The first occurrence of simple animals was approximately 600 million years ago. These were characterized as multi-cellular organisms with basic structures, but no true tissues. An example would be a jellyfish or marine sponge.
Multi-cellular organisms originated approximately 1 billion years ago, but lacked distinct structures that differentiated them as simple animal life. Complex animals began evolving approximately 550 million years ago, followed by the first life making its way onto land approximately 50 million years later. Life evolved at a steady pace on the Earth, developing a great deal of diversity as new environments and niches became populated by new forms of life. Life forms gradually became more complex as time progressed.
Single-celled organisms predate all of the above, having first developed approximately 3.5 billion years ago, after evolving from simple self-replicating molecules. This process of simple life becoming increasingly complex over time, through adaptation to its environment, has produced a great deal of bio-diversity and many complex creatures. In addition, the same process has allowed humanity to adapt and improve itself over time, becoming capable of manipulating the environment to suit their needs rather than adapting to it. In theory, evolution persists today, with many animals changing along with their environments or developing new adaptations which were not previously documented.