Complex animals are relative newcomers to Earth, writes Douglas Fox in the February 16th Issue of Nature. “Since life first emerged more than 3 billion years ago, single-celled organisms have dominated the planet for most of its history.” What we would now call extremophiles, these organisms thrived in anoxic and low oxygen environments—relying on carbon dioxide and sulfur-containing molecules or iron minerals that act as oxidizing agents to break down food. Much of Earth’s microbial biosphere still sustains itself through these anaerobic pathways, writes Fox. In my own research as a limnologist, I have observed a plethora of both microbial and complex life in these anoxic conditions—in water and in sediments beneath the water.
Until the onset of the Cambrian Era in what’s called the Cambrian Explosion, a slimy sheet of microbes covered the Ediacaran sea floor; a diverse blanket of “enigmatic animals whose bodies resembled thick, quilted pillows” dominated the ocean floor. Most were stationary, says Fox. “But a few meandered blindly over the slime, grazing on microbes.” Animal life at this point was simple.
And there were no predators.
But that was about to change. Within several million years, says Fox, this simple ecosystem would disappear and give way to an explosion of complex form and activity. Mobile animals with a new design—from legs and compound eyes to feathery gills and tooth-rimmed jaws. Among them was the metre-long Anomalocaris, an ancestral arthropod and a ferocious predator. First discovered by Charles Walcott in the Burgess Shale fossils, this beast propelled itself through the water by a kind of peristaltic waving of its flexible lobes—acting like fins—on the sides of its body. The Anomalocaris’s large head had stalked compound eyes with 16,000 lenses and a disk-like mouth with 32 overlapping plates. One of its favourite meals was apparently the hard-bodied Trilobite, another arthropod.
What caused this explosion of more complex animals that ate each other?
Complex multi-cellular animals, like the Trilobite and the Anomalocaris, generally depend on oxygen—even if just in little amounts—to metabolize food, which releases much more energy than most anaerobic pathways. “Animals” says Fox, “rely on this potent, controlled combustion to drive such energy-hungry innovations as muscles, nervous systems and the tools of defense and carnivory—mineralized shells, exoskeletons and teeth.”
Many paleontologists postulate that an increase in oxygen over an ecological threshold—from 3% to over 10% of current oxygen levels—triggered life to speed up. Large animals had emerged during the Ediacaran period, but they were slow or immobile. And didn’t eat each other. According to Guy Narbonne, a paleontologist at Queen’s University in Kingston, Canada, they were mostly soft-bodied and immobile sedentary forms that absorbed nutrients through their skin.
Shortly after oxygen levels increased beyond the threshold, Ediacaran organisms such as Dickinsonia and Chaniodiscus, went extinct and gave rise to Cambrian complex forms such as arthropods Anomalocaris and Marrella and chordates such as Pikaia—all predators that consumed other animals. And all a result of a small but significant rise in oxygen levels.
The change in grazing style contributed to a change in the ocean floor microbial mat. Previously forming a continuous “plastic wrap” sheet, which ensured that sediments remain largely anoxic (and off limits to animals), the mat started to break up in the Cambrian. This permitted animals to burrow into the sediments and create “habitat.” Animals now had access to previously untapped nutrients and a refuge from predators—just in time it seems.
The evolutionary question that was not discussed was: what triggered the increase in oxygen past the threshold? And what’s next in this fascinating story?
Fox, Douglas. 2016. “What sparked the Cambrian explosion?” Nature, February 16, 2016.
Nina Munteanu’s “Water Is…” (Pixl Press) on sale worldwide May 10, 2016