynthesizers were at work.

  • The age of slime: At roughly the same time we pick up Stromatolites, laminated bacterial mats. These were very common for until the end of the Proterozoic, when critters appeared that could eat them. Occur today only in unusual environments that protect them from grazers, like hypersaline Shark Bay in Australia.

    Proterozoic Eon - 2.5 - 0.542 Ga.

  • 1.8 Ga: Terrestrial red bed deposits begin to appear, indicating that the oceanic oxygen-sinks had become saturated and free oxygen was now building up in the atmosphere and reacting with terrestrial sediments. Confirming this, BIFs quit forming in the oceans.
  • Ozone: As it accumulated, free oxygen in upper atmosphere recombined to form ozone layer (O3), allowing life to colonize surface waters.
  • 2.7 Ga: Eukaryotes appear, probably formed by endosymbiosis (prokaryote-grade organisms joining together to form a more complex cell). Oldest biochemical markers of eukaryotes - steranes trapped in sediments.
  • 2.2 Ga: Oldest eukaryotic body fossils.
  • 1.7 Ga: Acritarchs, the resting stage of some eukaryotic organism. Common and diverse microfossils, reaching peak diversity around 850 Ma (million years ago) and lasting into the Phanerozoic.
  • 830 - 600 Ma: Snowball earth episode - a series of drastic ice ages (the most severe on record). Although controversial, some evidence indicates glacial and oceanic pack ice at the equator. This seems to have exerted a strong selective pressure, because soon afterward, we witness the independent derivation of multicellularity in:

    We now switch to a smaller time scale

    Ediacaran Period - 630 - 542 Ma. - The last period of the Proterozoic

  • Some controversial trace fossils from c. 800 Ma (some date to 1100 Ma, others think only 570 Ma)
  • Definite animal-generated trace fossils by 570 Ma

    The Ediacaran fauna, preserved only as impressions in sediment, records the first animal fauna:

    Cambrian and Ordovician Periods 542 Ma. - Beginning the Paleozoic Era of the Phanerozoic Eon with explosive diversification

    Cambrian 542 - 488.3 Ma

    Ordovician 488.3 - 443.7 Ma