Charles Darwin’s Theory of Evolution holds that all living things have descended from a common ancestor. If this true, we're not only related to monkeys, but also to fish, flowers, and trees.
Before the evolutionary process could start, there must have been a single living cell that sprang into existence from non-life, sustained itself, and reproduced. This life then evolved from simple to more complex creatures, some of which became male and female. This process was facilitated by random genetic mutations, with beneficial mutations remaining because they aided survival (natural selection) and, over time, caused transformation into entirely different creatures.