Roccamonfina volcano, about 60km northwest of Vesuvius, erupted violently around 350,000 years ago. Pyroclastic flows—deadly torrents of hot gas and volcanic ash—raced down the sides of the mountain. But within a few days, a small group of hominins trekked across the layer of ash and pumice that covered the steep mountainside. Recent analysis and some newly identified prints suggest that the intrepid (or reckless) hominins may have been Homo heidelbergensis who lived and hunted near the volcano.

Another layer of ash later covered the slope, sealing away at least 81 tracks until the early 1800s, when erosion revealed them to the local humans. The tracks record where at least five climbers, all with different foot sizes, walked down the steep, ash-covered hillside. One trail zigzags back and forth downhill, and you can easily picture climbers carefully working their way diagonally across the slope. Along another, more curving path, there are still handprints where the climbers reached out to steady themselves, and a slide mark reveals where one climber slipped.

The ash must have been cool enough to walk on but still soft enough to preserve tracks—very detailed ones, in a few cases. According to ichnologist Adolfo Panarello (of University of Cassino and Southern Latium) and his colleagues, that must have happened within a few days of the pyroclastic flow; Roccamonfina may even still have been erupting. In the 1800s, people living around the now-extinct volcano were sure that only the devil could have left those tracks.

These aren’t the devil’s tracks, but whose are they?

The 19th-century locals were right about one thing: the footprints weren’t left by our species. In fact, our species didn’t technically exist yet. Instead, the footprints most likely belong to an evolutionary relative of ours: Homo heidelbergensis, the species that gave rise to Neanderthals around 400,000 years ago.

One of the newly identified prints at the site records a surprising amount of detail about a climber's right foot: the wide heel, the low arch, and the base of the big toe. Overall, Panarello and his colleagues say it looks very similar to the feet of 430,000 year-old H. heidelbergensis fossils from Sima de los Huesos cave in Spain. That lines up with a 2016 study, which found that the short, wide shape of the footprints matched well with the size of fossil feet from H. heidelbergensis elsewhere in Europe.

Case closed, right? Not exactly; Panarello and his colleagues say their evidence isn’t conclusive. It's also possible that the climbers were actually Neanderthals, and there's even an outside chance that another hominin species may have lived in the area.

Prepare to split some hominin hairs

First, paleoanthropologists in general don’t yet agree on exactly when some of the hominins running around Europe stopped being H. heidelbergensis and started being Neanderthals. Evolutionary change is such a gradual process, and it's not clear whether there was a gradual replacement or a branching split. Around the time of the eruption, some H. heidelbergensis groups already looked a lot like Neanderthals—including the Sima de los Huesos group, whose feet look so much like the Roccamonfina prints. But at the same time, other groups in other parts of Europe still looked more like older hominins: including the 400,000-year-old skull from Ceprano Cave, which is only about 70km (42 miles) from Roccamonfina in northern Italy.

Because Ceprano is so close by, Panarello and his colleagues say that the hominins who climbed the volcanic debris at Roccamonfina probably belonged to the same species or subspecies. Some paleoanthropologists say that the Ceprano skull belonged to yet another hominin species called Homo cepranenis because its thick skull and heavy brow ridges looked like it belonged somewhere between Homo erectus and H. heidelbergensis on the evolutionary tree. On the other hand, a 2017 reconstruction of the skull suggested that it actually belonged to H. heidelbergensis—but a local population that had kept the species’ older features instead of evolving to look more Neanderthal-like.

If Panarello and his colleagues are correct, the hominins who ventured up Roccamonfina just after the eruption were probably H. heidelbergensis or a closely related species, but probably not the direct ancestors of the Neanderthals.

A few unanswered questions

Part of the reason footprints are so fascinating is that they record ancient people actually in the process of doing something at one specific moment in the past. And the soft volcanic ash of Roccamonfina caught this particular group of hominins at a really interesting moment. The tracks raise some interesting questions about how hominins living in volcanic regions related to the dangerous landscapes around them.

We don’t know where or how they took shelter when the volcano erupted and will never know how they explained the event to themselves and each other. But Panarello and his colleagues suggest the group probably lived and hunted in the area, and this stretch of mountainside was probably part of their normal hunting range. Archaeologists have found stone tools along the trackways in the same rock layer, and similar tools have turned up at a site nearby, although it’s not clear how old those tools are.

As they climbed the freshly buried slope, they weren't fleeing in terror (it would have been much too late for that anyway). Based on the shape of the impressions and the distance between them, the climbers were actually walking at a fairly leisurely pace.

They may even have been hunting. Hoofprints also dot the rock nearby, and a couple of large canine paw prints also pass close to the human tracks. But unlike at White Sands, New Mexico, the footprints don't show any confrontations between the hominins and their potential prey (or potential rivals). That means we can't say for sure that these hominins ventured onto the volcano in search of a meal.

Of course, Neanderthals weren't much different from us, as a growing pile of archaeological evidence shows. And that means they may have just been checking out the damage. Evidence at other sites makes it pretty clear that sometimes humans just want to look around, even if it's dangerous (sometimes especially if it's dangerous).

Journal of Quaternary Science, 2020 DOI: 10.1002/jqs.3186 (About DOIs).