Scientist Identifies Mysterious Fossils as the Oldest Animals on Earth

Life took root on Earth not long after the surface cooled following the late heavy bombardment period, but it was stuck at the level of single-cell life for billions of years. The division between simple microscopic organisms and complex animals is fuzzy because few fossils exist from this era. However, scientists have multiple examples of a group of… things called Dickinsonia. These round, ribbed organisms lived more than 500 million years ago, and we finally know what they were: animals. Possibly the oldest animals in the world.

Various species belonging to the genus Dickinsonia lived between 571 million to 541 million years ago. They have several hallmark features including an oval-shaped body a few inches long with bilateral symmetry and radiating ribs that leave distinctive fossils. They were the most numerous members of a group of organisms we now call Ediacaran biota, the earliest complex multicellular organisms. Even that has been in dispute, though. Until now, we haven’t known exactly how to classify Dickinsonia.

Like all life at the time, Dickinsonia were soft-bodied and therefore didn’t fossilize often. Researchers have asserted at various times that Dickinsonia were giant single-cell protists, colonies of bacteria, land-based lichens, and more. Many have suspected that Dickinsonia were among the very first animals to exist on Earth. They may have even given rise to some of complex life that appeared during the Cambrian explosion several million years later.

It’s tough to prove what Dickinsonia were because complex molecules like proteins and DNA won’t survive half a billion years in solid rock. Australian geochemistry grad student Ilya Bobrovskiy has now produced compelling evidence that the genus Dickinsonia is indeed part of the animal kingdom.

Bobrovskiy started by digging up eight new Dickinsonia fossils from Russia’s White Sea region. His hypothesis went that even if complex molecules were long gone, it should be possible to detect simpler molecules that point to what sort of creatures Dickinsonia were. He used solvents to extract chemical markers from the fossils and characterized what came out.

The breakthrough came in the form of steroids. These organic molecules come in many varieties, but the specific type of steroid molecule can tell you about the organism that produced it. The rock around the fossils contained stigmasteroids, which are indicative of green algae. The Dickinsonia fossils contained cholesteroids, and only one kingdom of life produces cholesteroids: animals.

The early response to Bobrovskiy’s analysis is positive. Many researchers hope to apply it to other confusing Ediacaran biota. However, we still don’t know if for sure if Dickinsonia were an evolutionary dead end for animals or our incredibly distant ancestors.

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