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A recent study published on July 2 in Nature Geoscience suggests that the Cambrian explosion, a rapid burst of evolution 540 million years ago, may have been triggered by only a small increase in oxygen levels in Earth's atmosphere and shallow ocean waters. This research was conducted by an international consortium of scientists from over 50 institutions.

"**Cambrian animals likely did not require as much oxygen as scientists used to believe. We found minor increases in oxygenation that are at the correct magnitude to drive big changes in ecology,**" said Erik Sperling, associate professor of Earth and planetary sciences at the Stanford Doerr School of Sustainability, who leads the consortium.

The study reconciles conflicting data sets globally and provides strong evidence that only a slight increase in oxygen occurred around the time of the Cambrian explosion. Additionally, researchers discovered that deep ocean oxygen levels did not reach modern levels until approximately 140 million years after the Cambrian explosion.

"**There was at least some increase in atmospheric oxygen around 540 million years ago, which impacted the oxygen availability in shallow marine environments where most marine biodiversity was hosted,**" stated lead study author Richard Stockey, PhD ’22, a University of Southampton paleobiologist who worked on this research during his doctoral thesis at Stanford. "**But from a global perspective, we didn’t see the full oxygenation of the oceans to near modern levels until about 400 million years ago, around the time that we see the appearance of large forests on land.**"

For decades, scientists have theorized that a sudden rise in atmospheric oxygen prompted the Cambrian explosion. However, evidence has been inconsistent and sometimes contradictory.

"**It’s one of these major evolutionary questions. We have 4 billion years of evolutionary history where not much is showing up in the rock record, and then within 20 or 30 million years, we get this burst of new body plans,**" said Sperling.

While only a small increase in atmospheric oxygen was found at the time of the Cambrian explosion, it may have been sufficient to propel significant evolutionary changes seen in fossil records. Most animals lived in shallow water then, and wind and wave mixing would have oxygenated these areas even as deeper oceans remained unchanged.

"**It’s not a huge increase in oxygen, but it might be enough to cross critical ecological thresholds based on what we see in modern areas with naturally low oxygen,**" Sperling noted.

To investigate changes over Earth's history spanning 700 million years, Stockey and Sperling examined trace metal levels such as uranium and molybdenum in black shale – sedimentary rock formed under low-oxygen conditions on ancient ocean floors. These elements accumulate differently depending on how widespread anoxic conditions are.

Previous research indicated increased trace metal concentrations around the Cambrian explosion period; however, other factors can affect these concentrations. Stockey applied statistical and machine learning techniques to analyze geochemical data from black shale samples extensively to separate various signals.

Stockey explained: "**We found that changes in organic carbon in black shale have driven many changes in trace metals observed for the last 15 or 20 years. It’s not until 140 million years after the Cambrian explosion when we see trace metals increasing at rates indicating whole ocean oxygenation.**"

The data were compiled through Sedimentary Geochemistry and Paleoenvironments Project (SGP), initiated by Sperling to standardize geochemical data for large-scale analysis – an approach inspired by biomedical research consortia studying diseases but novel within geology.

Sperling emphasized: "**It’s a very different approach than we’ve used before... Each individual research group still goes out to get snapshots but then comes together for analysis.**"

The analytical toolkit developed by Stockey could help understand ancient drivers like temperature or food supply impacting early evolution. The consortium also collects new data for undersampled periods while expanding analyses into older/younger intervals.

Stockey added: "**To harness advanced data science approaches for geological data effectively requires everyone speaking same language... This community-driven approach enhances confidence about reconstructing Earth’s evolution spatially/temporally.**"

Sperling is affiliated with Stanford Bio-X and Stanford Woods Institute for Environment alongside co-authors Christina R Woltz (postdoctoral scholar) & Samantha R Ritzer (PhD student). Additional contributors include Thomas H Boag (Columbia University), Malcolm S W Hodgskiss (UNESCO), among others across institutions worldwide funded by National Science Foundation/American Chemical Society.

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