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Ancestor Of All Animals Found In A Fossil

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Ancestor Of All Animals Found In A Fossil
Artist's rendering of Ikaria wariootia. Credit: Sohail Wasif/UCR

We all have our family trees. But do you know that we all have descended from one common ancestor on the family tree that consists of the most familiar animals today, including humans?

Named Ikaria wariootia, the tiny wormlike creature is the earliest bilaterian (one with a front and a back), two symmetrical sides, and openings at either end connected by a gut.

Known as Ediacaran Biota, the group of earliest multicellular organisms, examples include sponges and algal mats, had variable shapes and contains the oldest fossil fuels. Most of these are not related to animals today.

Bilateral symmetry development was one vital step in the evolution of animal life. A broad variety of animals are organized on this basic bilateral body design. The oldest ancestor of all bilaterians would have been simple and small, with rudimentary sensory organs as predicted by evolutionary biologists studying the genetics of modern animals.

For about 15 years, scientists agreed that fossilized burrows found in 555 million-year-old Ediacaran Period deposits in Nilpena, South Australia, were made by bilaterians. But there was no sign thus making them just to speculate but do nothing.

Scott Evans, a recent doctoral graduate from UC Riverside; and Mary Droser, a professor of geology, noticed minuscule, oval impressions near some of these burrows. Using a 3D laser scanner which revealed the regular, consistent shape of a cylindrical body with a distinct head and tail and faintly grooved musculature, the animal was found to range between 2-7 millimeters long and about 1-2.5 millimeters wide, with the largest the size and shape of a grain of rice—just the right size to have made the burrows.

Ikaria wariootia was named to acknowledge the original custodians of the land. Ikara, which means “meeting place” in the Adnyamathanha language. It’s the Adnyamathanha name for a grouping of mountains known in English as Wilpena Pound. The species name comes from Warioota Creek, which runs from the Flinders Ranges to Nilpena Station.

Burrows of Ikaria occur lower than anything else. It’s the oldest fossil we get with this complexity. Despite its relatively simple shape, Ikaria was complex compared to other fossils from this period. It burrowed in thin layers of well-oxygenated sand on the ocean floor in search of organic matter, showing rudimentary sensory abilities.

The burrows also preserve crosswise, “V”-shaped ridges, suggesting Ikaria moved by contracting muscles across its body like a worm known as peristaltic locomotion. Evidence of sediment displacement in the burrows and signs the organism fed on the buried organic matter reveals Ikaria probably had a mouth, anus, and gut.

“This is what evolutionary biologists predicted,” Droser said. “It’s really exciting that what we have found lines up so neatly with their prediction.”

The paper is published today in Proceedings of the National Academy of Sciences.

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