Scientists claimed the world's oldest rock art is 67,800 years old. But is the science behind that estimate flawed?

In a short communication published March 23 in the journal AOJ of Histoarchaeology and Anthropological Exploration, Sauvet criticized the published dates for some prehistoric rock art, stating that researchers are being less cautious in a "race towards the earliest rock art."

If these dates go unchallenged, Sauvet argued, it distorts our understanding of the intelligence of early Homo sapiens and human relatives, such as Neanderthals.

Sauvet thinks it is absolutely necessary to cross-date with other dating methods to ensure accuracy of the U-Th dating which on its own "is not acceptable," Sauvet said.

What is uranium-thorium dating?

When water washes over and breaks down limestone and then drips into caves, it slowly deposits calcite. As the calcite forms, it traps small amounts of uranium that is also dissolved in the water. The uranium then radioactively decays into several "daughter" isotopes, including thorium isotopes.

The method in question, U-Th dating, involves uranium-234, which contains 92 protons and 142 neutrons and will spit out two protons and two neutrons (called an alpha particle) to form thorium-230, which has 90 protons and 140 neutrons. It takes about 245,629 years for half of a given amount of uranium-234 to decay into thorium-230. So, assuming nothing is added to or taken away from the system since the uranium was deposited, the ratio of these two isotopes can determine the age of a calcite layer deposit.

In theory, this type of U-Th dating can be an incredibly powerful tool for archaeologists.

That's because in some cases, rock art contains carbon, an organic element that can be dated. But often, rock art is made from the mineral ocher or from etchings in stone, neither of which can be radiocarbon-dated.

This is where U-Th dating comes in handy. Calcite can grow over the art, creating a minimum age for its creation.

Questioning the age of rock art

However, in his paper, Sauvet argued that U-Th dating may provide unreliable dates and that all of them need to be cross-checked with other methods before a date can be estimated.

"My first alarm against the danger of the method was reading" a 2018 paper by Dirk Hoffmann, a researcher in the Department of Human Evolution at the Max Planck Institute for Evolutionary Anthropology, and colleagues, "in which they announce that three Spanish cave [drawings] were dated to 65,000 years and were due to Neanderthals," Sauvet told Live Science in an email. The drawings show red ocher dots, a ladder and hand stencils, and some researchers suggested they show Neanderthals' ability to think artistically.

In a response in 2020, Sauvet and 42 other researchers published a reply highlighting the drawbacks of U-Th dating. The problem, Sauvet said, is that the method relies on calcite deposits forming and remaining in a "closed system," meaning no uranium is leached from the calcite deposit from the time of its formation. In an "open system," rainwater and groundwater percolate through the calcite and leach out uranium, skewing the U-Th ratio and making the deposit appear artificially older than it is.

"Dating of rock art is a particularly challenging subject," Adelphine Bonneau, an assistant professor in the Department of Chemistry at the University of Sherbrooke in Quebec, told Live Science in an email. "In theory, Mr. [Dr.] Sauvet is right. U-series dating [which includes U-Th dating] can lead to overestimated dates."

According to Sauvet's paper, the hunt for increasingly older dates is in part due to grant awards and the prestige that comes with them. This has resulted in a series of increasingly older dates, including the supposedly 65,000-year-old art in Spain.

This date would "imply that paintings were made by Neanderthals, whereas there is absolutely no archaeological proof" that Neanderthals were capable of artistic creation, Sauvet said. (Other researchers, however, disagree, and think Neanderthals did create art.)

The debate was revived with the discovery of negative hand stencils in Sulawesi, Indonesia, that were U-Th-dated to around 67,800 years ago, which recently surpassed all previous records.

To highlight U-Th dating's unreliability, Sauvet cited several cases in his new paper in which U-Th and radiocarbon dates did not match when cross-checked. At Nerja Cave in southern Spain, a U-Th date yielded an age of 119,000 years, while a radiocarbon date of a charcoal mark that was part of the same drawing was around 19,000 years old; meanwhile, another radiocarbon date of a calcite layer from the same art was around 14,000 years old.

At Leang Balangajia, the outer calcite layer, which should be the youngest, was dated to 37,300 years old — roughly 7,800 years older than the layer underneath. In all of these cases, the suggestion is that the calcite layers were open systems.

In defense of U-Th dating

Not all researchers agree with Sauvet's assessment. João Zilhão, a research professor at the University of Lisbon and co-author of the Spanish rock art studies, said any dating method can be susceptible to errors. In an email to Live Science, he drew parallels to radiocarbon dating, in which the ratio of carbon-14 to carbon-12 is measured ‪—‬ a method that can be subject to contamination.

Similarly, optically stimulated luminescence, which measures when something was last exposed to light, can give an artificially old age if someone doesn't account for residual emissions from mineral grains in a sample, he noted.

There will always be ways that dating estimates can be subject to error, and U-Th is not particularly special in that regard, experts told Live Science.

"Saying that uranium-series dating does not work for rock art because there are cases where some calcite samples show open-system behavior is an overgeneralization," said Maxime Aubert, an archaeologist and geochemist at Griffith University in Australia, whose team was behind the Sulawesi dates.

Aubert acknowledged that uranium can leach out of or into calcite deposits, thereby distorting their true age. To get around this problem, his team has in recent years developed a laser ablation approach that allows zones altered by outside water seepage "to be identified and excluded from age calculation," he told Live Science.

To ensure their dates were accurate, including ones for the oldest hand stencil art in the world, Aubert's team used lasers to remove small sections within each sample. Next, they measured the isotope ratios in several of these sections to create a map of the isotope ratios across each calcite layer. Finally, they excluded layers where the isotope ratios varied a lot, which would have suggested contamination.

Scientists have developed other ways to ensure their dates are reliable, Bonneau added. "There are several ways to deal with these open-systems and correct the dates accordingly … Using laser-ablation and having a map of the calcite layers makes it possible to select the most reliable parts and then extract the dates," she said.

Bonneau noted that Hoffmann and colleagues did not have access to laser ablation at the time of their study, as the technology was still being developed.

To get around this, in past work Aubert "always published a lot of information regarding the different isotope ratios, the composition of the calcite layers, etc.," Bonneau said. "These data are needed to evaluate the reliability of the dates." Hoffmann's study is missing that data, she added, "so we cannot evaluate the reliability of the dates."

While U-Th dating can be subject to error, that doesn't make their age estimates worthless, she said. "Sauvet is right in principle, but if scientists do their job correctly, the dates are reliable," Bonneau concluded.

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