1. At what points during this timeline could extraterrestrial astronomers, observing Earth from distances of many thousands or millions of light years, demonstrate that life exists on Earth? How?
  2. At what points during this timeline could extraterrestrial astronomers, observing Earth from distances of many thousands or millions of light years, demonstrate that 'intelligent' life exists on Earth? How?

 
Years Before Present Event
4.56 Billion Formation of Sun, Earth & Solar System/Oldest meteorites
4.3 Billion Oldest identified mineral grains on Earth
4.0 Billion No more running water on Mars
4.0 Billion End of Early Bombardment
3.9 Billion Oldest known rocks on Earth; rich in uraninite (UO2), which forms in low O environments ( O2 at less than 1 millionth of present atmospheric oxygen levels); marginally "reducing" atmosphere & ocean
3.8 Billion First evidence of organically processed material on Earth
3.6 Billion Age of "fossil life" in ALH84001 (Martian meteorite)
3.5 Billion oldest fossil evidence of life on Earth: Stromatolites (oldest fossils); multicellular, prokaryotic, anaerobic, photosynthetic "cyanobacteria" or blue-green bacteria: ADVANCED! 
 beginning of oxygen productionplot of oxygen production
2.7 Billion oldest Banded Iron Formations
2.2 - 1.8 Billion explosion in production rate of BIFs (O2 reaches 1/100th present level - level required for ozone formation)
2.0 - 1.0 Billion formation of Redbeds
1.8 Billion First fossil evidence of Eukaryotes (cells with nuclei); require O2  above 1/100th
1.5 Billion  O2  and O3  build up in atmosphere: life at ocean surface; aerobic atmosphere & ocean 
1.3 Billion formation of SNC meteorite parent rocks on Mars
1.2 Billion sexual reproduction; diversity
1.0 Billion Venus greenhouse runs away; oxygen reaches present levels in Earth atmosphere 
700 million era of jellyfish & other soft bodied organisms
600 million Cambrian era: trilobites (among first multicelled organisms)
400 million  Devonian extinction: worst ever.  Dawn of era of trees & grasses
200 million Age of Dinosaurs begins; Breakup of Pangaea begins
150 million First flowers appear (quick growing food allows appearance of large dinosaurs); ammonites in ocean
65 million K-T extinction (end of ammonites); Era of mammals begins
3 million First apelike hominids walk upright
1 million First tools used by early humans
4,000  pyramids; first great cities; agriculture; animal farming
200 industrial revolution begins; CO2  begins slow buildup in atmosphere
*1889*  radio invented
*1945* atomic age begins
*1949* first interstellar signals sent by Earth (early TV)
*1959* first spaceflight; human induced Global Warming begins
*1969* Apollo moon landing
*1976* Viking landing on Mars
*1982* First hole in ozone layer
*1987* First spacecraft journeys beyond Pluto
Stromatolites and Cyanobacteria
  • Life, Life Everywhere, from Scientific American
  • Banded Iron Formations
  • Evolution of Oxygen in Earth's Atmosphere
  • Early Earth History - Precambrian Evolution of the Atmosphere and Oceans