World’s oldest animal fossil actually came from rotting algae

Hundreds of millions of years ago, one of the very first animals on Earth died at the bottom of an ancient ocean. In life, it was a humble sea sponge; in death, it had no bones, nor teeth, nor shell to leave behind as evidence of its brief, bottom-dwelling existence. But it did have fat molecules —  or so it seemed.

In 2009, a group of much later animals (human scientists) were studying a slab of ancient sea sediment when they discovered the fossilized remnants of what appeared to be those same sponge fat molecules, trapped among the rocks. The sediment dated to 635 million years ago —  roughly 100 million years earlier than the oldest confirmed animal fossil on record  — but the ancient molecules were unmistakably biological in origin, and matched those found in modern sponges, the researchers wrote. As more and more of these biomarkers were discovered across ancient seafloor samples, a question emerged: How could these early sponges be so widespread, and yet not leave a single body fossil behind?

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