A rise in oxygen more than half a billion years ago paved the way for the origin of the first carnivores. The meat eaters in turn triggered the Big Bang of animal evolution, researchers argue.
The major groups of modern animals — everything from insects to creatures with a backbone — popped up 540 million to 500 million years ago in a proliferation known as the Cambrian Explosion. Fossil and molecular evidence hint that the most primitive animals appeared a couple hundred million years earlier, leading scientists to wonder about the cause of the lag.
Now scientists have stitched together earlier theories to come to a comprehensive explanation. Erik Sperling, an earth scientist at Harvard University, and colleagues say an increase in oxygen in the geologic record at the onset of the Cambrian period allowed carnivores to evolve. The oxygen boost could have accommodated the high energy costs of pursuing and digesting prey, Sperling says.
Once carnivores arrived, an evolutionary arms race broke out between predator and prey, the team suggests July 29 in the Proceedings of the National Academy of Sciences. As prey evolved new defenses and predators developed new weapons, new kinds of animals sprung up.
Support for the oxygen-carnivore theory comes from modern polychaetes, tiny earthworm relatives that live on the seafloor and vary in their feeding habits. Combing through data from previous studies on polychaetes, Sperling’s team examined 962 worm species from 68 locations worldwide. The researchers found a clear association: The number of carnivorous species was lower in areas with the lowest oxygen levels. In some of these regions, predatory polychaetes were completely absent.
Previously, scientists either invoked an oxygen increase or an arms race to account for the Cambrian Explosion, says Guy Narbonne, a paleobiologist at Queen’s University in Kingston, Ontario. Linking oxygen to carnivores provides strong evidence that the two explanations are “intimately interrelated,” he says.
Paleobiologist Nicholas Butterfield of the University of Cambridge sees the data differently. He thinks the rise of oxygen was actually an effect of the animals on the environment. He contends that shallow marine areas, where early animals most likely lived, were probably well oxygenated and therefore a lack of the gas did not stifle their evolution. It just took a while for a burst of complex animals to arise from simpler ones, he says. “It takes a whole lot of tinkering and experimenting and false starts until you trip over something that works.”