A burst of evolutionary speciation which took place some 540 million years ago was likely triggered by eccentricities in Earth’s orbit around our Sun.
The Cambrian Explosion in which life on Earth underwent massive diversification was likely triggered by eccentricities in Earth’s orbit around our Sun. Or so say the authors of a new paper just published in the American Geophysical Union journalPlanet Earth is special in part because it orbits our yellow dwarf star in a very circular, predictable fashion.
But its orbit is not altogether predictable on time scales of thousands or even millions of years. In fact, Earth’s orbit can change, becoming what astronomers call eccentric . As a result, surges in oxygen in the ocean and atmosphere may have fueled the quick diversification of animal life called the Cambrian Explosion some 540 million years ago, the AGU says. And for the first time, the authors report that they have now made a quantitative link between what they term Earth's orbital forcing and evolutionary speciation and diversity during the Cambrian Explosion.In this paper, we propose that the long‐period “orbital forcing” is responsible for the pulses of oxygenation and biological radiation observed during the Cambrian Explosion, the authors write. Orbital forcing is the process where changes in the Earth’s orbit alter our planet’s climate, Benjamin Mills, a biogeochemist at the University of Leeds in the U.K. and the paper’s second author, tells me via email.Climate change caused by Earth’s long-period orbital cycles, the researchers argued, could have impacted the weathering of our planet’s land surfaces, releasing pulses of nutrients which flow into the oceans, prompting bursts of photosynthesis that released oxygen as a byproduct, says the AGU.The team used its newly developed Earth Evolution Model to compute changes in our planet’s climate and surface chemistry over millions of years. As the model ran, we noticed signals emerging over millions of years, driven by the changing size of the orbital changes and the slow response time of the model’s ocean chemistry, says Mills. These changes matched what we saw in the geological record, he says. These changes happened during a greenhouse climate condition at a time when Earth’s atmospheric oxygen levels had risen to an estimated 10 to 50 percent of their current levels, the authors note. These climate changes can increase weathering rates, which is the breakdown of surface rocks and minerals which then supply chemicals to the ocean, says Mills. When photosynthetic algae or bacteria die, some sink to the seafloor and become buried, he says. This ‘carbon burial’ removes the food that respiring organisms need, so it allows some O2 to stay in the atmosphere and oceans, says Mills.The Cambrian Period is the time when most of the major groups of animals first appeared in the fossil record; world climates were mild and there was no glaciation, the University of California’s Museum of Paleontology notes. The authors argue that changes in Earth’s orbit were responsible for a large part of this period’s climate. Orbital changes in Earth’s overall distance to the Sun, or even the tilt of the Earth can change the distribution of our planet’s surface temperatures as well as the water cycle and rainfall. Earth still orbits the Sun in a slightly eccentric orbit, and these forces are still acting today; they even caused the last ice age.The orbital changes tend to happen over thousands of years, but the size or amplitude of these cycles’ changes over a longer time – millions of years, says Mills. While these cycles occur all the time, only in the Cambrian period did they appear to lead to big changes in oxygen, he says.Our work introduces a previously unrealized linkage between orbital cycles and biological evolution, says Mills. At the broadest scale, this suggests that simple physical processes that should be present on many planets can ultimately lead to complex biological events, he says.
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