Skyrocketing animal diversity a half-billion years ago was linked to spikes and dips in marine oxygen levels, according to a detailed geological study.

Quote Originally Posted by Quanta Magazine
For more than a billion years after animal life arose, it stagnated in simplicity; sponges represented the height of complexity. Then around 541 million years ago, the pace of life’s transformation abruptly accelerated. This period, known as the Cambrian explosion, roughly bracketed the appearance of nearly all major animal groups alive today.

Within a few tens of millions of years, a geologic blink, the living world expanded into a semblance of its current fullness. Although recent work suggests that important and sudden expansions in animal biodiversity also happened before and after the Cambrian explosion, there is no doubt that it was an extraordinary and baffling episode in evolutionary history.

Why animal life got more complicated in such a hurry remains a burning question for paleontologists and evolutionary theorists. Was it just a matter of evolving the right combination of genes? Or were environmental factors limiting life’s possible forms, forcing it to wait until sufficient oxygen or other essentials could support more complexity?

A paper published earlier this week in Nature Geoscience offers detailed support for the idea that oxygen and animal diversity are positively linked, but with a twist.

Instead of oxygen levels gradually increasing, Cambrian seas underwent rapid periods of oxygen booms and busts, the paper argues. The international team of chemists, paleontologists and biogeochemical modelers behind the work tied these rapid fluctuations in oxygen to bursts of diversification and extinction during the Cambrian explosion.

Whether these leaps in animal diversity stemmed from a generally greater availability of oxygen or from the gas’s rapid boom-and-bust cycle during this period is unclear. But these results suggest that the linkage between increasing oxygen and the rise of animal complexity was no coincidence.

...

Geological slice by geological slice, the researchers compared the modeled oxygen levels and the diversity of fossils, revealing a volatile history. In the span of about 10 million years, between 524 and 514 million years ago, this shallow Siberian sea underwent five distinct oxygen spikes.

“These fluctuations are fairly extreme,” said He, explaining that each spike and dip constituted about a 50 percent increase or decrease in oxygen levels. “Previous studies suggest that oxygen levels during the Cambrian were about 40 percent of today’s atmospheric levels,” He said. “Fifty percent swings are pretty enormous.”

Each pulse of oxygenation corresponded with a local high in biodiversity, while dips in oxygen levels were associated with higher rates of extinction. For example, a pulse between 521 and 522 million years ago was associated with the appearance of numerous shelled animals, including trilobites and bivalved arthropods. A couple of million years later, the next pulse was coincident with a rise in large predatory arthropods and evidence of increased predatory behavior.

...