The honor goes to a minuscule worm-like creature that lived on the seafloor 555 million years ago. Researchers led by geologists at the University of California (UC) Riverside have identified it as the first known bilaterian, an organism with a front and back, symmetrical sides and front and back openings with a gut in between.
"It's the oldest fossil we get with this type of complexity," UC Riverside geology professor Mary Droser said in a press release.
Bilaterians are an important piece of the evolutionary tree that branches out to include the full diversity of animal life. The new body structure allowed creatures to move forward purposefully.
The fossil was dated to the Ediacaran Period 555 million years ago, when multi-celled, complex lifeforms began to emerge, BBC News explained. But other fossils found in the "Ediacaran Biota" are not directly related to today's animals. These include Dickinsonia, organisms shaped like lily pads that have no mouth or gut, according to UC Riverside.
Geologists thought that bilaterians must have been alive during this period, but direct fossil evidence was hard to find. Researchers had thought for 15 years that these creatures were the explanation for fossilized burrows found in Ediacaran deposits in Nilpena, South Australia, but there was no definitive proof.
Then, Droser and UC Riverside doctoral graduate Scott Evans honed in on barely discernible oval impressions near the ends of the burrows. They used a three-dimensional scanner to fill in the impression and a body emerged — a grain-of-rice sized cylindrical creature about 2-7 millimeters long and about 1-2.5 millimeters wide.
"Once we had the 3D scans, we knew that we had made an important discovery," Evans said in the press release.
Scientists called the creature Ikaria wariootia and introduced it to the world in an article published in Proceedings of the National Academy of Sciences Monday. The name comes from the language of the Adnyamathanha people, an aboriginal group who live in the part of Australia where the fossil was found, CNN reported. Ikaria means "meeting place" and Warioota is the name of a local creek.
But the creature didn't live by the riverside. Instead, it spent its life burrowing in sand on the ocean floor in search of food, BBC News described.
The creature is also important for another reason. Evolutionary biologists had long predicted early bilaterians would be small and simple like Ikaria wariootia, according to the press release.
"This is what evolutionary biologists predicted," Droser said. "It's really exciting that what we have found lines up so neatly with their prediction."