Our oldest ancestor and precursor to all animal-life, a 555-million-year-old worm?

New research may have solved one of the biggest mysterious in bio-geology by identifying the ancestor of all animal life: a small, worm-like creature that is over half a billion years old.

Sponges and algae are known to be the earliest multicellular organisms, but as they lack basic physiological features such as a mouth or gut, they are not considered to be the ancestors of animal life. The true precursor for animal life lies with the first bilateral organism.

The evolution of bilateral symmetry (symmetrical halves on both sides of a unique plane) was one of the most important early steps in the development of complex animal life. These early bilateral organisms would have been extremely simple with primitive sensory organs and basic locomotive mechanisms. Due to their physiology and the challenges that come with identifying specific features from primordial sediment, scientists have struggled to identify viable fossils or preservations of early bilateral organisms.

15 years ago, in Nilpena, South Australia, researchers discovered fossilised burrows in sediment that were carbon-dated to be 555-million-years old (from the Ediacaran Period). They believed that these burrows must have been created by some type of early bilateral organism; however, there was no technology available at the time that could distinguish individual features of the burrow that could lead to the identification of this mystery creature.

This was until University of California, Riverside doctorial graduate, Scott Evans, and professor of geology, Mary Droser, identified miniscule, oval impressions near some of the burrows. Through a grant from the NASA exobiology programme, they were able to use a three-dimensional laser scanner to reveal the body of this unknown burrowing creature.

The researchers identified a curved, cylindrical body, approximately the size of a grain of rice with faintly grooved musculature and a distinct head and tail. "We thought these animals should have existed during this [time] 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."

They have since named the creature Ikaria wariootia, from the genus 'Ikara' (meaning "meeting place" in the Adnyamathanha language of South Australia's indigenous people) and the Warioota Creek which runs from the Flinders Ranges to Nilpena Station (near where the fossils were originally discovered).

Despite its simple shape, Ikaria was incredibly complex for its geological time period. The research suggests that that it burrowed into thin layers of well-oxygenated sand on the primordial ocean floor in search of organic matter to feed on. This itself indicates that it had primitive sensory abilities as well as a developed mouth, gut and anus.

"Burrows of Ikaria occur lower than anything else. It's the oldest fossil we get with this type of complexity," Droser said. "We knew that we also had lots of little things and thought these might have been the early bilaterians that we were looking for."

The size and curvature of the creature also represents distinct front and rear ends, supporting the movement patterns of sediment that was found in the burrows. Furthermore, the openings of these burrows have V-shaped ridges, suggesting that Ikaria moved by contracting muscles across its body like a worm, known as peristaltic locomotion. "This is what evolutionary biologists predicted. It's really exciting that what we have found lines up so neatly with their prediction" said Droser.

The scientists want to continue their search for fossils from this primordial era in the hope that the future refinement of new, modern technology like the three-dimensional laser will bring more incredible discoveries like Ikaria wariootia - the possible ancestor of all animal life on planet Earth today.

Reference:

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