Five hundred and forty million years ago, a few groups of simple creatures began rapidly evolving into thousands of highly complex species. This Cambrian explosion, as it’s called, happened over the course of five million years — a blip on the evolutionary timeline. But what sparked the explosion?
The evidence that geochemist Justin Ries unearthed could change the way scientists think about the greatest evolutionary event in the history of animal life.
Decades ago, scientists believed that a sudden increase in atmospheric oxygen had triggered the Cambrian explosion. This was the going theory until scientists in the 1990s figured out how to use chemical signatures in ancient rocks to reconstruct the composition of the atmosphere. They found that oxygen levels had begun to rise about 2.5 billion years ago and seemed to increase gradually until modern times. That meant the level of oxygen would’ve been high enough to spur evolution long before the Cambrian explosion. So scientists rejected the idea that oxygen had driven diversification, and several other theories emerged, including one about jawed fish and other predators forcing sea animals to evolve more sophisticated defenses.
“For nearly every person in the field there’s a different theory for what caused the Cambrian explosion,” Ries says. “And most of them emerged only after the simple oxygen theory had been rejected.”
But a few years ago, a team of researchers uncovered peculiarly high sulfate-isotope levels in the Precambrian limestone formations of Oman — levels high enough to cast doubt on what scientists had believed about ancient oxygen levels.
“Something interesting was going on with sulfur and oxygen at that period in time, but it was unclear whether the trend was global,” Ries says. “That’s why I went to Namibia — to look at limestones that are the same age as those in Oman.”
There aren’t many places to study well-preserved limestone from the time immediately before the Cambrian explosion. One of the best is southwestern Namibia, where canyons have left pristine limestones exposed for hundreds of miles in all directions. German and Dutch expatriate ranchers own millions of acres in this region, including the canyons of the Zebra River Valley, and they rarely see outsiders. They’ve been known to shoot first and ask questions later, Ries says.
“It’s like the American West of Germany over there,” he says. “Abandoned pickup trucks. Country music. The northern part of Namibia is lush like Tanzania, but where we were was more like Arizona or, in some places, the surface of the moon.”
Fortunately, Ries was there with the right sort of guide: John Grotzinger, a geologist from the California Institute of Technology who is friends with several Namibian geologists and ranchers. Grotzinger has spent thirty years mapping and dating several rock formations in the Zebra River Basin and other sites. “He’s kind of a legend in the field,” Ries says. So when Grotzinger invited Ries to accompany him to Namibia, Ries gladly accepted.
Before dawn each day, Ries, Grotzinger, and UC Riverside professor Gordon Love would strike out from the Zebra River Lodge, a no-frills outpost built into stony hills, and drive as far as they could until the terrain became impassable by SUV.
One morning, during the first mile on foot, Ries heard a bloodcurdling shriek. He sprinted through the high, bristly grass, and didn’t look back until he caught up with Grotzinger and Love.
“The sound was like a mix of a snarling leopard and screaming hyena,” Ries says. He turned and saw a lone baboon, madder than hell, strutting back and forth while pounding his chest and slapping the ground. Grotzinger told Ries that the baboon had been the alpha male of a tribe. According to the locals, a younger and burlier baboon deposed the graying dictator and forced him into a nomadic life in the canyon lands.
Three miles down the river valley, the canyon walls grew taller and the landscape morphed into a scene from The Planet of the Apes. On the rocky mounds, fifteen baboons appeared — none happy to see the humans.
“They walk on two feet and have very expressive faces,” Ries says. “They resemble people — small, hairy, and very angry people. And they get right up in your face.” Grotzinger reassured Ries that the baboons were harmless: they might bark a good game, but they weren’t likely to attack humans.
The animals followed them for several miles, all the way to one of the cliffs that Grotzinger had previously mapped and dated. The rocks there were 545 million years old. The researchers began climbing and chiseling out samples, but it wasn’t long before the baboons figured out Ries’s path up the slope. They relieved themselves on the rocks that the researchers were sampling, and when that didn’t deter the humans, the baboons kicked rocks down the slope. “We threw rocks back at them and they eventually went away,” Ries says. “But they always came back.”
