Ever come across something out in nature that looks unnatural? Perhaps it's a strikingly symmetric array of trees or stones, or a mysterious ring of darker grass, or what appears to be a set of carefully designed paths through the undergrowth.
If you have, the chances are good that you've puzzled over what those signs meant, and who or what built them, and why. Sometimes they will turn out to be the product of another human, or a machine. Others will be the result of either some busy animal or swarm of insects, or perhaps just the natural tendency of phenomena to self-organize or to follow patterns and symmetries. Trees may grow along a seam of rich soil, or track a subterranean stream. Grasses can propagate outwards from a point of fertility, or fungi can deposit their spores in a circular front of growth.
Even if you have stumbled across the remains of human activity; deducing the original intent, the plan, the motivation is enormously difficult. It's what archaeologists, anthropologists, and historians do for a living, and I don't think many of them will tell you it's so easy.
Now Jaubert et al. have published the results of their analysis of a remarkable set of structures discovered some 336 meters deep inside Bruniquel Cave in France. Made from the broken-off spears of roughly 400 stalagmites, these circular patterns and seemingly careful piles span between 2 and 7 meters in diameter, and strongly suggest the deliberate actions of someone, or something.
The astonishing aspect is the age that Jaubert et al. find for the formation. The broken pieces of the stalagmites have continued to grow layers since they were snapped off in the wet cave environment, and so the researchers were able to identify where the original calcite surfaces were in their core samples. They then used uranium-series radioisotope dating to come up with a time for the event of around 176,500 years ago (give or take about 2,100 years).
So who or what did this? There is (for example) no apparent evidence of cave bears - and of course the idea of bears constructing a nest or a symbolic artifact out of stalagmites pushes our credulity, as lovely a picture as it would make.
There is little evidence of modern humans building anything so elaborate much beyond 20-40 thousand years ago in any location in the world. The prime suspects that the researchers point to are Neanderthals - for whom there is already good evidence of sophisticated tool use, fire use, and construction. Neanderthals had also already been around for more than 220,000 years at this point.
It is a spooky and evocative thought. A hominid species that was not us may have got together and built something in this cave - a carefully considered pattern and structure. And on top of that, they must have lit their workspace - because it's too far from the cave entrance now (or then) for daylight to seep in.
What were they thinking? Why did they do it? Is there something to be decoded from this?
Apart from the inherent fascination for us, there are some striking parallels between these questions and the kind of challenges we face in looking for evidence of other intelligent, technological life in the universe. Perhaps that similarity presents an opportunity for progress in both areas - from our ancient hominid relatives, to life much further afield.
A critical issue for the question of life elsewhere is whether we can apply any of what we know about our own evolution or behavior to hypothetical aliens. Over the years there has been a vast amount of fairly glib conjecture of the "well of course any intelligent life would do this..." variety, to loose speculation about ways in which aliens might reveal themselves, deliberately or otherwise.
There's nothing inherently wrong with all of that, but if we are to make genuine progress I think we need to learn to be both more agnostic and better at quantifying the possibilities. For example, it could be that the fundamental way in which we think, and the way that our intelligence works and makes decisions is not actually universal at all.
The 176,000 year old structures in a cave in France offer a superb opportunity for SETI-leaning astrobiologists to try some detailed thought experiments. If you found this in (for example) a cave on Mars, how would you go about proving its origins and figuring out why it was constructed, especially if you didn't know anything about the nature of the intelligence that made it?
The skilled researchers working in the Bruniquel Cave are likely to unearth more clues over time. Perhaps there will be evidence pointing towards recognizable rituals or behavioral patterns. But it might be worth considering that Neanderthals need not have been a great fit to any recognizable mold of modern human cognition. If there was a way to demonstrate that (and I honestly don't know how you would do this), it would be truly fantastic, and would give our search for extraterrestrial intelligence a very useful jolt.