Wormlike Creature That Lived 555 Million Years Ago May Be the Ancestor of Almost All Living Animals

Scientists may have discovered one of the earliest ancestors on the family tree—a "simple and small" wormlike critter named Ikaria wariootia after the Adnyamathanha word for "meeting place."

Ikaria was a bilaterian, like dogs, dinosaurs and almost all living animals, including humans. This means it had a front, a back, two symmetrical sides and openings at both ends connected to one another by a gut.

There are very few animals around today that do not fit this mold, comb jellies and cnidarians being two exceptions, but that wasn't always the case. According to geologists writing in Proceedings of the National Academy of Sciences, the development of bilateral symmetry was a crucial evolutionary step.

The Ediacaran Period, 635 to 542 million years ago, marked an important biological transition for the planet, as simple microscopic organisms made way for more complex lifeforms.

The Ediacaran Biota describes the earliest multicellular organisms from sponges and algal mats to Dickinsonia—creatures shaped like lily pads that contained no mouth and no gut. But missing from the record was the oldest bilaterian; thought to be a small, simple animal with only rudimentary sensory organisms.

The closest evolutionary biologists came to discovering the remains of this mysterious creature were the traces of burrows (called Helminthoidichnites); lines, hoops and squiggles that show their movements. But thanks to advances in 3D scanning, geologists have been able to detect not just one but dozens of cylindrical-shaped bodies in sandstone deposits at Nilpena, South Australia—a single bed surface contained 108 Ikaria with heads, tails and grooved musculature.

"This is what evolutionary biologists predicted," Mary Droser, Professor of Geology at the University of California Riverside, said in a statement. "It's really exciting that what we have found lines up so neatly with their prediction."

The Ikaria identified were small and simple, as predicted—ranging 1.9 to 6.7 millimeters in length and 1.1 to 2.4 millimeters in width. The researchers believe the largest individuals would have reached no more than 7 millimeters, making them no bigger than a grain of rice. Their size, they say, matches up perfectly with the Helminthoidichnites.

But despite their simplicity from today's standpoint, Ikaria were ahead of their time and "remarkably complex" in comparison to their peers. Their ability to dig into and burrow along well-oxygenated sand on the seabed to find food shows they had senses, even if those senses were incredibly primitive. The authors also point to evidence in the fossil record—including displaced sediment—which suggests the species had a mouth, a gut and an anus, as well as the fact that they appear to have eaten organic matter.

The shape of the burrows even offers clues into how they moved. The researchers suspect they contracted their muscles to move forward, like worms do today.

"It's the oldest fossil we get with this type of complexity," Droser explained. "Dickinsonia and other big things were probably evolutionary dead ends."

The creature has been named Ikaria wariootia, in recognition of the indigenous people and original custodians of the land where the deposits were found. Ikaria translates to "meeting place" and refers to a group of mountains in South Australia, while wariootia references the nearby Warioota Creek.

Dr. Tim Ewin, Senior Curator in the Earth Sciences department of the Natural History Museum in London, says the organism described fits our current best hypothesis of what early bilaterians looked like—"It is a fascinating glimpse of what our earliest ancestor might have looked like," he told Newsweek.

The article has been updated to include comments from Dr Tim Ewin.