: Around 600 million years ago, Earth's first multicellular moving animals evolved. Known as the Ediacaran fauna, these early slug- and worm-like creatures fed off microbial mats that covered the ocean floor. For years, scientists have debated how these animals kept themselves from suffocating because the ocean at the time is thought to have had less than half of its current oxygen levels. Looking at modern environments that are also oxygen-depleted, scientists have discovered that oxygen levels spike near biomats, plant-like bacteria that pump out oxygen as a waste product of photosynthesis. "We think that animals used the small but highly oxygenated zones as oases," lead author Murray Gingras told Nature, giving the world's first complex animals the kick-start they needed to evolve. "This is a really neat solution to an old problem," Ediacaran researcher Jim Gehling told New Scientist.How the Heck:
What's the News
The researchers found a proxy for the ancient, oxygen-depleted environment by traveling to lagoons in the Los Roques archipelago in Venezuela. Just like the ocean 600 million years ago, the oxygen levels in these modern-day lagoons are often less than half of the current global levels.
Despite the overall low oxygen environment, the scientists discovered that the biomats on the lagoon floor harbored as much as four times the amount of oxygen as the surface waters. This is enough oxygen to support complex life, such as a community of worms.
The researchers' theory is strengthened by the fact that past researchers have found Ediacaran fossils alongside biomats. What's more, the researchers found animal burrows in the modern-day lagoon that were similar to fossilized animal burrows from the Ediacaran period.
What's the Context:
Paleontologists think the Ediacaran fauna represent the first moving, multicellular animals because the oldest rocks with animal burrows date to roughly 600 million years ago. Although, based on fossil evidence, they've known for a while that early animals fed on microbial mats, "this is the first time that researchers have shown how mats can function as important oxygen resources for fauna."
The Ediacaran fauna thrived for millions of years but later vanished from the fossil record, replaced by the organisms of the Cambrian explosion around 542 million years ago.
Today, microbrial mats form in harsh environments that host few large animals; more large animals means more mixing of sediments, which halts microbrial growth. So the idea that early microbrial mats supplied the oxygen for multicellular animals is interesting because it means that biomats "provided the seed for their eventual demise." Once spread across wide swaths of ocean floor during the Ediacaran period, biomats diminished as more complex organisms started mixing up sediment at the start of the Cambrian.
Not So Fast: Some paleontologists point out that the early animals would have still struggled to survive at night, when the biomats would have stopped photosynthesizing and pumping out oxygen. At the present-day lagoon, some organisms, such as insect larvae, get around this hitch by hibernating at night, but there is no evidence yet that ancient organisms did the same. The Future Holds: Now the research team is studying Cambrian and Ediacaran rocks in Canada and Uruguay to understand how details of the animal-biomat-sediment interactions are recorded in the geological record. Reference: Gingras, M. , et al. "Possible evolution of mobile animals in association with microbial mats." Nature Geosci. doi:10.1038/NGEO1142