Hundreds of millions of years ago, before animals began to emerge en masse during the Cambrian period, the Earth’s seas were filled with mysterious, soft-bodied organisms known as “Ediacara biota.”
The first Ediacaran fossils were discovered in 1946, and ever since then, paleontologists have been grappling with how to classify these strange creatures. Some experts think Ediacarans were algae, others believe they were fungi, and still others have posited that they were a distinct kingdom of life unrelated to anything living today. Ediacarans have been described as a “failed experiment” in evolution, since they were believed to have died out before the emergence of animals. But as Colin Barras reports for Science, new research suggests that Ediacarans may have actually been the first animals to appear on Earth.
“Ediacara biota” is a collective name for a large group of around 200 types of fossils that have been found across the globe. Ediacaran fossils are diverse in appearance: some resemble “simple blobs,” some look more like worms, and some have an unusual, plant-like appearance—with branched fronds that take the form of fractals and subunits replicating the pattern of the entire frond itself—but have in fact been categorized as animalia. These creatures are believed to have died out just before the “Cambrian explosion” around 541 million years ago, when most major animal groups began to appear.
But a new study published in the journal Palaeontology offers evidence to suggest that Ediacarans may have survived into the Cambrian period. Jennifer Hoyal Cuthill of the Tokyo Institute of Technology and the University of Cambridge and Jian Han of Northwest University in Xi’an, China, noticed similarities between the plant-like Ediacarans and a type of marine creature called Stromatoveris psygmoglena.
Found only in China’s Chengjiang county, Stromatoveris psygmoglena is a Cambrian-era animal. After examining 200 Stromatoverisi fossils, Hoyal Cuthill and Han concluded that the creature has a very similar anatomy to seven members of the Ediacara biota. Like these Ediacarans, Stromatoverisi have “multiple, branched fronds which radiate outwards like seaweed,” Hoyal Cuthill writes in the Conversation.
The researchers also used a computer analysis to determine the evolutionary relationship between Ediacarans and a number of other groups, including Stromatoveris psygmoglena. They found that Ediacarans and Stromatoverisi belonged to their own branch on the evolutionary tree of life, which has been named “Petalonamae.” The analysis also revealed that Petalonamae are distinct from any other living animal group. But, according to Hoyal Cuthill and Han, both Ediacarans and Stromatoverisi were indeed animals.
“[W]e found that Stromatoveris psygmoglena provides a crucial link between the older period and the animals which appeared in startling number and diversity during the Cambrian period,” Hoyal Cuthill writes.
The new study has been met with some doubts; for instance, Simon Darroch, a geobiologist at Vanderbilt University, tells Barras that he is not entirely convinced Ediacarans and Stromatoverisi have the same fractal architecture. But Hoyal Cuthill and Han’s findings could have major implications for our understanding of evolutionary history. If Ediacarans can be correctly classified as animals, that means animals began to diversify some 30 million years before the Cambrian explosion; the earliest frond-like Ediacarans appear in the fossil record 571 million years ago.
“This could mean that the petalonamids adapted more successfully to the changes of the Cambrian period than had been thought,” Hoyal Cuthill writes in the Conversation, “or that the Ediacaran period and its animals were less alien and more advanced than previously realized.”