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An artist's rendering of the ancient wormlike creature called Ikaria wariootia. Source: Sohail Wasif/UCR

Rice-Sized Ancient Worms are the Ancestors of All Animals

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Geologists have discovered the first ancestor on the family tree that contains most animals today, including humans. The ancient worm-like creature , Ikaria wariootia , is the earliest bilaterian, or organism with a front and back, two symmetrical sides, and openings at either end connected by a gut. It was found in Ediacaran Period deposits in Australia and was 2-7 millimeters long, with the largest the size of a grain of rice.

A team led by UC Riverside geologists has discovered the first ancestor on the family tree that contains most familiar animals today, including humans.

The tiny, wormlike creature, named  Ikaria wariootia , is the earliest bilaterian, or organism with a front and back, two symmetrical sides, and openings at either end connected by a gut. The paper is published today in  Proceedings of the National Academy of Sciences .

Ikaria wariootia impressions in stone. (Droser Lab/UCR)

Ikaria wariootia impressions in stone. ( Droser Lab/UCR )

The earliest multicellular organisms , such as sponges and algal mats, had variable shapes. Collectively known as the Ediacaran Biota, this group contains the oldest fossils of complex, multicellular organisms. However, most of these are not directly related to animals around today, including lily pad-shaped creatures known as  Dickinsonia that lack basic features of most animals, such as a mouth or gut.

We’re All Organized Around the Same Basic Body Plan

The development of bilateral symmetry was a critical step in the evolution of animal life, giving organisms the ability to move purposefully and a common, yet successful way to organize their bodies. A multitude of animals, from worms to insects to dinosaurs to humans, are organized around this same basic bilaterian body plan.

Evolutionary biologists studying the genetics of modern animals predicted the oldest ancestor of all bilaterians would have been simple and small, with rudimentary sensory organs. Preserving and identifying the fossilized remains of such an animal was thought to be difficult, if not impossible.

For 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 of the creature that made the burrows, leaving scientists with nothing but speculation.

Finding the Ancient Wormlike Creature

Scott Evans, a recent doctoral graduate from UC Riverside; and Mary Droser, a professor of geology, noticed miniscule, oval impressions near some of these burrows. With funding from a NASA exobiology grant, they used a three-dimensional laser scanner that revealed the regular, consistent shape of a cylindrical body with a distinct head and tail and faintly grooved musculature. The animal ranged 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.

"We thought these animals should have existed during this interval, but always understood they would be difficult to recognize," Evans said. "Once we had the 3D scans, we knew that we had made an important discovery."

A 3D laser scan of an Ikaria wariootia impression. (Droser Lab/UCR)

A 3D laser scan of an Ikaria wariootia impression. ( Droser Lab/UCR )

The Oldest Fossil with this Type of Complexity

The researchers, who include Ian Hughes of UC San Diego and James Gehling of the South Australia Museum, describe  Ikaria wariootia , named to acknowledge the original custodians of the land. The genus name comes from 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 type of complexity," Droser said. " Dickinsonia and other big things were probably evolutionary dead ends . We knew that we also had lots of little things and thought these might have been the early bilaterians that we were looking for."

In spite of 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, indicating rudimentary sensory abilities. The depth and curvature of  Ikaria represent clearly distinct front and rear ends, supporting the directed movement found in the burrows.

It burrowed in thin layers of well-oxygenated sand on the ocean floor. (CC0)

It burrowed in thin layers of well-oxygenated sand on the ocean floor. ( CC0)

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 buried organic matter reveal  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."

Top image: An artist's rendering of the ancient wormlike creature called Ikaria wariootia. Source: Sohail Wasif/UCR

The article, originally titled Ancestor of all animals identified in Australian fossils ,’ was first published on Science Daily.

Source: University of California - Riverside. "Ancestor of all animals identified in Australian fossils: A wormlike creature that lived more than 555 million years ago is the earliest bilaterian." ScienceDaily. ScienceDaily, 23 March 2020.

References

Scott D. Evans, Ian V. Hughes, James G. Gehling, and Mary L. Droser. ‘Discovery of the oldest bilaterian from the Ediacaran of South Australia.’  PNAS, March 23, 2020 DOI:  10.1073/pnas.2001045117

Comments

I'm not religious, so this isn't coming from a evolution denier position. My degrees are in science.

I have a hard time believing 555 million years is enough time for this to evolve and speciate into a complex mammal. Well , stronger than that… I believe it's preposterous. In laboratories we have a hard time even showing one advantageous mutation in a colony of bacteria even with applying specific pressures and massive gene pushing stimulus. I mean sure you can have colonies of bacteria and add an antibiotic to eventually get a colony that is resistant. But that is something else. That is not adding new functioning proteins that are advantageous and that change the physiology of the organism.

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