Meet the granddaddy of the animal kingdom, including humans: Ikaria wariootia, a humble worm-like creature that lived an estimated 555 million years ago.

This extinct creature is the oldest known “bilaterian,” meaning its body exhibited bilateral symmetry on its left and right sides, in addition to front and rear openings (a mouth and an anus) connected by a through-gut. These features distinguish Ikaria wariootia as the first clear common ancestor of most animals alive today.

Over hundreds of millions of years, animals have diversified into a dazzling range of forms that includes humans, dinosaurs, elephants, sharks, spiders, birds, snakes, and countless other species. Despite this mind-boggling variety, all animals share the same extended lineage, inspiring scientists to probe the deepest reaches of this labyrinthine family tree.

Tantalizing traces of early animals are etched in ancient sandstone near Nilpena, South Australia, in the form of tiny fossilized burrows called Helminthoidichnites. For more than a decade, scientists have speculated about the creatures that might have dug these tunnels, but have not been able to spot their fossilized bodies.

Now, a team led by Scott Evans, a recent doctoral graduate from UC Riverside, has identified more than 100 body fossils of these ancient burrowers using 3D laser scanning, according to a study published on Monday in the Proceedings of the National Academy of Sciences.

“It was really exciting to finally find them!” Evans said in an email, adding that it was a “drawn-out process” even before his team began acquiring laser scans.

“We knew from what we could see that these were definitely fossils and that they might fit what we predicted made Helminthoidichnites and that was really the motivation for going out and getting the scanner,” Evans explained. “Once we had 3D scans, that was our 'aha' moment of fully realizing what we had and how impactful it was.”

The team revealed that the largest Ikaria wariootia animals could grow to about the size and shape of a grain of rice. The new genus is named for the word “Ikara,” which means “meeting place” in the Indigenous Adnyamathanha language. The species name refers to Warioota Creek, which runs close to the fossil site.

These burrowers weren’t the only animals on Earth during this era, which is known as the Ediacaran Period, and not all living animals today are bilaterian—for instance, sponges, jellyfish, and anemones are animals that do not share this body plan. However, the majority of animals have bilaterian bodies and Ikaria wariootia is “potentially the oldest, definitive bilaterian” found in South Australia, the study said.

In addition to pushing back the timeline of early bilaterians, the specimens may present the oldest direct evidence of scavenging in the fossil record. As they tunneled through the seafloor, the animals likely fed on algae or dead lifeforms they came across, a foraging strategy that may have helped give rise to predation.

The fossils not only offer a glimpse of Earth’s first riffs with the animal kingdom, they can also help constrain theories about the emergence of life on alien worlds. “If we want to identify potential planets that could host complex life we need to better understand the environments in which such life evolved on our planet more than a half a billion years ago,” Evans said.

Indeed, the research was funded in part by a NASA Exobiology Program Grant because of its relevance to understanding the origins of complex organisms. “We can also see that Ikaria specifically sought out oxygen rich environments and avoided oxygen poor ones,” Evans explained. “These indicate that having an appreciable amount of oxygen was really important for the evolution and success of these types of organisms and that we should be looking for exoplanets with similar, well-oxygenated conditions.”

The identification of Ikaria wariootia represents a major milestone in the reconstruction of our shared animal ancestry, and it is likely to be followed by more discoveries from the ancient Ediacaran-era rocks of South Australia, South China, and Russia, among others. Evans also hopes to reconstruct some of the “genetic machinery” that led to this influential organism.

“Work that I am currently involved with is combining the characters we can identify in Ikaria, and other animals from this period, with our understanding of the genes that produce those characters in animals today,” he said.