Approximately 540 million years ago, life rapidly diversified in an evolutionary burst — a biological “Big Bang” that witnessed the emergence of nearly every modern animal group. Scientists have long sought to determine what caused the Cambrian explosion, and to explain why animal life didn’t take this step at any point about a billion years earlier.
The most popular narrative puts oxygen front and center. The geological record shows a clear link, albeit an often subtle and complicated one, between rises in oxygen levels and early animal evolution. As Quanta reported earlier this month, many researchers argue that this suggests low oxygen availability had been holding greater complexity at bay — that greater amounts of oxygen were needed for energy-demanding processes like movement, predation and the development of novel body plans with intricate morphologies.
“It’s a very attractive, intuitive explanation,” said Nicholas Butterfield, a paleobiologist at the University of Cambridge. “And it’s wrong.”
Butterfield — “a lone voice in the wilderness,” he calls himself — has what many others might consider an unusual take on the oxygen story. He’s essentially turned the idea on its head. According to his theory, changes in environmental conditions weren’t the cause, but rather the consequence, of animals migrating and perturbing their surroundings. “We have to appreciate that animals have a powerful, powerful impact on the carbon cycle and on how everything goes around,” he said.
In a paper published in the January issue of Geobiology, Butterfield braided fluid dynamics and ecology to present his case for animals driving oxygenation instead of the other way around. First, he argued, if there was enough oxygen to power unicellular eukaryotes 1.6 billion years ago — which was indeed the case — then there would have been enough to run a whole assortment of animals. He believes early multicellular organisms would have consisted of flagellated cells moving in unison, collectively whipping their appendages to create currents that would have made it easier for them to obtain dissolved oxygen. “I make the case that if there’s enough oxygen to run a single-celled eukaryote, there’s enough oxygen to run a fish,” Butterfield said. “So oxygen limitation cannot be invoked to explain the billion-year delay in the evolution of animals.”
Instead, his hypothesis focuses on diurnal vertical migration, a daily process during which sundry organisms, ranging in size and complexity from zooplankton and sponges to fish and squids, migrate between shallow and deeper waters to find food and avoid predators. By feeding up above and metabolizing down below, the animals scrub and help ventilate the ocean, raising oxygen concentrations at the surface while driving anoxic regions to greater depths. This redistribution of oxygen would also have improved the transparency of the water column, allowing light to penetrate farther down and escalating predators’ reliance on vision at deeper and deeper levels when hunting. The subsequent evolution of larger, deeper-diving visual predators would then have pushed the “oxygen minimum zones” to even lower depths, creating a feedback loop.
Eventually, this cascading interplay between animals’ inadvertent re-engineering of ocean structure and their adaptive responses to those changes reached a tipping point. “The system went critical,” in Butterfield’s words, resulting in the sudden eruption of animal diversity and complexity during the Cambrian.
The delayed appearance of animals in the ocean was therefore not caused by a lack of oxygen, according to Butterfield, but rather because blind Darwinian evolution needed time to arrive at that tipping point. “The gene regulatory network to build an animal is the most complex algorithm that evolution has ever produced,” he said. “And it’s only ever happened once, [just as] it’s only ever happened once in land plants,?
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