Life was bland on Earth until an event triggered an event that rapidly changed life on Earth as we know it! The Cambrian explosion, produced arthropods with legs and compound eyes and predators that could destroy prey with their jaws! Above the grassy plains of Namibia, there exists a series of craggy pinnacles rising 80 meters above. The stone formations are monuments of a faded empire—but not of the human kind as we see with the pyramids. Instead, they are pinnacle reefs, built by cyanobacteria on the shallow sea floor 543 years ago. By today’s standards, the ancient world that occupied these reefs during the Ediacaran period were alien. At the time, the oceans held so little oxygen that modern fish would have quickly perished. Most animals were stationary, but some wandered around a layer of gooey microbes. That is—until an evolutionary storm dramatically changed this world.
Over a several million years, this ecosystem disappeared, giving way to a world ruled by highly mobile animals. The process, which is known as the Cambrian explosion, produced arthropods with legs and compound eyes and predators that could destroy prey with their jaws. But for decades, the cause of this evolutionary burst has been widely debated by biologists. Some believe a rise in oxygen parked the change, but others believe it was the result of a key evolutionary innovation.
In the past few years, discoveries have started yielding clues about the end of the Ediacaran period. Evidence suggests that earlier theories were perhaps too simple—that the Cambrian explosion was actually the result of a complex combination of small environmental changes that triggered evolutionary developments. The increase in oxygen enabled the emergence of predators—which sparked an arms race that led to the complex beings we see today.
“This is the most significant event in Earth evolution,” Guy Narbonne, a paleobiologist at Queen’s University in Kingston, Canada. “The advent of pervasive carnivory, made possible by oxygenation, is likely to have been a major trigger.”
The discovery is interesting because it reminds us that we are still relatively new to Earth. It has been single-celled organisms dominating the planet for most of its history. Given the importance of oxygen for more complex animals, researchers have speculated that a sudden increase in the gas spurred the Cambrian explosion. To test that idea, they have studied ancient ocean sediments running from about 635 million to 485 million years—analyzing the amounts of iron, molybdenum and other metals in them. Of course, the metals’ solubility depends on the amount of oxygen present, so the type of metals give clues to how much oxygen was in the water when the sediments form.
Originally, the evidence supported the idea that oxygen was a key trigger for the explosion. But last year, Erik Sperling, a palaeontologist at Stanford University, compiled a database of 4,700 iron measurements taken from rocks around the world and challenged that view. His colleagues did not find a statistically significant increase in the proportion of oxic to anoxic water between the Ediacaran and Cambrian periods.
Donald Canfield, a geobiologist at the University of Southern Denmark, also doubts that oxygen was a limiting factor for early animals. His team suggests that oxygen levels were already high enough to support simple animals hundreds of millions of years before they actually appeared. Though Cambrian animals would have needed more oxygen, “you don’t need an increase in oxygen across the Ediacaran/Cambrian boundary.” Oxygen could have been abundant enough.
A study from Timothy Lyons, a geobiologist from California, reveals that the increases in oxygen just before the Cambrian explosion were temporary peaks that last a few million years before gradually stepping upward.
Sperling suggests that biologists have been approaching oxygen and animal evolution the wrong way. By pooling and analyzing previously published data with his own, he found that tiny worms survive in areas of the sea where oxygen levels are incredibly low and feed directly on microbes. In places where sea-floor oxygen levels are a bit higher, the animals still feed on microbes rather than on each other. But between 3% and 10% oxygen levels, predators start to consume other animals. The implications suggest that even a small oxygen rise could have been enough to trigger the change.
“There’s just so much in animal ecology, lifestyle, and body size that seems to change so dramatically through those levels,” Sperling notes.
Of course, the gradual emergence of predators driven by a small rise in oxygen meant serious trouble for the Ediacaran animals that lacked defenses. They began to fall prey to predators.
Sources: Nature, Scientific American