The planet we all know today as Earth started out as just an explosion in outer space. The Big Bang Theory is a theory that supports this idea. Even though this is only a theory, there is much rock, fossil, and glacial evidence that seem to prove this theory true.
Later, the atmosphere was formed. When the ozone layer was later formed, this opened up room for gases such as carbon dioxide and oxygen to exist. This made it possible for photosynthesis to occur, creating the start of all life on Earth including cyanobacteria.
Soon there were many new species on Earth. We keep track of them by keeping a fossil record, and we keep track of how long ago the species was created by keeping a geologic time scale.
As time went on, the supercontinent Pangaea started breaking apart into multiple new, smaller continents that we live on today. The theory of plate tectonics is a theory that supports the idea that the Earth's crust is made up of several tectonic plates that move under the Earth's surface, causing plate boundaries to occur and land masses to move and break apart.
Thanks to igneous, metamorphic, and sedimentary rock types we can discover so much about the life on earth from the beginning. The law of superposition states that younger rock is laid on top of newer rock, which helps scientists today find the age of certain species or fossils.