A 2,600-year-old human skull from the Iron Age was unearthed in a muddy pit back in late 2008. Upon closer examination, the soil-caked cranium contained an unusual yellow substance that turned out to be Britain’s oldest surviving human brain. Now, researchers have figured out how this remarkable brain had been preserved so well and for so long: The mud provided an oxygen-free burial.

The York Archaeological Trust was commissioned by the University of York to excavate in Heslington East for a planned expansion of the campus. The discovery of the skull—complete with a jaw and two vertebrae still attached—was made in an area that had been farmed and developed since at least 300 BC. As Rachel Cubitt from the trust was cleaning the skull, she noticed something loose inside. “I peered though the hole at the base of the skull to investigate and to my surprise saw a quantity of bright yellow spongy material,” Cubitt recalls in a news release. “It was unlike anything I had seen before.”

The survival of ancient brains is rare enough, since its fatty tissue is typically absorbed (or feasted on) by microbes in the soil. Furthermore, “the survival of brain remains where no other soft tissues are preserved is extremely rare,” Sonia O’Connor from the University of Bradford said in a statement at the time. She helped confirm that it was, indeed, a brain.

Next, Philip Duffey at the York District Hospital took CT scans of the skull. “It's exciting that scanning has shown structures which appear to be unequivocally of brain origin," he told BBC in 2008. To the right is a representation of the skull generated from those CT scans. "There is something unusual in the way the brain has been treated, or something that it's been exposed to that has preserved the shape of it,” he added.

In the years since, dozens of researchers have studied the brain. Radiocarbon dating a sample of the jawbone determined that the skull belonged to a person who lived around 2,600 years old. The teeth and the shape of the skull suggests that it was a man between 26 and 45 years old. Based on the vertebrae, he was struck hard on the neck, and then decapitated with a small sharp knife.

The severed head must have been immediately buried in a pit of wet, clay-rich ground. For decomposition and rotting to occur, there needs to be water, oxygen, and a suitable temperature for bacteria to be active—but if even one of these things is missing, preservation occurs instead. The hair, skin, and flesh outside of the skull rotted away as usual, but the inside was preserved, thanks to the sealed, oxygen-free burial environment provided by the fine-grained sediment. The fats and proteins of the brain tissue must have linked together to form a mass of complex molecules, and while the brain shrank, no new oxygen meant that its shape and various microscopic features were preserved.

Images: York Archaeological Trust