Earth's Timeline and Early Life
Earth's 4.6 billion-year history is organized into different time periods based on rock layers and fossil evidence. The Precambrian makes up a whopping 80% of Earth's history and contains three major eons: Hadean, Archean, and Proterozoic.
The Hadean eon (meaning "hell-like") was Earth's fiery beginning with planetary formation, massive volcanoes, and a dim sun. No fossils exist from this tumultuous time when Earth's surface was mostly molten. During the Archean eon, Earth's surface finally cooled enough to form solid rock and the first oceans appeared. The oldest rocks we've found date to this period, along with potentially the first simple bacteria about 3 billion years ago.
The Proterozoic eon brought major changes to our planet. Bacteria began producing oxygen, transforming the atmosphere. The first eukaryotic cells (complex cells with nuclei) appeared, along with algae and simple invertebrates. During this time, Earth experienced extreme ice ages sometimes called "Snowball Earth."
Fun Fact: The oxygen that allows us to breathe today was actually "pollution" created by ancient bacteria! This oxygen was toxic to many early life forms but eventually enabled the evolution of more complex organisms.
