If we compare Earth's whole life in a year that will be like this 1 Jan (midnight) Big Bang - Universe forms 15 Ma r first starts and galaxies form 1 May Milky Way galaxy forms 8 Sep Sun forms 9 Sep Solar system forms 12 Sep Earth forms 13 Sep Moon forms 20 Sep Earth's atmosphere forms 1 Oct Earliest known life on Earth 7 Oct Earliest known fossils 18 Dec First many-celled life forms 19 Dec First fish 21 Dec First land plants; first insects 23 Dec First reptiles 24 Dec First dinosaurs 26 Dec First mammals 27 Dec First birds 28 Dec First flowering plants 28 Dec Dinosaurs extinct 31 Dec (11:55 pm) Modern humans