They were the commonest animals then and have been ever since. They played a key role in the history of life. Origin They first appeared around 570 million years ago during the Cambrian period. Because they belong to the group of animals called Protostomia , to which the annelid worms also belong, we assume that arthropods evolved from annelids or shared a common ancestor with them. Early arthropods included crabs, spiders and extinct animals like the sea scorpion shown here.
Today there are more types of arthropod than all the other types of animals put together.
To grow, arthropods had to shed their hard skin, pump themselves up with air or water, and grow another skin. This way of growing was wasteful, and probably limited the size that they could grow to. So although they are the commonest animals on earth, they have never been a dominant type of animal. However they have played a major role in evolution. For example flowering plants probably could not have evolved without them.