The human brain is an amazing thing – especially when you consider its humble beginnings. And a new discovery may have pushed those beginnings even further back into the historical record: it is, the researchers responsible believe, “the oldest fossilized brain we know of, so far.”

So says Nicholas Strausfeld, a Regents Professor in the University of Arizona Department of Neuroscience and study leader, in a statement. And the creature responsible for this update to the science textbooks? A tiny, half-billion-year-old creepy crawly named Cardiodictyon catenulum.

It is – or was, since it’s been extinct for at least a good 250 million years now – a type of armored lobopodian, an ancient group of animals that used to scuttle around the Cambrian-era sea floor on dozens of soft, stubby legs. The individual specimen responsible for this new discovery is perhaps even less assuming: originally discovered in 1984, it’s barely half an inch long, or less than 1.5 cm, and had been basically minding its own business for the last four decades or so.

That makes it a surprising place to have found such an important discovery. “Until very recently, the common understanding was ‘brains don’t fossilize,’” said co-lead researcher Frank Hirth, a reader of evolutionary neuroscience at King’s College London.

“So you would not expect to find a fossil with a preserved brain in the first place,” he explained. “And, second, this animal is so small you would not even dare to look at it in hopes of finding a brain.”

But with the discovery of a tiny, perfectly preserved nervous system, brain included, inside of the fossilized critter, a century-old debate may have finally been resolved. That’s because of one very particular aspect of Cardiodictyon: its non-segmented head.

“From the 1880s, biologists noted the clearly segmented appearance of the trunk typical for arthropods, and basically extrapolated that to the head,” Hirth said. “That is how the field arrived at supposing the head is an anterior extension of a segmented trunk.”

Cardiodictyon does indeed have a segmented body, throughout which can be seen repeating arrangements of neural structures known as ganglia. But its head, and its brain, are not – which kind of clashes with the picture scientists had of brain evolution in arthropods.

“This anatomy was completely unexpected because the heads and brains of modern arthropods, and some of their fossilized ancestors, have for over a hundred years been considered as segmented,” Strausfeld explained. “But Cardiodictyon shows that the early head wasn’t segmented, nor was its brain, which suggests the brain and the trunk nervous system likely evolved separately.”

But the identification of the Cardiodictyon brain wasn’t the only breakthrough found in the study. The ancient brains may have looked dramatically different from those of modern arthropods – that is, the most species-rich group in the animal kingdom which comprises insects, crustaceans, arachnids, millipedes and centipedes, and so on – but as detailed anatomical studies and genomic analysis showed, they were pretty similar under the hood.

“By comparing known gene expression patterns in living species, we identified a common signature of all brains and how they are formed,” Hirth said. “We realized that each brain domain and its corresponding features are specified by the same combination genes, irrespective of the species we looked at.”

“This suggested a common genetic ground plan for making a brain,” he added.

So, after resolving a decades-old dispute about an epochs-old mystery, what message do the researchers want us to take from their discovery? Perhaps surprisingly, a warning for the future.

“At a time when major geological and climatic events were reshaping the planet, simple marine animals such as Cardiodictyon gave rise to the world’s most diverse group of organisms,” said Strausfeld. 

“The euarthropods… eventually spread to every emergent habitat on Earth,” he said. But thanks to human-made climate change, he cautioned, they “are now being threatened by our own ephemeral species.”

The study was published in the journal Science.