The Proterozoic Eon, which began some 2.5 billion years ago and ended about 542 million years ago, was traditionally regarded as the last era before the evolution of what we would recognise as animals. But more recent fossil discoveries have repainted this picture, and it now appears that animals roughly midway through the Ediacaran Period.
The earliest animals were very simple creatures- small invertebrate multi-cellular forms like the earliest worms and such. Arthropods evolved shortly thereafter, and would go on to become the most dominant type of animal on the planet – just think how many insects and spiders and such you see each day if you don’t believe that.
As mentioned in:
The River of Dreams — Billy Joel