Five hundred million years ago, strange, mouthless marine creatures called Ediacarans may have soaked up dissolved nutrients exclusively through their skin.
Today, only single-celled organisms, such as bacteria, obtain all their food through their membranes, and some larger creatures, including sponges and corals, get a fraction of their sustenance via this process.
But there are physical limitations to this strategy—namely absorbing and distributing nutrients in an ever-larger body cavity—which is why animals evolved a branching circulatory system to take nutrients digested in the gut and transport them to the places where they are needed.
And that is why the Ediacarans, which could grow to up to one meter in length and are considered Earth's first complex organisms, have been such an enigma to paleontologists.
Although these creatures show some similarities to modern-day sponges and corals, Ediacarans were entirely soft-bodied and lacked mouths or other feeding orifices, and scientists are unsure of their relationship to modern animals. They existed prior to trilobites and the animal menagerie that blossomed during the Cambrian explosion. Some Ediacarans, called erniettomorphs, possessed branching structures, whereas others, called rangeomorphs were cylindrical in shape and quilted. The latter appear to have contained pockets of sand inside their bodies.
Marc Laflamme of Yale University and colleagues wanted to test the theory that these creatures were absorbing nutrients through their skin. To do that, the researchers built computer models based on specimens that varied in size and estimated the surface area of their skin that was exposed to seawater and could potentially exchange nutrients with it.
Remarkably, the team found that the repeated branching structures allowed the erniettomorph Ediacarans to maintain a ratio of surface area to body volume as high as some of today's giant bacteria. The quilted Ediacarans, on the other hand, could only achieve such a favorable surface area–to-volume ratio if they filled their bodies with dead space that did not need to be fed—hence, the sand.
"The biology of these organisms has been up in the air," Laflamme says. "People have presented ideas and this is the one case where we modeled and tested the theories."
But what were these odd organisms eating exactly? Back then, the ocean was replete with dissolved organic carbon, which came from decaying algae and other primitive organisms that filled the oceans. "Organisms weren't packaging poop into carbon modules that will sink to the ocean floor," Laflamme explains, "and so carbon is being diffused and circulating in the water." The Ediacarans could just slurp this nutrient-rich soup up through their skin, a process called osmotrophy. The results appear this week in Proceedings of the National Academy of Sciences.
Paleontologist Guy Narbonne of Queen's University in Kingston, Ontario, says that he's "enthusiastic about the idea." Bing Shen of Rice University, who studies early Earth geochemistry, says Laflamme's theory is certainly "quite possible" and that Laflamme's nutrition models will provide constraints on how scientists interpret the lives of these cryptic critters.