Denis's cave in southern Siberia was home to Neanderthals and Denisovans for thousands of years, but questions remain about the timing of their stay. A pair of new studies traces the history of archaic human occupation at the site, showing who lived there and when-including a possible era during which two now-extinct species hanged together.
Two papers published today in Nature present an updated timeline for the occupation of Denis's cave by Neanderthals and Denisovans. The new research suggests that Denisovans – a sister species to the Neanderthals – made this cave their home for a longer period than Neanderthals, first venturing into the cave as far back as 287,000 years ago. Neanderthals arrived at the site about 140,000 years ago, possibly sharing the space with the Denisovans for thousands of years. It's further evidence that Neanderthals and Denisovans interbred – and that this co-mingling happened at or near Denisova.
Archaeologists and paleontologists have carefully sifted through Denis's cave for the past 40 years, pulling out various animal and Neanderthal bones. But the real bombshell came in 2010 with the discovery of a finger bone from previously unknown human species, the so-called Denisovans. Genetic analysis suggests that Denisovans were a related species to the Neanderthals, but pretty much everything else about them remains a mystery, such as when they first appeared on the scene and when they died out.
Denisova cave, located at the foot of the Altai mountains in southern Siberia, is thus a critical resource for improving our understanding of not only the Denisovans, but also the Neanderthals as well. And possibly our own species, Homo sapiens-Though the cave, perhaps strangely, has not produced a single shred of evidence showing that anatomically modern humans ever lived in there. For Neanderthals and Denisovans, however, Denis's cave served as an important refuge for vast swaths of time.
Vast swaths of time, indeed. We're not talking about a thousand years here or a thousand years there. Rather, we're talking about hundreds of thousands of years of occupation. Compiling a timeline of events, such as when the cave was first occupied and by whom, has proved tough, partly because of the large size of the cave and its complex layers of sediment; The cave's stratigraphy encompasses both the Siberian Middle Paleolithic period (between 340,000 and 45,000 years ago) and the Initial Upper Paleolithic period (roughly 45,000 to 40,000 years ago).
Scientists have also been confronted with the limits of radiocarbon dating, which can only go back 50,000 years. The cave has been inhabited for much longer than that, requiring the use of less reliable dating methods, and by consequence, the positing of unconvincing or controversial timelines.
To overcome these hurdles and limitations, a multidisciplinary team of researchers from around the world, including from Russia, UK, Australia, Germany and Canada, spent the last five years analyzing bones and artifacts found in the Denisova cave. The researchers used multiple dating techniques, both well-established and cutting-edge, and statistical techniques to date thousands of items at the site, allowing them to piece together the most accurate and detailed timeline to date human occupation at Denisova cave.
The first study, led by Zenobia Jacobs and Richard Roberts of the University of Wollongong in Australia, presented new dates for Denis cave sediment deposits. The researchers used a relatively new technique called stimulated luminescence, in which scientists can tell the last time that a mineral grain, such as quartz, was exposed to sunlight. Dates were provided for 103 sediment deposits spanning 280,000 years of history in the cave.
The results of this work showed that Denisovans first occupied the cave around 287,000 years ago, and continued to live in the cave until about 55,000 years ago. Neanderthals arrived at the cave about 193,000 years ago, and they continued to live there until about 97,000 years ago – a 96,000-year overlap. The bones of 27 animals, including mammals and fishes, along with 72 species of plants were also analyzed, pointing to a variable climate in the region during the millennia of occupation at the cave. At times, the region was relatively warm, featuring forests of broad-leaved trees, but at other times it was a harsh and desolate tundra-steppe habitat.
A major implication of the Jacobs and Roberts study is the suggestion that Denisovans and Neanderthals hunkered down in the cave together. Now, it's possible that the two species did not share the space simultaneously, but recent evidence suggests they probably did. In an astounding find from last year, a group of scientists, some of whom are co-authors of this new study, uncovered genetic evidence of a hybrid archaic hominin, dubbed Denisova II, who lived in the cave 90,000 years ago – a girl with a Denisovan dad and a Neanderthal mum.
