Archaeologists Keep Finding Preserved Human Brains. But How Do the Organs Remain Intact?

If they’re lucky, archaeologists find human bones or teeth that have been preserved for hundreds or thousands of years. But they very rarely find tendons, muscles and skin, because soft tissue tends to disintegrate over time. However, this rule comes with one big exception: human brains.

More than 4,400 preserved human brains—some that are 12,000 years old—have been discovered across much of the world, according to a new paper published Wednesday in the journal Proceedings of the Royal Society B. And in more than 1,300 cases, the brain was the only soft tissue to survive—often, without an obvious explanation.

Study co-author Alexandra Morton-Hayward, now a paleobiologist at the University of Oxford in England, first became intrigued by brains while working as an undertaker. She noticed that brains typically decomposed faster than other organs, turning to liquid and leaving behind only an empty skull. But sometimes, the “brain was still perfect, like a jelly,” she tells Science’s Andrew Curry.

Later, when she became a PhD student, Morton-Hayward read about intact brains found inside the skulls of 8,000-year-old human skeletons in Florida. Intrigued, she decided to go looking for more papers that referenced preserved human brains.

In her search, she repeatedly came across language that described finding preserved brains as a rare and unusual phenomenon. But as she read more and more papers—and brought specimens into her lab—she realized they’re much more common than previously thought.

For the new paper, Morton-Hayward and her colleagues cataloged the known preserved human brains that had been described in the archaeological record. Creating this comprehensive database allowed them to zoom out and look for patterns.

Preserved brains look like “normal, perfect, fresh” human brains, except they’re usually about a fifth of the usual size, Morton-Hayward tells Science. They typically have a “tofu-like consistency,” she adds. They’ve been found with bog bodies and Incan human sacrifices on top of South American volcanoes, as well as in medieval cemeteries, in Egyptian necropolises and in mass graves from the Spanish Civil War.

Care to see a carload of preserved #brains? Our latest sampling trip unearthed 162 new beauties - now in the fridges @OxUniEarthSci, after 100s of years in the ground! ... Can we stop calling them 'unique' and 'exceptionally rare' finds in yet? #Archaeology pic.twitter.com/prUUh0VoC7

— Alexandra Morton-Hayward (@MortonHayward) August 28, 2022

In many cases, the brain’s preservation could easily be explained by known forces, such as dehydration or freeze-drying. Nearly 38 percent were dehydrated, and 30 percent were saponified, a chemical process that produces a preservative substance known as grave wax from fats in the body. Less than 2 percent were frozen, and less than 1 percent were tanned.

But a little more than 30 percent of the brains were preserved by some mysterious, yet-to-be-discovered process.

“This unknown mechanism is completely different,” Morton-Hayward tells New Scientist’s Michael Le Page. “The key feature of it is that we only have the brain and the bones left. There’s no skin, no muscle, no gut.”

Researchers don’t know what happened to those brains, but they do know they were found in a wide array of different settings and climates—from shipwrecks and lead coffins to burial mounds and shallow graves.

“The absence of unifying environmental factors points to something in the brain itself,” that allows for its preservation, Morton-Hayward tells El País’ Miguel Ángel Criado.

One possibility is that the presence of certain substances, such as iron, at the archaeological sites might be triggering a chemical reaction that makes the brain tissue more stable. Researchers still need to test that theory, but if it’s true, it might offer new insights into neurodegenerative diseases like dementia, per New Scientist.

More broadly, the new paper suggests archaeologists should be on heightened alert for preserved brains when conducting field research. These organs can be easily overlooked, because they’re often the same color as their surroundings.

“It is therefore highly likely that brain material is frequently discarded during archaeological excavation as it is not recognized for what it is,” says Brittany Moller, an archaeologist at James Cook University in Australia who was not involved in the research, to New Scientist.

Since the work also shows that preserved brains are not as uncommon as previously thought, it might also encourage researchers to analyze the organs, rather than keeping them carefully tucked away. Right now, less than 1 percent of the preserved brains have been studied.

“If they’re precious, one-of-a-kind materials, then you don’t want to analyze them or disturb them,” Morton-Hayward tells Science News’ Nora Bradford.

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Sarah Kuta is a writer and editor based in Longmont, Colorado. She covers history, science, travel, food and beverage, sustainability, economics and other topics.