Animals present today show a vast diversity in their bodies so, they must have diverse ancestry too. Common ancestry would greatly limit the extent of diversity.
For example, all birds are closely related and have common ancestors, but bats too have wings (analogous organs) and are closely related to them.
Therefore, if we classify species according to the evolutionary relationship, we can build up small groups of organisms with recent common ancestors. These can be further classified as super-groups with most distinct common ancestors. Likewise, we can keep going backwards in this classification to come to a single species at the very beginning of evolutionary time.
But having a single ancestor would limit the diversity seen in the organisms today.
Thus, it can be said that the occurrence of animals on Earth suggests their diverse ancestry also.