Deep in the Altai mountains of southern Siberia sits a very choice piece of real estate. It’s nothing so newfangled as a ski lodge or one of the traditional wood houses that dot the local countryside. Rather it’s a primeval limestone cave, called Denisova, that overlooks a rushing river and the surrounding forest. Multiple human species, or hominins, have sought shelter in this cave over the past 300,000 years, such is its allure. Artifacts, bits of bone and ancient DNA found in its chambers testify to the presence of these peoples. The site thus offers a rare window on a particularly fascinating period of human evolution, one in which other human species coexisted with our own kind.
Researchers have long wondered how these groups interacted and influenced one another culturally when they met up, and Denisova could be a key to answering this question. But figuring out which hominin species was present when at the cave and which artifacts they made has proved challenging. Now new efforts to date the remains from Denisova are at last bringing that picture into sharper focus. Two studies published in the January 31 Nature provide a time line of human occupation of the cave. The results raise intriguing questions about the origins of symbolism and certain technologies traditionally considered to be inventions of Homo sapiens alone.
Archaeologists have been unearthing artifacts from Denisova Cave since the 1980s. The site contains frustratingly little in the way of hominin fossils, however. Most of the bones from the site are mere scraps, too incomplete to assign to a particular species on the basis of their physical characteristics. But in the last decade researchers have managed to recover ancient DNA from some of these fossil bits and from sediments in the cave. The DNA shows both Neandertals and another archaic group known as the Denisovans hung out there. And last year a team reported they had retrieved DNA from what was apparently a hybrid individual who had a Neandertal mom and a Denisovan dad. But for all that scientists have been able to piece together about Denisova, the timing of hominin occupation of the site has remained uncertain, thanks to certain quirks of site formation and preservation as well as the limitations of various techniques used to date archaeological and fossil material.
In the new studies, two groups of researchers obtained a raft of fresh dates for the stratigraphic levels of interest at the site using a combination of techniques. One group, led by Zenobia Jacobs and Bo Li of the University of Wollongong in Australia, used a method called optically stimulated luminescence to date sediments from the cave. The team also reconstructed the environmental conditions at the site between 300,000 and 20,000 years ago. In the second study, Katerina Douka of the Max Planck Institute for the Science of Human History in Germany, Thomas Higham of the University of Oxford and their colleagues used radiocarbon dating to ascertain the ages of artifacts spanning the transition from the simpler Middle Paleolithic material culture to the more elaborate Upper Paleolithic one. Most of the human fossils are too old for radiocarbon dating, which maxes out at around 50,000 years. So the team determined the so-called relative genetic ages of the human fossils from the site by comparing their DNA sequences with those obtained from other human fossils and counting the differences between them. Such mutations accumulate at a known rate in modern humans. Using that rate the researchers were able to convert the ancient DNA differences to time. Douka, Higham and their colleagues then fed all of the ages obtained from the radiocarbon and optically stimulated luminescence methods, along with timing data from the genetic studies and the stratigraphic layers themselves, into a statistical model that calculated the most probable ages for the human fossils.
The results of these studies reveal Denisovans and Neandertals occupied the cave intermittently from at least 200,000 until around 50,000 years ago during both cooler and warmer climate phases. Denisovans were the first of the two groups to move into the cave and the last to abandon it. They probably overlapped there around 120,000 years ago, and possibly at other times as well.
The time line hints at a tantalizing possibility for who made the early Upper Paleolithic artifacts at the site, which include animal tooth pendants, a stunning stone bracelet and a needle and other tools crafted from bone. The radiocarbon dates obtained from some of the pendants and bone tools put them at 43,000 to 49,000 years old. There are no known hominin remains of that age from the site—the youngest specimen is a Denisovan fossil that dates to between 52,000 and 76,000 years ago. Yet Douka, Higham and their colleagues think creators of these artifacts are likely to have been Denisovans. Neandertals appear to have checked out of the cave by around 80,000 years ago. A fossil from the site of Ust’ Ishim documents the presence of our species in western Siberia around 45,000 years ago, which is the right time for it to be the maker of these artifacts. But that site is hundreds of kilometers from Denisova. “Our Russian colleagues have rightfully argued that we have no modern human fossils at Denisova and no modern human DNA from Denisova sediments, so why invoke modern humans” to explain the onset of the Upper Paleolithic at the site, Douka says. “One might say that given [the Ust’ Ishim fossil], we should assume modern humans made the pendants and bone tools at Denisova, but we don’t have modern human fossils in the Altai at that time.” Higham adds: “It could be modern humans but the most parsimonious explanation for the moment is that it’s Denisovans.”
The suggestion Denisovans developed the Upper Paleolithic artifacts at the site bears on a hot topic in paleoanthropology: the origins of modern behavior and cognition. Once upon a time, archaeologists thought only H. sapiens made symbolic items such as jewelry and advanced technology such as standardized bone tools. Then discoveries in the 1970s ignited debate over whether Neandertals also might have invented such items. In recent years evidence has mounted in support of a more sophisticated Neandertal. For instance, last year researchers reported cave paintings in Spain pre-date the arrival of H. sapiens to the region by thousands of years and must therefore be Neandertals’ handiwork. Neandertals, however, are not the only archaic hominin species to show signs of advanced cognition: In 2015 archaeologists unveiled their discovery of a shell that was engraved with a geometric design some 500,000 years ago—long before the origin of modern humans or Neandertals—the implication being that an earlier human ancestor known from this time period, Homo erectus, must have been the designer.
Could Denisovans have independently developed modern cognitive capabilities, too? Archaeologist Francesco d’Errico of the University of Bordeaux in France, who was not involved in the new studies, contends the paucity of relevant archaeological material from the site and insufficient description of these remains “make it difficult to reach a firm conclusion.” But “I’m not against the idea,” he says. “I do not see why archaic hominins could have not invented personal ornamentation independently and repeatedly, and a lot of evidence from Europe is now supporting this view.”