If I tell you that scientists have studied the nervous system of a creature that’s half a billion years old, it’s hard to appreciate what that means. Half a billion years is, to paraphrase Douglas Adams, a vastly, hugely, mind- bogglingly big span of time, when even last week seems like an age ago.

So here’s (a concise history of) what happened since a little creature called Alalcomenaeus died:

Its body sinks to the ocean floor, gets covered in sediment and slowly turns into a stony fossil. Meanwhile, all the world’s land has time to glom together into a mega-continent called Pangaea before breaking up again. Life, was restricted to the oceans, invades the land. Plants and fungi go first, producing thin coverings of mosses and lichens and eventually giant forests. The insects appear, and take to the skies. Other marine animals evolve familiar traits like bones and jaws, and their descendants diversify across the land. Dinosaurs come, see and conquer, before (mostly) dying out. Mammals get their day and one of them, armed with technology and knowledge, unearths Alalcomenaeus from its ancient resting place in what is now China.

As I said: a vastly, hugely, mind-boggling big span of time. Lots happened.

And through all of it, the nervous system of this buried animal remained intact.

A team of scientists have now reconstructed it. There it is in the images above and below —a network of nerves that drove an animal’s behaviour in a time before life on land.

View Images Different images of Alalcomenaeus. (a) is the fossil. (b) is an outline of iron desposits. (c) is a CT scan. (d) is the previous two overlaid on each other. (e) is the nervous system.

This is the second such discovery. The first was published last year, when Xiaoya Ma and Nicholas Strausfeld described the brain of a 520-million-year old animal called Fuxianhuia protensabrain of a 520-million-year old animal called Fuxianhuia protensa. It consisted of three clusters of nerves (ganglia) that had fused together. Nerves from the second ganglion reached into the creature’s antennae, while nerves from the third one led into a pair of claws. Each of the animal’s eyes was served by three further nerve bundles, known as optic lobes.

“In other words, the specimen had a brain like that of a modern crustacean,” says Strausfeld. Fuxianhuia was clearly an early relative of modern crabs, lobsters and shrimp—a relationship that was unclear from its body alone.

But the team saw the result as just half of a bigger story. Today, the arthropods—successful animals with hard external skeletons and jointed legs—split into two major groups. There’s the mandibulates, which includes all insects, crustaceans, centipedes and millipedes. And there’s the chelicerates, which includes spiders, scorpions and horseshoe crabs. The nervous systems of the two groups look very different. Fuxianhuia exemplified the mandibulate pattern. What about the chelicerate one?

“Last year, we speculated that one day in future, [someone should find] a fossil that would show evidence of a chelicerate brain,” says Strausfeld. “To our surprise, that fossil turned up the very next year.”

The inch-long specimen was found at the fossil-rich Chengjiang site in southwest China. It was clearly an Alalcomenaeus, which one of the most widespread arthropods at the time. The large, claw-like appendages on its head gave it and its relatives their name—the megacherians, meaning “great hand”.

Gengo Tanaka from the Japan Agency for Marine-Earth Science and Technology created a 3-D model of the specimen using a CT-scanner, while using an X-ray microscope to measure the distribution of chemical elements in its body. Iron was especially informative, and revealed the outline of the nervous system but not other tissues like muscles. (“We don’t know why, and I doubt if anyone else does,” says Strausfeld.)

The animal’s brain consists of three fused ganglia, and blends into more ganglia that extend down the length of the animal’s body. It has four eyes, each of which is served by just one optic lobe. That’s a chelicerate layout—in mandibulates, the body ganglia would be more distinct and separated by long nerves, and there would be two to four optic lobes per eye.

The results show that the mandibulate and chelicerate lineages had well and truly split 520 million years ago, and already had the distinctive nervous systems that their modern descendants do. And this means that the ancestors of these groups, and the earliest arthropods, were around well before that. That’s what the team is now looking for.

And another question remains: how could a soft nervous system last for half a billion years? Strausfeld says that the nerves of invertebrates are dense and rich in fats, which makes them water-repellent. This, combined with their hard external skeleton, might have slowed the process of decay long enough for them to fossilise. Indeed, in an earlier study, Strausfeld’s team buried marine worms in mud and put them under high pressure to simulate the start of fossilisation—and their nerves lasted while their muscles decayed.

That’s just a guess, though. “Scientists like to demonstrate how dead animals decay and how they can thus provide misleading information as fossils,” says Strausfeld. “However, fewer scientists try experiments that might imitate conditions leading to extraordinary preservation. And extraordinary preservation that is the hallmark of Chengjiang fossils.”

Reference: Tanaka, Hou, Ma, Edgecombe & Strausfeld. 2013. Chelicerate neural ground pattern in a Cambrian great appendage arthropod. Nature http://dx.doi.org/10.1038/nature12520