This evidence, together with other lines of research, suggests the two species interbred regularly, and that this was not just an isolated case.
The second study, led by Katerina Douka from the Max Planck Institute for the Science of Human History in Germany, offered new dates for Neanderthal and Denisovan fossils, along with tooth pendants and bone points found at the site. Douka's team used multiple techniques to indirectly and directly date thousands of bone fragments and artifacts, including radiocarbon dating and uranium-series dating, both of which take advantage of the known rates of radioactive decay.
"This is the first time we can confidently assign an age to all the archaeological sequence of the cave and its contents." Tom Higham, an archaeologist at the University of Oxford and a co-author of the new study, said in a statement.
The oldest Denisovan fossil suggests that this group was present at the site as early as 195,000 years ago, while all Neanderthal fossils, including Denis II, were dated to between 80,000 and 140,000 years ago. The youngest Denisovan fossil was dated between 52,000 and 76,000 years ago.
"The Douka paper was exciting because we knew that Neanderthals and Denisovans both used Denis's cave, and that the two groups interbred in or near there, but we did not know much about the length of time that each group frequented the cave or the length of time that the two groups overlapped in using the cave, "Sharon Browning, a research professor at the Department of Biostatistics at the University of Washington, who was not involved in the new study, told Gizmodo.
Many of the dates provided in the Douka paper had a large margin of error, a consequence of complex stratigraphy (eg, some items were drifted to lower stratigraphic layers) and a lack of willingness to go beyond available data. But while there is "considerable uncertainty in some estimates, and the possibility of visits from either the group that were earlier or later, but did not leave detected trace," Browning said these results still "help to establish the likely pattern of use over time. "
The artifacts found at the site, such as bone points, pierced teeth and pendants, were dated between 49,000 and 43,000 years ago, and they are now the oldest artifacts ever uncovered in northern Eurasia, according to the Douka paper. Trouble is, these dates are thousands of years after the last evidence of human occupation appears at the cave.
"On the basis of current archaeological evidence, it can be assumed that these artefacts are associated with Denisovan population," the authors contemplated in the study. "It is currently not possible to determine whether anatomically modern humans were involved in their production, as modern-human fossils and genetic evidence of such antiquity have not yet been identified in the Altai region."
Denisovans were the likely producers of these items because it is the simplest explanation, given that Neanderthals were long gone from the cave by then, and no evidence of modern humans exists at the cave, according to the new research. But anthropologist Chris Stringer from the Natural History Museum in the UK is not convinced that these items belonged to the Denisovans.
"My money would be on early modern humans, which can be mapped elsewhere at this date, for example at Ust'-Ishim in Siberia, but the authors of the Douka paper rather surprisingly argue that it is most parsimonious to assume that Denisovans were responsible, even though no Denisovans are still known as late as that in the sequence, "Stringer told Gizmodo. "Only more discoveries and more research can solve that question satisfactorily."
Stringer said he liked the two new studies, saying they "bring the latest dating techniques to bear on stratigraphy, palaeoclimatic records, and human fossils," but he said many unresolved issues remain. There is the possibility, for example, that some, if not all, of the bones were dragged into the cave by carnivores who hunted humans, he said, or that the bones shifted dramatically over the years from their original resting place, throwing off the dating to a considerably degree.
"But at face value it looks like the Denisovans can be placed at least intermittently at the site for about 250,000 years, from close to 300,000 years ago to about 50,000 years ago, with the Neanderthals also there for periods in-between," said Stringer. "Occupations seem to concentrate in the warmer periods, reinforcing the view that Denis's cave was probably at the northern limits of occupation for both of these populations.
The fact that Neanderthals and Denisovans both were present at times greatly complicates disentangling which humans were responsible for for which elements of archeology-perhaps sediment DNA studies will eventually help to better map their presence in the cave. "
These uncertainty and the large margins of error are undeniably frustrating, but these two papers help to clear away a lot of ambiguity. As time passes, we're steadily gaining a clearer picture of archaic human occupation at the Denisova cave. And damn, is it ever fascinating.[Nature, Nature]