Oxygen and animal evolution: Did a rise of atmospheric oxygen trigger the origin of animals?

I read a paper, Oxygen and animal evolution: Did a rise of atmospheric oxygen trigger the origin of animals? by Daniel B. Mills and Donald E. Canfield, BioEssays, Volume 36 (12) - Dec 1, 2014

I can’t find a good, free, PDF of this one, mateys.

Mills and Canfield give an overview of the “Oxygen Control Hypothesis”, which is basically that atmospheric concentration of molecular oxygen (O₂) had to attain certain values for animal life to radiate and diversify. The theory supposes two thresholds for oxygen concentration.

I was vaguely acquainted with the second of the two thresholds, which is “enough oxygen to enable animals to have an energetic lifestyle”. The first threshold was new to me. Apparently the belief is that without something like the Ozone Layer, our Sun puts out enough ultraviolet radiation that life on land would be impossible, and that life in the first few feet of water would either be impossible or nearly so.

Mills and Canfield discuss these two thresholds, whether they exist and what the values might be, along with the history of looking for atmospheric composition by geologic proxies. Just like everything paleontological, there’s alternate theories and automatic gainsaying. Even a small oxygen concentration might give the Earth an ozone layer good enough to filter UV, but so might an atmosphere full of weird organics, like the early Earth is supposed to have. The oxygen concentration that early animals needed is also up for grabs. Some sponges can get by on very little oxygen, but it’s not clear exactly how little.

In a 2018 paper, Animal origins and the Tonian Earth system, Mills, Francis and Canfield try to put the nails in the Oxygen Control Hypothesis’ coffin: “the origin of animals was not obviously correlated to any environmental-ecological change in the Tonian Period.”

Mills and Canfield also discuss what an “animal” is, and how fossils strengthen or weaken the Oxygen Control Hypothesis. Animal (metazoan) phylogeny might strengthen the OCH, if radiations occur when geology says oxygen levels increase. They introduce the concept of “Last Common Ancestor of Metazoans”, as part of the discussion about what OCH might entail LCAM goes along with a lot of other similar concepts:

• First known life forms
• First Universal Common Ancestor
• Last Universal Common Ancestor
• LUCELLA, the Last Universal Cellular Ancestor, which was also the ancestor of the archaic virocell, too.
• First eukaryotic common ancestor
• Urmetazoan
• Urbilateran
• Mitochondrial Eve

These are all cool to think about, they should point to things to look for in the fossil record. But they also imply certain mysteries. Echinoderms famously have 5-fold symmetry, like starfish and crinoids, but evolved from a mirror-symmetry Urbilateran. Luckily, we’ve got fossil echinoderms that don’t have 5-fold symmetry. Mollusks are not segmented, where the Urbilateran was. We have no fossils for this transition. Some folks seem to think that Kimberella is a mollusk, but it shows up maybe 250 million years after mollusks diverged from other animals.

But all of that points out that the Oxygen Control Hypothesis rests in the subject of “geobiology” - these guys have exactly zero fossils. Stepping back just a little, to get a wider perspective, you’re forced to acknowledge what remarkable effect living things, mostly bacteria and algae, have had on planetary geology

The lead author of the paper, Daniel B. Mills, has an extensive publications list. He seems dead set on determining if the Oxygen Control Hypothesis is true.

Donald Canfield has an equally impressive list of publications.