Why some human brains don’t break down after thousands of years

Research papers often describe human brain fossils as exceptionally rare. But a new study that amasses data from thousands of preserved brains suggests that’s not the case. In this trove, scientists have identified five processes that preserve this soft tissue, in some cases up to 12,000 years (Proc. R. Soc. B 2024, DOI: 10.1098/rspb.2023.2606).

“Everyone decays differently and different parts of the body decay differently,” says Alexandra Morton-Hayward, a forensic anthropologist at the University of Oxford. In previous work as an undertaker and embalmer, Morton-Hayward saw that human brains usually liquefied within days. But as she came across reports of preserved brains in the archaeological record, she questionedwhether these samples were actually rare.

Morton-Hayward and her colleagues searched 400 years of literature for archaeological reports of discolored, shrunken brains. They paired records of 4,405 brains from 213 sources with information about the conditions the specimens were exposed to, including climatic data such as temperature and precipitation. This revealed trends about five mechanisms by which brains can be preserved for hundreds and sometimes thousands of years.

Frozen brainshad the shortest longevity, with most enduring less than 500 years. In temperate peat bogs, brains were found preserved for up to 2,800 years. The mechanism at work there is tanning, which occurs when compounds from sphagnum moss react with free amino groups in decaying tissue. Dehydrated brains, mostly discovered in sweltering deserts, persisted for up to 5,200 years. Some brains in the literature were identified as saponified, a process that transforms triglycerides, a group of fats, into a smelly substance known as grave wax.

These four processes—freezing, tanning, dehydrating, and saponifying—are well known to forensic anthropologists, Morton-Hayward says. But the team found a fifth preservation type that included some of the oldest brains. In these cases, usually found in watery environments, brains were the only preserved soft tissue reported. “There’s no skin, no eyes, no liver, no lungs, nothing—just a brain rattling around in a skull,” Morton-Hayward says.

This prompted the researchers to consider what might be special about neural tissues. “The brain has a really unusual ratio of proteins to lipids. It’s almost one-to-one with very little carbohydrate component,” Morton-Hayward says. Most other organs contain more sugars. And the brain’s transmembrane proteins, which play roles in signaling, are rich in sulfur-containing cysteine and methionine. Those amino acids would be susceptible to cross-linking with lipids—an action that would form bonds to make large stable molecules in a way that researchers have found in even older vertebrate soft tissues. This process may be catalyzed by iron that accumulates in the brain during life, the authors suggest.

“This is a really important overview,” says Jasmina Wiemann, a molecular paleobiologist at the Field Museum of Natural History who was not part of the work. Seemingly vulnerable tissues may be longer lasting than expected. Many questions remain to investigate, including the roles of transition metals and environmental factors. “What everyone really would love to see is chemical data,” she says.

The prevalence of preserved human brains doesn’t surprise Mary H. Schweitzer, a molecular paleontologist at North Carolina State University who has found soft tissue in far older specimens, including dinosaur fossils. “I think if we really look, we’re going to find all kinds of things in the fossil record that we have been told are not possible,” says Schweitzer, who wasn’t part of the work. She suspects that archaeologists may not have been looking hard enough for soft tissues, such as blood vessels, which she found preserved in a fossilized bone of a Tyrannosaurus rex.

The new work suggests that there are many brains waiting to be found. “They’re quite obvious when you find them. They have these incredible rusty orange and red and yellow surface colors,” Morton-Hayward says. Analyzing these brains could provide new clues about how humans lived and died. The cross-linking mechanism creates an ideal way to preserve proteins and genetic information for millennia, she says. And her team is starting to analyze the proteome that’s locked up in these brain fossils.

That examination could provide insights into the spread of epidemics or about the health and diet of individuals, Morton-Hayward says. “These things could be a huge, valuable resource of biomolecular information.”