Well, we know that animals did evolve on this planet, there are also ALOT of different kinds of them...
What Peter Ward said:
"It has always been assumed that attaining the evolutionary grade we call animals would be the final and decisive step. Once we are at this level of evolution, a long and continuous progression toward intelligence should occur. However, recent research shows that while attaining the stage of animal life is one thing, maintaining that level is quite another. The geologic record has shown that once evolved, complex life is subject to an unending succession of planetary disasters, creating what are known as mass extinction events. These rare but devastating events can reset the evolutionary timetable and destroy complex life while sparing simpler life forms. Such discoveries suggest that the conditions allowing the rise and existence of complex life are far more rigorous than are those for life's formation. On some planets, then, life might arise and animals eventually evolve - only to be soon destroyed by a global catastrophe."
So it seems like evolution of animals is not really that
difficult to attain but its harder to maintain...also I think these "rare and devastating events" only sped up our ancestors evolution to intelligence, maybe it might do the same for another species on another planet?