Geologists say they have discovered a fossil of the first ancestor on the family tree that contains most animals today, including humans. The fossil of the extinct worm-like creature was discovered in deposits in Nilpena in South Australia belonging to the Ediacaran Period, an interval of geological time ranging 635 to 541 million years ago.
The tiny creature, called Ikaria wariootia, lived about 555 million years ago and was about the size of a grain of rice. According to the study, published March 23, 2020, in the peer-reviewed journal Proceedings of the National Academy of Sciences, it is the earliest bilaterian, or organism with a front and back, two symmetrical sides, and openings at either end connected by a gut.
The development of bilateral symmetry, scientists say, 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.
For the new research, University of California/Riverside scientists Scott Evans and Mary Droser used a 3-D laser scanner to analyze miniscule oval impressions near fossilized burrows found in 555 million-year-old Ediacaran Period deposits in Nilpena, South Australia. Their research 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 (up to about a quarter of an inch) long and about 1-2.5 millimeters (up to about a tenth of an inch) wide, just the right size to have made the burrows. Evans said in a statement:
We thought these animals should have existed during this interval, but always understood they would be difficult to recognize. Once we had the 3-D scans, we knew that we had made an important discovery.
According to a statement from the researchers:
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.
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.
Bottom line: Geologists have discovered the fossil of a worm-like creature that they say is the first ancestor on the family tree that contains most animals today, including humans.
Eleanor Imster has helped write and edit EarthSky since 1995. She was an integral part of the award-winning EarthSky radio series almost since it began until it ended in 2013. Today, as an EarthSky.org Editor, she helps present the science and nature stories and photos you enjoy. She and her husband live in Tennessee, where they enjoy guitar playing and singing. They have 2 grown sons.