This worm-like creature is the first ancestor on the human and animal family tree

(CNN)Evidence of a worm-like creature about the size of a grain of rice has been uncovered in South Australia, and researchers believe it is the oldest ancestor on the family tree that includes humans and most animals. The creature lived 555 million years ago. It's considered to be the earliest bilaterian. Bilaterians are organisms with a front, back, two openings on either end and a gut that connects them.

They were an evolutionary step forward for early life on Earth. Some of the oldest life on Earth, including those sponges and algal mats, is referred to as the Ediacaran Biota. This group is based on the earliest fossils ever discovered, providing evidence of complex, multicellular organisms. But those aren't directly related to animals living today.

And researchers have been trying to find fossilized evidence of the common ancestor of most animals. Developing bilaterian body structure and organization successfully allowed life to move in specific, purposeful directions. This includes everything from worms and dinosaurs to amphibians and humans. But for our common ancestor, they knew that fossils of the tiny, simple creatures they imagined would be nearly impossible to find because of its size and soft body.

Then, they turned to fossilized burrows, dated to the Ediacaran Period some 555 million years ago, found in Nilpena, South Australia. For 15 years, scientists knew they were created by bilaterians. But there was no evidence of what made the burrows and lived in them. That is, until researchers decided to take a closer look at the burrows.

Geology professor Mary Droser and doctoral graduate Scott Evans, both from the University of California, Riverside, spotted impressions shaped like ovals near the burrows. A 3-D laser scan revealed the impressions contained evidence of a body shaped and sized like a rice grain, with a noticeable head, tail and even V-shaped grooves suggesting muscles. Contractions of the muscles would have enabled the creature to move and create the burrows, like the way a worm moves.

“Patterns of displaced sediment and signs of feeding led the researchers to determine that it had a mouth, gut and posterior opening.”