I am a geobiologist curious about the diversification of animals on Earth, an event we know very little about. Tumors may reveal clues to animal evolution, since they so well manage the leap from unicellularity to multicellularity. Animals are one of the most successful versions of multicellular organisms on Earth, but their rise remains enigmatic. Animals evolved late in Earth history (after ~85% of its current time had passed) and then their diversification was dramatic. Why? We do not understand this dramatic event and explore it primarily through clues preserved in the geological record, such as chemical or visible fossils. The geological record, however, struggles with its fragmented evidence, low resolution and its inability to shed light on causality between observations (what of the evidence that actually represents cause and effects). Therefore, we use tumors as a model for animal evolution to get at the mechanisms that were fundamental for tissue maintenance in an evolutionary perspective. These mechanisms likely still remain fundamental for tissue maintenance, whether or not the tissue is malignant or non-transformed. An evolutionary view on tissue management, therefore, can also assist us in understanding the development of tumors.