The oxygen requirements of the earliest animals. D. B. Mills, et al. 2014.PNAS, Feb. 17, 2014.
New studies of a small sea sponge fished out of a Danish fjord shows that complex life does not need high levels of oxygen in order to live and grow.
Complex life evolved is believed to have evolved after the atmospheric levels of oxygen began to rise app. 630 – 635 million years ago. Halichondria panicea closely resembles the first animals on Earth & shows that animals can live and grow even when the atmosphere contains only 0.5 per cent of the oxygen levels in today's atmosphere.