Meeting Description: Since the 1960s the rise of animals during the late Neoproterozoic and Cambrian has been ascribed to increased atmospheric and marine oxygen levels. Over the past decade a much richer fossil record as well as molecular clock evidence has greatly increased our knowledge of the early history of animals. An array of geochemical proxies for oxygen levels have also been developed. Participants in this working group represent a variety of opinions about the relationship between oxygen levels and evolutionary change. The purpose of the meeting is to draft a joint paper outlining the available evidence, areas of agreement and disagreement and to identify what new studies are required to resolve remaining questions. Publication of the paper has already been invited by Geobiology.