During the time before the Cambrian period (known as the Precambrian), evolutionary and geologic development was very slow.
It took millions and billions of years for any changes to happen. With the Cambrian period, however, life forms evolved at a relatively rapid rate and have not stopped since.
Imagine, between 4.2 and 3.5 billion years ago, life originated on Earth.
It took more than 3 billion years to get from these earliest life forms to those of the Cambrian period, but only 0.5 billion years to get from Cambrian life forms—such as trilobites—to human beings.