Researchers from UC Riverside have discovered the oldest known ancestor of almost all animals, including humans – and it’s a worm. Evolutionary biologists have long been searching for fossils from the oldest multicellular organisms, but until now, none of the creatures found could have been our own first ancestors. That’s because they all lacked bilateral symmetry, in which an organism can be divided into left and right mirror images. This important property is shared by 99% of animals in existence today (excluding jellyfish, comb jellies, placozoans, and sponges), yet no one could find the first bilateral organism.

That all changed when the UC Riverside research team found tiny oval impressions in 555-million-year-old deposits in southern Australia. These impressions were only about a couple millimeters large, but the researchers were able to use a laser scanner to more precisely characterize the organisms leaving those indents. Named Ikaria wariootia, these worms show bilateral symmetry, with evidence that they contracted muscles across their body to move from one place to another. There are also signs that they would burrow through the deposits to find and eat organic matter, which suggests they might even have had some sensory capabilities.

Before this discovery, researchers had already predicted that an organism like this must have existed in that approximate time period. Of course, there’s more work left to do. The researchers must now further investigate these deposits to both confirm these findings and to shed more insight into the capabilities and behaviors of these worms. Understanding our oldest ancestors will fill in a long-missing piece of the evolutionary puzzle, and ultimately allows us to better understand our own species’ history.

Managing Correspondent: Isabella Grabski

Press Article: Ancestor of all animals identified in Australian fossils. Phys.org.

Scientific Article: Discovery of the oldest bilaterian from the Ediacaran of South Australia. Proceedings of the National Academy of Sciences.

Image Credit: Sohail Wasif/UCR