For nearly a billion years, the Earth had the capacity to produce oxygen without ever accumulating enough to transform its atmosphere. This stagnation delayed the emergence of complex life until a sudden shift occurred in the Cambrian, around 520 million years ago. Understanding this trigger remains essential to trace the conditions that allowed the explosion of biodiversity.
Researchers from the Nanjing Institute of Geology and Palaeontology (Chinese Academy of Sciences), the Institute for Planetary Materials (Okayama University) and the University of Peradeniya have published work combining geochemistry, climate and astronomy in Geophysical Research Letters and Communications Earth & Environment. They reveal how orbital cycles, associated with erosion-regulated nutrient flows and ancient chemical balances, enabled rapid oxygenation of the environment. These results shed light on a key stage in the evolution of life on Earth.
The Cambrian explosion, a shift linked to oxygen
About 520 million years ago, the planet experienced one of the greatest biological transitions in its history: the Cambrian explosion. In the space of 10 to 20 million years, the diversity of life forms experienced a spectacular jump. Many large modern animal groups (or phyla) then appeared for the first time in the fossil record. This transition, well documented, remains astonishing in its rapidity on a geological scale.
For several decades, scientists have suspected that rapid increases in oxygen levels in the atmosphere and oceans played a determining role in this diversification. Indeed, oxygen constitutes a crucial factor for the metabolism of multicellular organisms. However, the cause of these oxygen “pulsations” remained poorly understood until now.
Researchers from the Nanjing Institute of Geology and Palaeontology (NIGPAS), China, analyzed carbon and sulfur isotope data sets from Lower Cambrian sedimentary rocks. They found that fluctuations in atmospheric oxygen followed a cyclical rhythm, every 2 to 3 million years. These cycles corresponded to variations observed in the diversity of marine animals, revealing a strong correlation between chemical environment and evolutionary dynamics.
These results suggest that the oxygenation of the globe occurred in fits and starts, rather than gradually. Biodiversity then unfolded in “evolutionary waves”. Each facilitated by a temporary but significant increase in oxygen, linked to mechanisms yet to be explored.
The decisive role of Earth's orbital cycles
To understand the origin of these periodic increases in oxygen, researchers explored a hitherto underestimated avenue: variations in the Earth's orbit. These changes, called Milankovitch cycles, modify the distribution of solar energy received by the Earth on long time scales — from several hundred thousand to several million years.
By integrating these cycles into a climate and geochemical model, the Sino-Japanese team simulated the effect of variations in insolation on the global climate. Result: the observed oxygen peaks closely coincide with orbital cycles of 1.2 to 4.5 million years. They particularly affect high latitudes. This astronomical modulation led to alternating phases of warming and cooling, with direct consequences on the erosion of the continents.
Periods of warming have intensified continental weathering, a process where rocks break down under the influence of weather. This phenomenon allowed the increased release of essential nutrients, such as phosphorus, into the oceans. These inputs stimulated marine photosynthesis, leading to increased oxygen production.
The novelty here lies in the integration of orbital forcing into geochemical modeling, a first according to the authors. This establishes a clear link between slow but powerful astronomical factors and rapid shifts in the Earth's environment. Cambrian oxygenation therefore appears to be an amplified response of the Earth system to a recurring external signal.
Unstable marine chemistry prone to upheaval
If orbital cycles triggered climatic pulsations, a sensitive chemical system was still needed to amplify their effect. This is precisely what the study reveals. During the Cambrian period, ocean chemistry exhibited particular instability. This vulnerability is explained in particular by a low concentration of marine sulfate, which has modified the biogeochemical balances.
The researchers demonstrated that this instability reinforced the amplitude of the responses of the global cycle of carbon, sulfur and oxygen. With each influx of nutrients from the continents, the system responded with a rapid increase in marine photosynthesis. So oxygen production. This mechanism would have allowed photosynthetic organisms, mainly cyanobacteria, to proliferate in waves.
Isotopic analyzes on marine carbonates have highlighted cyclic variations of δ¹³C (carbon) and δ³⁴S (sulfur) which coincide with the detected orbital cycles. These signatures confirm the presence of repeated events of organic carbon storage and pyrite formation. These are the two key processes for the accumulation of oxygen in the atmosphere.
By modeling these dynamics, the team found that the system responded strongly to small disturbances. Like an ecosystem ready to rock at the slightest signal. This explains why the Cambrian, unlike other periods subject to the same orbital cycles, saw such a biological acceleration occur.
This instability, far from representing a handicap, has therefore played a structuring role in the transformation of the terrestrial environment. It allowed a rare synchronization between climate, geochemistry and biological evolution.
Nickel, urea and cyanobacteria: the key to the origins of oxygen
Long before the Cambrian explosion, the Earth should have become an oxygenated planet. Cyanobacteria, capable of producing oxygen via photosynthesis, have existed for more than two billion years. However, the atmosphere remained anoxic for almost a billion years. Why such inertia?
This is the question that Dr Dilan M. Ratnayake (University of Peradeniya, Sri Lanka) and his colleagues from the Universities of Okayama and Nagoya looked into. In the study published in
Communications Earth & Environmentthey identified two major chemical brakes: nickel and urea.
At high concentrations, these compounds inhibit the growth of cyanobacteria. Nickel, in particular, interferes with enzymes related to photosynthesis. Urea, in excess, also disrupts nitrogen metabolism. The researchers reconstructed Archaean conditions in the laboratory and confirmed that these substances limited photosynthetic productivity.
Over time, the natural decrease in nickel in the oceans – due to the decline in underwater volcanism – and the stabilization of urea have removed these barriers. The cyanobacteria were then able to multiply, leading to a massive release of oxygen. This is the Great Oxidation Eventabout 2.3 billion years ago.
These discoveries show that the evolution of the Earth's atmosphere also depends on the balance between trace elements and microbial activity. They shed light not only on the origins of complex life on Earth, but also on the conditions necessary for oxygenation on other planets. As Ratnayake points out, in a press release, “ understanding these processes helps us better target biosignatures in the search for extraterrestrial life “.
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