New research has identified Isteuria Warioa tiny wormlike creature from more than 555 million years ago, as the earliest known bilaterian animal. This discovery, detailed in a study published in Proceedings of the National Academy of Sciencessheds light on a crucial stage in animal evolution during the Ediacaran period. Found in South Australia, this fossil provides key evidence of the development of bilateral symmetry — a defining characteristic of the vast majority of animals alive today, including humans.
Ikaria Wariootia: A Glimpse Into the Earliest Bilaterians
Bilaterians are animals that develop bilateral symmetry, meaning their bodies have distinct left and right sides that mirror each other. This body plan includes a defined head, tail, back, and belly, allowing for purposeful movement and complex organization. Scientists have long theorized that the oldest ancestor of bilaterians would be small and simple, with basic sensory organs, but until now, no fossil evidence had confirmed this. Ikaria wariootia fills this gap, measuring only 2 to 7 millimeters long, resembling a grain of rice in size.
“We thought these animals should have existed during this interval, but always understood they would be difficult to recognize. Once we had the 3D scans, we knew that we had made an important discovery,” Dr. Scott Evans from the University of California, Riverside, explained. The team used advanced 3D laser scanning to reveal the fossil’s cylindrical body with clear bilateral symmetry and signs of musculature, marking a significant breakthrough in identifying early bilaterians.
Fossilized Burrows Link Ikaria Wariootia to Purposeful Movement
The discovery is closely tied to fossilized burrows called Helminthoidichnitesfound in the same geological layers in Nilpena, South Australia. For over 15 years, paleontologists suspected these burrows were made by bilaterians, but the maker remained a mystery. The size and shape of Ikaria wariootia perfectly match these burrows, supporting the idea that this creature actively burrowed in oxygenated ocean-floor sand in search of organic matter.
“Burrows of Ikaria wariootia occur lower than anything else. It’s the oldest fossil we get with this type of complexity,” said Professor Mary Droser. The fossil evidence also shows V-shaped ridges in the burrows, suggesting that Ikaria moved by peristaltic locomotion, contracting muscles across its body much like modern worms. This mode of movement indicates a level of coordination and sensory input that was previously undocumented in such early animals.
Implications for Understanding Ediacaran Fauna and Animal Evolution
This finding also reshapes how scientists view other Ediacaran organisms. Large, iconic creatures like Dickinsonia have been seen as evolutionary dead ends without descendants. In contrast, the smaller, simpler organisms like Ikaria might represent the earliest ancestors of all bilaterians, the lineage that led to most modern animals.
“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,” Professor Droser said. The discovery of Ikaria wariootia bridges a gap between genetic predictions and fossil evidence, confirming that early bilaterians had the body plan and capabilities needed for complex behavior such as directed movement and burrowing.
