From ScienceDaily:
In the beginning, life was small. For billions of years, all life on Earth was microscopic, consisting mostly of single cells. Then suddenly, about 570 million years ago, complex organisms including animals with soft, sponge-like bodies up to a meter long sprang to life. And for 15 million years, life at this size and complexity existed only in deep water.
Why did life forms exist at that size only in deep water, where light, food, and especially oxygen must have been scarce. A new paper tests the hypothesis that stable temperature was the key — cold but stable:
Previously, scientists had theorized that animals have an optimum temperature at which they can thrive with the least amount of oxygen. According to the theory, oxygen requirements are higher at temperatures either colder or warmer than a happy medium. To test that theory in an animal reminiscent of those flourishing in the Ediacaran ocean depths, Boag measured the oxygen needs of sea anemones, whose gelatinous bodies and ability to breathe through the skin closely mimic the biology of fossils collected from the Ediacaran oceans.
“We assumed that their ability to tolerate low oxygen would get worse as the temperatures increased. That had been observed in more complex animals like fish and lobsters and crabs,” Boag said. The scientists weren’t sure whether colder temperatures would also strain the animals’ tolerance. But indeed, the anemones needed more oxygen when temperatures in an experimental tank veered outside their comfort zone.
Together, these factors made Boag and his colleagues suspect that, like the anemones, Ediacaran life would also require stable temperatures to make the most efficient use of the ocean’s limited oxygen supplies.
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In a world with low oxygen levels, animals unable to regulate their own body temperature couldn’t have withstood an environment that so regularly swung outside their Goldilocks temperature. Paper. (open access) – Thomas H. Boag, Richard G. Stockey, Leanne E. Elder, Pincelli M. Hull and Erik A. Sperling. Oxygen, temperature and the deep-marine stenothermal cradle of Ediacaran evolution. Proceedings of the Royal Society B: Biological Sciences, 2018 DOI: 10.1098/rspb.2018.1724 More.
On that theory, when there was more oxygen, creatures living in shallow water could be big too.
See also: Planets With Oxygen Not Necessarily Good Candidates For ET Life?
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Life exploded after slow O2 rise?
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