Dec. 13 (UPI) -- Scientists have long wondered why the planet's first complex organisms emerged in the cold, dark depths of the ocean, where food and sun are in short supply.
Now, researchers at Stanford University think they've discovered the answer. According to a new study in the journal Proceedings of the Royal Society B, the deep ocean offered the animals more stable temperatures.
When developing strategies for subsisting in low-oxygen conditions, temperature stability is essential, researchers determined.
The new study is one of several efforts to interpret the early fossil record using scientists' understanding of animal physiology. In addition to detailing the origins of complex life, the analysis could help scientists better predict the species most likely to adapt to and thrive in changing ecosystems.
"Bringing in this data from physiology, treating the organisms as living, breathing things and trying to explain how they can make it through a day or a reproductive cycle is not a way that most paleontologists and geochemists have generally approached these questions," study co-author Erik Sperling, an assistant professor of geological sciences at Stanford, said in a news release.
Previously, scientists assumed the first complex animals, which emerged 570 million years ago during the Ediacaran period, could only thrive under specific temperature and oxygen conditions.
"We assumed that their ability to tolerate low oxygen would get worse as the temperatures increased," said Tom Boag, the study's lead author. "That had been observed in more complex animals like fish and lobsters and crabs."
But when researchers took a closer look at the temperature and oxygen tolerances of anemones, organisms with biologies similar to those of Ediacaran species, they determined both hotter and colder temperatures made low-oxygen living more difficult.
Low-oxygen conditions in the deep ocean did make life more difficult, as animals had to use more energy to move oxygenated sea water through their bodies. But the latest research suggests the advantages offered by stable conditions outweighed the benefits of life in the shallows, where oxygen was plentiful but temperatures could fluctuate.
Because the world's earliest complex organisms didn't have to regulate their body temperature, they were able to focus their evolutionary energies on developing efficient oxygen-processing abilities.
"The only place where temperatures were consistent was in the deep ocean," Sperling said. "That's why animals appeared there."