The baboons would’ve been more bearable had Ries needed only a few samples. But he was determined to conduct the most detailed sampling of limestone from the Precambrian period that anyone had ever done. So his team chiseled out rock every ten feet until they reached the top of the twelve-hundred-foot cliff. Then the team climbed down the slope at dusk and navigated the dark canyon, each man carrying sixty to eighty pounds of rock.
The baboons followed them all the way out of the canyon, prowling just out of flashlight range and hissing through the darkness. They never attacked — but the elements did. One afternoon, a storm swept in, stranding the men atop a mesa and turning their rocky pathway into a torrent of rainwater. They had to scramble down a more dangerous slope, where the wet, sandy rock ripped their fingertips to shreds.
“You don’t realize how much you need your fingertips until they’re gone,” Ries says. “We bandaged them with duct tape for the rest of the trip.”
Back at the lodge, the team packed up the rocks, and after two weeks in the field they returned home to study the chemical signature of the limestones.
Using a mass spectrometer — the standard way to measure sulfur-isotope levels in ancient rock — Ries found that the sulfur-isotope signal in the ocean before the Cambrian explosion was much higher than he had expected. Surprisingly, it was even higher than the sulfate-isotope signal. That meant the sulfate levels in the oceans — and by extension, the oxygen levels in the atmosphere — must have been much lower than they are today, and probably much too low to support complex animal life.
“At first, I was skeptical of the data,” Ries says. The findings clearly contested the accepted theory that the atmosphere’s oxygen level had been high enough to support complex animal life before the Cambrian explosion. “We kept redoing our extractions and rerunning the analyses, but we got the same results each time.”
Ries then decided to see what other researchers had found when they studied sulfur isotopes in limestone from other formations around the world. Some studies had similar findings, but most of these results had been rejected as anomalies. Even the researchers who had conducted the studies thought that their results were off-base.
Here’s why. If you only sample two or three rocks from a cliff face, there’s a chance you’ll sample a vein of pyrite or an area that’s been contaminated by groundwater. These dense pyrite formations and contaminated rocks no longer contain original chemical signatures from the ocean. They can distort results and lead researchers to misinterpret the geological record.
But when Ries read the other studies, he saw that the presence of heavy sulfide isotopes between 543 million and 549 million years ago was a global phenomenon. And he knew that he hadn’t sampled pyrite veins or contaminated rocks, because he’d collected samples every ten feet up the slopes of four separate limestone formations.
“We knew we were looking at a global seawater signal because the carbon-isotope signatures in our rocks were identical to those in marine limestone from other continents, deposited across the same period of time,” he says. And geologists agree that the limestone formation in southwest Namibia is one of the most pristine in the world.
Ries asked David Fike, then a graduate student in Grotzinger’s lab, to run more tests of his samples. Fike validated the findings: oceans before the Cambrian explosion contained sulfide that was isotopically heavier than sulfate, which meant that low sulfate and oxygen levels persisted much longer than previously thought. Such low oxygen levels would have kept a damper on the diversification of animal life in the years immediately preceding the explosion.
Then Ries went a step further. He plotted the results of every study of sulfide and sulfate isotopes in ancient limestones dating back 2.5 billion years, including his own work and the studies that had been tossed out because they were anomalous to the accepted theory.
Ries saw an amazing trend. Atmospheric oxygen levels did rise 2.5 billion years ago, as scientists in the 1990s had discovered. But later, these levels decreased. Then, about 1.25 billion years ago, oxygen levels increased again before dropping one more time right before the Cambrian explosion. Finally, about 540 million years ago, oxygen levels increased dramatically to near-modern levels.
Ries says that such an increase could have been just what anatomically simple species needed to become more complex.
“This doesn’t prove that oxygen was the driving force behind the Cambrian explosion,” he cautions. “But it undoes the rejection of that theory.”
The simplest explanation for the Cambrian explosion might be the right one after all.
Justin Ries is an assistant professor of marine sciences in the College of Arts and Sciences. His work in Namibia was supported by John Grotzinger’s grants from the Agouron Institute and the California Institute of Technology. The mass spectrometer analysis was performed at the University of Indiana and the University of California, Riverside, with funding from NASA. Ries’s paper on this research appeared in the August 2009 issue of Geology.