Lets say we have a bundle of cells, it is the extent of life on the planet.
It was created an evolutionary defence mechanisim where by instead of normal mitosis where the cell splits in two and they go there seperate ways, this cell splits in two and the new cell stays attached to the parent because whateve enviroment it is in and whatever threats it faces, this is the best way to survive.
These cells work together the survive and after a few million years with enviromental changes included, the cells have evolved to be a bigger bundle, eventualy over millions of years this bundle has to change to survive and cells on the outside of the bundle have to move the bundle around on its search for energy and cells on the inside are needed for other jobs.
Eventualy you will have the very basic sea creatures, plankton->shrimps->small crustaceans
and then things start to get interesting, cells are developed for hunting/defence, brains are developed, starting as a few cells to keep other cells working and this develops into the brain keeping the organs working.
After that point, the big development that I dont get is self awareness, I can understand why a dog looks in the mirror and sees another dog and not itself, and i understand how we can look in the mirror and not see a different person, but i dont understand how we went from one to the other.