So basically the first organisms were unicelular lifeforms that arose at the bottom of the ocean through a poorly understood process by the name of abiogenesis.
They quickly spread and began growing and diversifying, taking over the oceans of the world.
The first lifeforms to spread on land were some ocean weed plants.
Eventually some larger vertebrate lizard-looking based creature that lived near shallow bodies of water had to adapt to survive on land due to the sea levels constantly rising and falling. This creature is probably the ancestor of most (the fuck are bugs?) modern land-dwelling creatures, though the same process might have occured independently with other sea creatures.
Forms of life on the planet constantly shrunk and expanded due to mass extinctions and shit creating fossils and genetic bottlenecks we can observe but eventually some ugly ass tree apes had to adapt to living on land due to their trees dying out from climate changes and to do this they had to form groups and cooperate to forage and shit thus slowly selecting for intelligence.
These apes became homo erectus and neanderthals and those hobbit-looking motherfuckers but eventually the homo sapiens branch of them became the most dominant and either genocides or raped to death all the other ones. And thus boom modern humans.
Have I got anything wrong? I'm not a biology student but this is my understanding of how things went down.