The Cueva de Ardales, southern Spain, is filled with handprint stencils and paintings of humans and animals. It also has red-stained flowstones, that have inspired debate as to whether the coloring is natural, or something far more important: the cave's earliest art, providing an origin story for the more complex work that came later. New research appears to settle the question in favor of human intervention, offering an opportunity to explore the development of cave painting from its very beginning. The timing of the painting, some up to 65,000 years old, proves the first artists were Neanderthals, not Homo Sapiens.

More familiar cave art used the flattest surfaces available, but Cueva de Ardels also contains a dome of stalagmites, a few of which are colored in a way that looks almost natural but leaves room for questions. Dr Africa Pitarch Marti of Barcelona University collected microscopic samples from three segments of the panel and analyzed their composition to try and determine if the red pigment application was anthropogenic in nature.

In Proceedings of the National Academy of Sciences, Marti and co-authors report the pigment is of mineral, rather than microbial, origin. Iron, in the form of hematite, provides the color. The material varies by stalagmite, with calcium always present, and at least one of carbon and silicon in every sample, sometimes accompanied by other trace metals. Crucially, while iron-rich deposits exist in the cave, they have different compositions to these stalagmites' pigments; most didn't even contain hematite. The limestone formations could not have got their color from naturally occurring iron formations, the authors conclude.

This means the coloring must have been done by human hand, using pigments brought from an unknown source outside the cave. The location of some of the stained stalagmites rules out an accidental brushing against an outcrop. Although in most cases the authors are unsure how the color was applied, some of it appears on folds so inaccessible the could only have reached by airborne droplets, suggesting a blowing mechanism seen often in later hand stencils.

The authors posit this may have been a way for Neanderthals to highlight the cave's natural formation for themselves. 

“It would seem to us that the carrier of the symbolic information is, in this case, the large stalagmitic dome harboring the panel, not the panel itself,” the authors write; “Put another way, treating the dome as the canvas is useful shorthand but should not be taken to imply that this large formation is no more than a convenient surface used to appose markings and that these markings are in and of themselves the repositories of symbolic information irrespective of where made. Instead, we believe that the dome is the symbol, and the paintings are there to mark it as such, not the other way around.”

Dating cave art is often difficult. One of the paintings has been measured to be more than 65,000 years old, preceding previous records by almost 20,000 years and establishing Neanderthals as the artists. Another can only be definitely said to be more than 46,000 years old, but could be equally ancient and was made with very similar paint. Consequently the pair could represent the product of a sudden burst of creative activity, or be made thousands of years apart. Yet there are commonalities, such as strongly colored central regions fading to the outskirts. 

However, a third stalagmite, using a differently composed paint, can be dated to between 45,000 and 49,000 years ago, proving similar art was made thousands of years apart. Some of the paint is layered, indicating it may have been restored by descendants of those who originally made it.

To our eyes, there is an immense gap between staining a limestone column red and painting animals so realistically we can match the images to fossils of extinct species. However, identifying these early efforts as art, and a timeline of development, can help us understand where more familiar works came from.