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.