page_title: Prehistory of the United States
section_title: Precambrian
hierarchical_section_title: Prehistory of the United States / Precambrian
caption_attribution_description: Estructura cellular dels cianobacteris Català a. Membrana exterior b. Capa de peptidoglicà c. Membrana plasmàtica d. Citoplasma e. Grànul de cianoficina f. Ribosoma g. Grànul de glicogen h. Cos lipídic i. Carboxisoma j. Ficobilisoma k. Grànul de polifosfat l. Vacúol gasífer m. Tilacoide n. Nucleoplasma Estructura celular de las cianobacterias Español a. Membrana celular b. Capa de peptidoglucanos c. Membrana plasmàtica d. Citoplasma e. Gránulo de cianoficina f. Ribosoma g. Gránulo de glicógeno h. Cuerpos lipídicos i. Carboxisoma j. Ficobilisoma k. Gránulo de polifosfat l. Vacuola gasífera m. Tilacoide n. Nucleoplasma
context_page_description: The prehistory of the United States comprises the occurrences within regions now part of the United States of America during the interval of time spanning from the formation of the Earth to the documentation of local history in written form. At the start of the Paleozoic era, what is now "North" America was actually in the southern hemisphere. Marine life flourished in the country's many seas, although terrestrial life had not yet evolved. During the latter part of the Paleozoic, seas were largely replaced by swamps home to amphibians and early reptiles. When the continents had assembled into Pangaea drier conditions prevailed. The evolutionary precursors to mammals dominated the country until a mass extinction event ended their reign.
  The Triassic, first period of the Mesozoic era followed. Dinosaurs evolved and began their rise to dominance, quickly spreading into the United States. Soon Pangaea began to split up and North America began drifting north and westward. During the latter Jurassic, the floodplains of the western states were home to dinosaurs like Allosaurus, Apatosaurus, and Stegosaurus.
context_section_description: The oldest known eukaryotes lived around 1.4 billion years ago during the Precambrian near Beck Spring, California. These were photosynthetic organisms and produced oxygen as a byproduct of their physiology. The Beck Spring eukaryotes and other Precambrian photosynthesizers released the oxygen that now makes the planet's atmosphere breathable. Precambrian Blue-green algae remains from this age were preserved between Copper Harbor and Eagle Harbor on the shoreline of Lake Superior in the Upper Peninsula of Michigan. The appearance of the first eukaryotic cells in the fossil record were relatively followed by evidence of complex multicellular life. Rocks in Nevada dating back to a billion years ago preserve trace fossils left behind by worms as they burrowed below the sediment. Other complex Precambrian lifeforms were preserved in North Carolina and Arizona.
caption: Blue-green alga
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