Reply to Saint-Antonin: Low-oxygen-tolerant animals predate oceanic anoxic events

It is has been assumed for over half a century that the earliest animals were obligate aerobes with relatively high oxygen requirements. However, the conserved biochemistry and widespread phylogenetic distribution of anaerobic energy metabolism in animals suggests a deep ancestral possession of the genes and enzymes necessary for a facultative anaerobic lifestyle. Additionally, non-bilaterian bodyplans are not expected to require particularly high environmental oxygen levels. This is consistent with experimental evidence demonstrating the low-oxygen tolerance of the sponge Halichondria panicea. While it is conceivable that low-oxygen-adapted animals evolved only sometime during the past 541 million years, perhaps in response to oceanic anoxic events, they most reasonably date back to the first animals themselves, as the last common ancestor of animals likely emerged in a relatively lowoxygen world, possessed the genetic means for anaerobiosis, and exhibited a bodyplan conducive to aerobic growth under oxygen levels less than 4% of modern atmospheric saturation.