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First Ever Bilaterian Discovered

Humans, it’s been said, are like donuts. They have an opening at each end, and a single continuous hole running through their middle. (Note: This theory has yet to appear in a peer-reviewed journal.)

It’s a crude simplification of our species, sure, but look far enough back on the animal family tree and you’ll find an ancestor organism that’s little more than a digestive tract with some meat wrapped around it. Limbless and hungry like a sentient macaroni, this ancient creepy-crawler was the first bilaterian — an organism with two symmetrical sides, a distinct front and back end, and a continuous gut connecting them.

While bilaterians run rampant today (insects, humans and most other animals among them), the identity of that progenitor organism has long eluded discovery. Now, researchers believe they’ve found it in the fossil record for the first time.

In a study published March 23 in the journal Proceedings of the National Academy of Sciences, a team of scientists analyzed a chunk of rock containing an ancient undersea burrow found deep below Australia. They found several fossil organisms preserved near the burrows, each creature about the size and shape of a grain of rice and dating to roughly 555 million years ago.

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