Neanderthals could use feathers and bird claws as ornaments, archaeologists found. But some scientists were skeptical about what these findings meant.
Neanderthals might have lived near modern humans, after all, and spotted them making things. Neanderthals were smart enough to copy the ornaments, the thinking went — but not enough to invent them.
This debate was fueled in part by technological uncertainty: It can be very difficult to pin down a firm date for human fossils and artifacts.
To determine the age of cave paintings, for example, researchers have traditionally relied on radiocarbon dating. But that method only works if the paint contains carbon-bearing ingredients, such as charcoal. Red ocher, by contrast, can’t be dated this way.
Making matters worse, radiocarbon dating becomes increasingly unreliable beyond about 40,000 years.
Dr. Zilhão joined with archaeologists Alistair G.W. Pike of the University of Southampton and Dirk L. Hoffmann, now at the Max Planck Institute for Evolutionary Anthropology in Germany, to see if the prehistory of European art could be brought into sharper focus.
Instead of studying radiocarbon, they would use a different clock to tell time.
As water seeps into caves, it may deposit milky crusts of minerals on the walls known as flowstones. Flowstones contain tiny amounts of uranium, which slowly breaks down into thorium. The older a flowstone gets, the more thorium builds up inside it.
Credit J. Zilhão
A flowstone covering a piece of cave art might give Dr. Zilhão and his colleagues a minimum age for its creation. The problem was that scientists usually needed big chunks to find enough uranium and thorium to measure. The flowstones on cave art were typically very small.
But Dr. Hoffman had been working on ways to drastically increase the sensitivity of the technology so that he could work with much smaller samples.
The researchers returned to caves in Spain where ancient paintings had been discovered over the past century. The artists had drawn abstract images on the cave walls, including long lines, patterns of dots, and the outline of a human hand.
The team found flowstones covering parts of the artworks and scraped away samples for dating. In three caves, it turned out, some of the art was over 64,000 years old — about 20,000 years earlier than the first evidence of modern humans in Europe.
“They must have been made by Neanderthals,” said Dr. Pike.
Wil Roebroeks, an archaeologist at Leiden University who was not involved in the new study, said the evidence was conclusive. “This constitutes a major breakthrough in the field of human evolution studies,” he said. “Neanderthal authorship of some cave art is a fact.”
Dating flowstones is a big advance on previous techniques for determining the age of cave art, but the technology has one major limitation: it can only assign a minimum age to cave paintings. Flowstones may have begun forming the day after a painting was finished — or 10,000 years afterward.
Credit J. Zilhão
But a second study, which Dr. Zilhão and his colleagues published Thursday in the journal Science Advances, hints that Neanderthals might well have been painting long before 64,000 years ago.
The scientists traveled to a cave on the coast of Spain where Dr. Zilhão had earlier discovered shells that had been drilled with holes and painted with ocher.
In 2010, he and his colleagues had used radiocarbon dating to estimate the age of other shells in the same layer of rock at 45,000 to 50,000 years old.
That result did not tell the team who made the ornaments. Neanderthals might be responsible, but it was also possible that the earliest modern humans in Europe made them.
And the uncertainties of radiocarbon dating also had left open the possibility that the shells were, in fact, far older.
Dr. Zilhão returned to the cave in order to try uranium dating. He and his colleagues discovered a layer of flowstone sitting atop the rock where they had found the shell jewelry. That flowstone turned out to be about 115,000 years old.