For a brief span of time, about 542 million years ago, the world belonged to the Ediacarans, a group of life forms so physiologically unique that biologists have considered giving them their own taxonomic kingdom.
An essay by Richard Corfield in Astrobiology Magazine points up the strange history of the Ediacarans, a group of anatomically diverse organisms that lived during the Ediacaran period (between 635 and 542 million years ago). These creatures, which predated nearly every form of animal life that exists today, stood rooted in bacterial bases on the seafloor, drawing nutrients from the water.
As best we can tell, the Ediacarans lacked mouths and recognizable digestive systems, and their bodies are thought to have looked like "sacks of mud, disks, hubcaps and mattresses." They were among the first complex life forms to appear on the planet, but they bear no discernible resemblance to anything else in the fossil record:
...they have none of the characteristics of the bilaterian animals, which evolved during the Cambrian explosion 542 million years ago. Since then, bilateral animals have provided the basic body plan for every animal that has occupied and dominated the Earth.
The Ediacarans came and went in a remarkably brief interval, geologically speaking. Corfield believes their population exploded when a mass-oxygenation of Earth's oceans coincided with the end of the Cryogenian ice age. Before long, Ediacarans had spread across the globe, as indicated by fossils everywhere from England to Australia. But the rise of mobile, bilaterian animals introduced too much competition to the oceans, and the Ediacarans died off almost as quickly as they'd flourished.
Guy Narbonne, a paleontologist at Queen's University in Ontario, recently told reporters that Ediacaran-descended life could still be with us today, in the form of certain worms and mollusks. For Corfield, though, the interesting thing about Ediacarans isn't whether they still have a presence on Earth, but what their evolutionary arc suggests about whether and how life might develop on other worlds:
the story of the Ediacarans... tells us that evolution can happen very quickly. The idea — first credited to Darwin — that vast amounts of deep time are required for evolution to occur may not be correct. The speed with which the Ediacarans arose in the aftermath of the final Cryogenian glaciation suggests strongly that the evolution of complex, multicellular organisms was on the blocks and just waiting for the starting pistol.
Fossils on the Edge of Forever [Astrobiology Magazine]