Cambrian Explosion: role of oxygen (Introduction)

by David Turell @, Wednesday, January 18, 2017, 01:10 (1174 days ago) @ David Turell

The Cambrian Explosion started about 520 million years ago with increased oxygen in the atmosphere, but there was brief spike in oxygen at 2.3 billion years ago without complex life. What other factor might have been at play?:

"It was a sign of things to come. About 2.3 billion years ago, our primitive planet was an oxygen-poor world profoundly different from now – but then it briefly and mysteriously gained an oxygen-rich atmosphere.

"This so-called Lomagundi Event could have provided a fleeting opportunity for complex, animal-like creatures to evolve billions of years before the ancestors of all animals we know today appeared.

"Earth is thought to have begun to develop its modern, oxygen-rich atmosphere as recently as 800 million years ago. This is roughly when biologically complex, oxygen-breathing animals first appear in the fossil record, leading many to suggest that animal life was made possible by the rise in atmospheric oxygen.

"Before 800 million years ago, there may have been little gaseous oxygen around – one 2014 estimate suggests there may have been as little as 0.1 per cent of the present level.

"The Lomagundi Event – between 2.3 and 2.1 billion years ago – is an exception to this early oxygen-poor world. Chemical analysis of “Lomagundi” rocks hints that the amount of organic carbon buried in the deep ocean suddenly spiked.


"Kipp and his colleagues measured selenium isotope ratios in Lomagundi rocks formed at various places around the world, and then worked back to estimate the dissolved oxygen level in the shallow seas of this time. Their calculations suggest the level may have been at least 5 micromoles per litre of water.

"This is considerably below the modern level of about 325 micromoles per litre, but is still well above the minimum oxygen requirements of some simple marine animals alive today, which can make do with about 0.9 micromoles per litre of water.


"It’s the implications of the research that will attract most attention, though. “The take-home message is that the oxygen level was high enough to support eukaryotic life and, by some arguments, maybe even animal life,” says Timothy Lyons at the University of California Riverside, who collaborates with Kipp and his colleagues, but was not involved in the new study.

"This confirms that the Lomagundi Event was what Lyons calls an “oxygen oasis in time”. It has implications for our understanding of how, or if, animal evolution was influenced by available oxygen.

“'It’s like a perfect thought experiment,” says Lyons. “Let’s predict what a dominantly anoxic community of life would do in the face of getting a large injection of oxygen early in its history.”

"So far, however, it appears there was little response: although there are hints that life became more complex during the Lomagundi Event, there is no really convincing evidence.

“'But that doesn’t mean that those organisms didn’t exist,” says Kipp. “With palaeontology, it’s difficult to argue that absence of evidence is evidence of absence.”

"Life’s apparent failure to become complex during the Lomagundi Event despite having the oxygen to do so is unsurprising, says Nicholas Butterfield at the University of Cambridge. Rather than a lack of oxygen delaying the appearance of animals until the past 800 million years, he thinks the reason was that it took evolution aeons to “work out” how to develop such biologically complex organisms.

"In other words, life simply wasn’t ready to become complex at this time. “I think it does support the view that there is far more to the story than oxygen,” Butterfield says."

Comment: The speed of evolution appears to vary. It didn't take long to make humans from the earliest possible beginning in monkeys about 22 million years ago. It took about 3.5 billion years to make the Cambrian animals. Perhaps a long period of high oxygen was necessary.

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