Explanations of the dramatic Cambrian explosion of life forms (540 million years ago) are a cottage industry, with arguments about oxygen a staple of the discussion. See, for example,
Maverick theory: Cambrian animals remade the environment by generating oxygen
Did a low oxygen level delay complex life on Earth?
There was only a small oxygen jump
Animals didn’t “arise” from oxygenation, they created it, researchers say
Theory on how animals evolved challenged: Some need almost no oxygen
New study: Oxygenic photosynthesis goes back three billion years
Enough O2 long before animals?
Life exploded after slow O2 rise?
So the Cambrian really WAS an explosion then?
and finally,
Researchers: Cambrian explosion was not an explosion after all (When in doubt, insist that nothing happened.)
And this summer, we learn from ScienceDaily:
Five hundred and forty million years ago, during the Cambrian period, life suddenly went nuts. “Blossomed” is far too mild a word: instead, geologists call this sudden diversification an “explosion.” But what exactly sparked the Cambrian explosion?
Now, a new study suggests that wild swings in oxygen levels may have sent life scrambling to adapt, leading to a major burst of diversity. That, says lead author Guangyi Wei of Yale University, challenges the long-held explanation that gradually rising oxygen simply reached a life-fueling tipping point. The study was just published online ahead of print in Geology.
Wei explains that the goal was to reconstruct a continuous record of global marine oxygen levels from the late Ediacaran (latest pre-Cambrian) into the early Cambrian. To do that, the team, from Yale and Nanjing University in China, measured changes in uranium isotope ratios that reflect changing marine oxygen levels, as recorded in carbonates deposited during that interval, from China.
The results revealed huge global swings from anoxic-completely depleted in oxygen to oxygenated conditions over periods of two to ten million years. Such big ups and downs in oxygen, the scientists suggest, could have destabilized ecosystems, fragmented habitats, and triggered an explosion of changing life forms.
The study provides some of the first direct evidence that the Cambrian explosion came on the heels of major variations in oxygen, says Wei. “It would be great to get more data from other regions and time intervals,” he adds. Paper. (paywall) – Guang-Yi Wei, Noah J. Planavsky, Lidya G. Tarhan, Xi Chen, Wei Wei, Da Li, Hong-Fei Ling. Marine redox fluctuation as a potential trigger for the Cambrian explosion. Geology, 2018; DOI: 10.1130/G40150.1 More.
These discussions, featuring many conflicting views about the causes of the explosion, take place against the background assumption that there is no design in nature. At times, it feels like trying to understand World War II without allowing for the possibility that any intention underlay any of the events. Maybe that’s why the issues can’t be resolved.
For a different view, see Steve Meyer’s Darwin’s Doubt.