Prokaryotes, the first single cells, basically did nothing more than eat and reproduce.
A prokaryote ate by allowing the passage of nutrients, substances used for food, from the water through its membrane wall. It released byproducts back through the membrane. It reproduced by a method called mitosis.
In mitosis, the DNA splits apart and the cell membrane folds in on the middle of the cell, cutting the one cell into two cells with identical DNA codes.