title: Oxygen requirements of the earliest animals.
abstract: A rise in the oxygen content of the atmosphere and oceans is one of the most popular explanations for the relatively late and abrupt appearance of animal life on Earth. In this scenario, Earth's surface environment failed to meet the high oxygen requirements of animals up until the middle to late Neoproterozoic Era (850-542 million years ago), when oxygen concentrations sufficiently rose to permit the existence of animal life for the first time. Although multiple lines of geochemical evidence support an oxygenation of the Ediacaran oceans (635-542 million years ago), roughly corresponding with the first appearance of metazoans in the fossil record, the oxygen requirements of basal animals remain unclear. Here we show that modern demosponges, serving as analogs for early animals, can survive under low-oxygen conditions of 0.5-4.0% present atmospheric levels. Because the last common ancestor of metazoans likely exhibited a physiology and morphology similar to that of a modern sponge, its oxygen demands may have been met well before the enhanced oxygenation of the Ediacaran Period. Therefore, the origin of animals may not have been triggered by a contemporaneous rise in the oxygen content of the atmosphere and oceans. Instead, other ecological and developmental processes are needed to adequately explain the origin and earliest evolution of animal life on Earth.
authors.affiliation: Department of Biology and Nordic Center for Earth Evolution, University of Southern Denmark, 5230 Odense M, Denmark.
authors.forename: Daniel B
authors.initials: DB
authors.lastname: Mills
authors.forename: Lewis M
authors.initials: LM
authors.lastname: Ward
authors.forename: Carriayne
authors.initials: C
authors.lastname: Jones
authors.forename: Brittany
authors.initials: B
authors.lastname: Sweeten
authors.forename: Michael
authors.initials: M
authors.lastname: Forth
authors.forename: Alexander H
authors.initials: AH
authors.lastname: Treusch
authors.forename: Donald E
authors.initials: DE
authors.lastname: Canfield
mesh_terms: D000220:Adaptation, Biological / Q000502:physiology*; D000818:Animals; D001272:Atmosphere / Q000737:chemistry*; D001483:Base Sequence; D005075:Biological Evolution; D003718:Denmark; D007700:Kinetics; D064307:Microbiota / Q000235:genetics*; D008969:Molecular Sequence Data; D010100:Oxygen / Q000032:analysis*; D010101:Oxygen Consumption / Q000502:physiology*; D012150:Polymorphism, Restriction Fragment Length; D011161:Porifera / Q000382:microbiology / Q000502:physiology*; D012336:RNA, Ribosomal, 16S / Q000235:genetics; D017422:Sequence Analysis, DNA
chemical_list: D012336:RNA, Ribosomal, 16S; D010100:Oxygen
keywords: Metazoa; hypoxia