A new discovery challenges one of the strongest arguments in favor of the idea that animals with bilateral symmetry—those that, like us, have two halves that are roughly mirror images of each other—existed before their obvious appearance in the fossil record during the early Cambrian, some 542 million years ago. In the November 25th issue of Current Biology, a Cell Press publication, researchers report the first evidence that trace fossils interpreted by some as the tracks of ancient bilaterians could have instead been made by giant deep-sea protists, like those that can still be found at the seafloor to this day.
Protists are a diverse group of predominantly microscopic organisms. They are commonly single-celled with a single nucleus, but they may attain larger size by having many nuclei or forming colonies of identical, unspecialized cells. In the new study, the team describes macroscopic groove-like traces produced by living giant protists, known as Gromia sphaerica, which look something like a grape in terms of shape and size. Those grooves bear a remarkable resemblance to the trace fossils from the Precambrian, including ones as much as 1.8 billion years old.
"Our paper gives the precedent of a protozoan that is motile, produces macroscopic traces, and has a large hydrostatically supported body," said Mikhail Matz of the University of Texas at Austin. "With these possibilities demonstrated, pretty much anything within the Precambrian fossil record can in principle be attributed to large protozoans, from the earliest traces and fossils of the Stirling formation that are 1.8 billion years old to the weird Ediacaran biota with which the Precambrian culminated."
This new "protozoan option" takes the edge off the most compelling evidence of primitive bilaterians in the Precambrian that is so important for what has been called the "ancient school," he says. That line of thinking holds that the apparently explosive diversification of multicellular body plans during the Cambrian is an artifact of the fossil record; it suggests that bilaterians actually existed long before the Cambrian and evolved gradually over time. Others think instead that the Cambrian explosion really happened the way it appears that it did and that evolutionary mechanisms must therefore be sought to explain the rapid diversification.
"Previously one could say, 'There were traces, therefore there must have been bilaterians,' whereas now it is 'There were traces, therefore there may have been bilaterians,' which is, obviously, not nearly as strong a statement," Matz said.
He calls the findings a "classic case of scientific serendipity." They stumbled upon the giant protists while working on a project exploring the interaction between light and life in the ocean. "We were looking for pretty animals that have eyes, are colored, or glow in the dark," Matz said. "Instead, the most interesting find was the organism that was blind, brainless, and completely covered in mud."
Almost nothing is known about G. sphaerica, he added. His team is now deep sequencing the genes expressed in this giant protist and a few related protozoans to get a better idea about their evolutionary relationships to one another. They also plan to initiate a project on "deep-sea paleontology" to create a catalogue of traces produced by a variety of present-day animals. "There is surprisingly little data on this, so paleontologists have to resort to speculations a lot when interpreting fossil traces," Matz said.
The researchers include Mikhail V. Matz, University of Texas at Austin, Austin, TX; Tamara M. Frank, Harbor Branch Oceanographic Institute at Florida Atlantic University, Fort Pierce, FL; N. Justin Marshall, The University of Queensland, Brisbane, Queensland, Australia; Edith A. Widder, Ocean Research and Conservation Association, Fort Pierce, FL; and Sonke Johnsen, Duke University, Durham, NC, USA.