As an Explanation of the Cambrian Explosion, the Oxygen Theory Takes a Lethal Blow
The oxygen idea had already taken blows, including from us, but this would seem to be the end of it. The notion was that rising oxygen levels permitted and therefore somehow spurred the Cambrian explosion, the geologically abrupt emergence of complex animal life less than 600 million years ago. It is as if the possibility of breathing deep was so enticing as to coax a vast infusion of biological information, needed to generate most known animal body plans, from nonexistence into existence. "Oh, whistle, and I'll come to you, my lad."
Now a team reports in Proceedings of National Academy of Sciences that there was adequate oxygen long before, so long as to decisively blunt the oxygen theory. This seems definitive. From Science Daily:
Animals evolved by about 600 million years ago, which was late in Earth's history. The late evolution of animals, and the fact that oxygen is central for animal respiration, has led to the widely promoted idea that animal evolution corresponded with a late a rise in atmospheric oxygen concentrations.How did they make the determination? The Abstract summarizes ("Sufficient oxygen for animal respiration 1,400 million years ago"):
"But sufficient oxygen in itself does not seem to be enough for animals to rise. This is indicated by our studies," say postdoc Emma Hammarlund and Professor Don Canfield, Nordic Center for Earth Evolution, University of Southern Denmark.
Together with colleagues from the China National Petroleum Corporation and the University of Copenhagen, Hammarlund and Canfield have analyzed sediment samples from the Xiamaling Formation in China. Their analyses reveal that a deep ocean 1.4 billion years ago contained at least 4% of modern oxygen concentrations.
The Mesoproterozoic Eon [1,600-1,000 million years ago (Ma)] is emerging as a key interval in Earth history, with a unique geochemical history that might have influenced the course of biological evolution on Earth. Indeed, although this time interval is rather poorly understood, recent chromium isotope results suggest that atmospheric oxygen levels were <0.1% of present levels, sufficiently low to have inhibited the evolution of animal life. In contrast, using a different approach, we explore the distribution and enrichments of redox-sensitive trace metals in the 1,400 Ma sediments of Unit 3 of the Xiamaling Formation, North China Block. Patterns of trace metal enrichments reveal oxygenated bottom waters during deposition of the sediments, and biomarker results demonstrate the presence of green sulfur bacteria in the water column. Thus, we document an ancient oxygen minimum zone. We develop a simple, yet comprehensive, model of marine carbon?oxygen cycle dynamics to show that our geochemical results are consistent with atmospheric oxygen levels >4% of present-day levels. Therefore, in contrast to previous suggestions, we show that there was sufficient oxygen to fuel animal respiration long before the evolution of animals themselves.
Emma Hammarlund encapsulates the bad news:
The sudden diversification of animals probably was a result of many factors. Maybe the oxygen rise had less to do with the animal revolution than we previously assumed.
But you know what, failed scientific theories that seem to lend support to materialism have a way of defying death. They stick around even well after the evidence has turned against them. I'd bet you that popular and even professional science sources will still be touting the oxygen theory years from now, as if nothing had happened.
Image credit: NASA Earth Observatory [Public domain], via Wikimedia Commons.