Life in the early earth

Precambrian
* first million years - no life - the atmosphere formed; ocean formed
* first cells appear (monerans) about 3,500 million years ago - they release oxygen
        as a byproduct
*  2,500 million years ago - we had oxygen
*  1,900 million years ago complex cells evolve
*  1,000 million years ago - many celled organisms evolve
*  545 million years ago numerous organisms live in the ocean
*  fungi and plants evolve - very few fossils

Paleozoic
*  animals with hard shells appear
*  animals are invertebrates at first
*  vertebrates appear
*  later, organisms leave the oceans and live on land (similar to millipedes)
*  amphibians evolve next
*  land - swamp forests with seedless fernlike plants
*  reptiles appear
*  mass extinctions happen - probably because Pangaea came together at this time
*  end of the Paleozoic Era - 245 million years ago

Mezozoic Era
* some reptiles evolve into dinosaurs
* next 160 million years - dinosaurs rule
* first mammals evolve- small, active at night
* birds evolve a little later
* flowering plants
* continents of Pangaea break apart
* 66 million years ago - mass extinctions again - probably due to meteor impact

Cenozoic Era
* We live in this now
* Mammals continued to evolve - maybe because the dinosaurs were gone
* Humans only about the last 2 million years