A 2,600-year-old human brain is a wonder to behold. Dubbed Heslington Brain for where it was unearthed in England in 2008, it is one of the oldest naturally preserved brains ever found in Europe and Asia. It is an oddity to be certain: human brains consist of 80 percent water and neural tissue typically degrades rapidly after death – but not so for this Iron Age man.
The brain was found inside a skull tossed into a pit dated to between 482 and 673 BCE and likely belonged to a man in his thirties. Previous examination found fractures in the neck vertebra, suggesting he died by hanging. It appears his head was then sliced off with a thin-bladed knife and his blood drained.
"The preservation of this human brain tissue remains a mystery in view of decomposition and autolysis starts within minutes after death," write the team in the Journal of the Royal Society Interface. They add that such preservation "of human brain proteins at ambient temperature should not be possible for millennia in free nature."
To deepen the mystery, open wounds often hasten the rate of putrefaction, rather than slow the disintegration process. However, a swift burial into a pit of fine-grained wet sediment may have reduced infection and slowed decomposition, note the researchers. If the ground temperature was just a few degrees above freezing, it would have taken a week before the first signs of putrefaction occurred. No signs of artificial preservation techniques such as smoking or embalming were found.
To further unravel the mystery of the Heslington brain, a team led by neurologist Dr Axel Petzold from University College London had to unfold its proteins – a process that took a year to perform. The team allowed both the Iron age brain and a modern one to decompose over the course of an entire year to see how long it took for their proteins to unfold. The modern brain dissolved but the Heslington's intermediate filaments, a form of connective structure, didn’t degrade much in the face of proteases typical after death. Instead, protein aggregates helped to increase structural integrity and thus preservation.
"The human brain is in particular need of structural stability because the key components, neurons and axons do not normally regenerate," the team write, adding that "the ability to form protein aggregate is thought to be relevant for the progression of neurodegeneration."
However, much remains puzzling. Why the filaments reacted in this way is unknown. It’s possible some sort of natural preservation compound is aiding the process. “The data suggest that the proteases of the ancient brain might have been inhibited by an unknown compound which had diffused from the outside of the brain to the deeper structures," write the team.
"There was no evidence for pathological prion proteins in the ancient brain tissue using state-of-the art methods. The quality of the DNA was too low to permit for screening of other human diseases of pathological protein aggregation or neurodegeneration. These data are consistent with the notion that DNA degrades about 10 times faster than proteins."
Further exploration could hold implications for neurodegenerative diseases in which protein aggregates are front and center, such as Creutzfeldt-Jakob disease or Alzheimer’s disease. In this way, the dead man’s brain may help extend the life of others 2,600 years post-mortem.