How to Fossilize a Brain

A new study shows that brains can can fossilize like bones

fossil brain
This image shows a Fuxianhuia protensa specimen from the Chenjiang fossil beds in southwest China. The ancient arthropod was just under 5 inches in length. Xiaoya Ma, London Museum of Natural History

For years, paleontologists thought it was impossible for organs like the brain to last long enough to become fossils. After all, squishy body parts like brains usually decompose a lot quicker than bones and teeth. But now, a group of scientists have upended this idea with the discovery of seven fossilized brains more than 500 million years old.

This isn’t the first time a fossilized brain has been discovered: Four of the scientists who worked on this new find were involved with the first ever reported finding of a fossilized brain in 2012 in the journal Nature. But at the time, many paleontologists were skeptical of the find, saying that it was more likely to be an artifact left behind by experiments or an implausibly rare one-off.

But while that study relied on a single specimen, the team has since uncovered seven new examples of fossilized brains from the same species of an ancient arthropod, according to the new study published in Current Biology.

"People, especially scientists, make assumptions. The fun thing about science, actually, is to demolish them," Strausfeld said in a statement.

In order to become fossils, the creatures were most likely buried in an underwater mudslide, like other well-preserved fossils from the Cambrian, Mo Costandi reports for The Guardian. That way, their bodies (and brains) would have been sealed off from scavengers looking for a snack as low levels of oxygen in the soil kept microbes from decomposing the carcasses.

Time alone doesn’t make something fossilize: It also takes extreme pressure to squeeze out water. This is one of the reasons that it’s much more common to find fossilized bones and teeth than fossil tissue, which tend to just pop under the pressure.

The fossils in question belong to an extinct shrimp-like creature called Fuxianhuia protensa. Discovered in southwest China’s famously fossil-rich Chengjiang Shales, the arthropods probably lived during the Cambrian Period around 520 million years ago and possessed brains similar to modern-day crustaceans, which is probably one of the reasons their brain cells survived for so long instead of being pulverized under pressure, Kiona Smith-Strickland writes for Gizmodo.

F. protensa's tissue density appears to have made all the difference," Strausfeld said in a statement.

To test this theory, Strausfeld and his colleagues examined the seven new fossils with an electron microscope, discovering their brains had been flattened over time into a thin film of carbon. Even after millennia, their neural pathways were still identifiable, co-author Xiaoya Ma tells Smith-Strickland.

The team then ran experiments to mimic the fossilization process, like burying live sandworms in clay and seawater to see whether their nervous systems survived being entombed (they did). In a similar test burying live cockroach brains, the scientists discovered they were flattened just like the fossilized F. protensa brains.

In the process of proving their find, the scientists also stumbled on some interesting clues to how modern arthropod brains might have evolved. When Strausfeld’s team first discovered the fossils, they discovered that F. protensa had a complex brain similar to some modern insects, which suggests that some arthropods regressed to a simpler nervous system over time, Costandi reports. Previously, most paleontologists believed that arthropods evolved from a clam-like species with a simple brain.

Get the latest stories in your inbox every weekday.