All major players spent time in Denisovan Cave
At various times over the past 300,000 years, Denisova Cave has housed three different species of hominids. But with fossils of just eight individuals – four Denisovans, three Neanderthals, and the daughter of a Neanderthal / Denisovan couple – it’s difficult to tell a detailed story about when each species lived in the cave. According to a recent genetic study, however, the Denisovans were the first, arriving around 250,000 years ago. And they may still have been there when the first members of our species arrived about 45,000 years ago.
This timeline is the result of a recent study of mitochondrial DNA (genetic material passed directly from mother to child) mixed in the deep layers of sediment covering the cave floor. According to archaeologist Elena Zavala of the Max Planck Institute for Evolutionary Anthropology, lead author of the study.
“We know that DNA can bind to minerals found in sediment and we have also seen microfossils when examining sediment under a microscope,” she told Ars in an email. “Future studies linking specific elements of the sediment to DNA preservation will help improve our understanding of this process.”
Zavala and his colleagues sampled the sediment from each layer of the cave, in all three chambers, at intervals of 10 to 15 centimeters. Next, they isolated the mitochondrial DNA sequences from hominids and other species of mammals, such as mammoths, bears, and hyenas. To identify which hominin each of the 175 hominin mtDNA samples belonged to, they compared the DNA fragments to specific parts of the genome that differ between Denisovans, Neanderthals, and us.
New hominids on the block
In a layer of sediment that began to accumulate on the cave floor 250,000 years ago, they found the first traces of Denisovan’s mtDNA. This is significantly older than the cave’s first Denisovan fossil, which dated from 194,000 to 123,000 years ago. The first Neanderthals appeared in the cave some time before 170,000 years ago, and for about 40,000 years the two groups of hominids appear to have more or less shared the cave. It’s hard to say for sure, because the layers of sediment only break down over time into pieces that are thousands of years old. This means that archaeologists cannot tell whether the two species were roommates or whether they alternated possession of the cave every few years, decades, or centuries.
“The chronology resolution is too coarse to distinguish events even at a 1000-year resolution, so we cannot say whether Denisovans and Neanderthals sometimes coexisted in the cave during the Middle Paleolithic,” archaeologist from the University of Wollongong, Richard Roberts, co-author of the study, told Ars.
In any case, the arrival of the Neanderthals follows a major climate change. About 190,000 years ago, the climate of the Altai Mountains turned colder as the world moved out of a relatively warm interglacial period into another ice age. We know this from the chemical records trapped in ice caps, cave formations and sea sediments around the world. The floor of Denisova Cave holds its own record of life changing in Altai. In layers from this period, Zavala and his colleagues found less mitochondrial DNA from bears and wolves, and more hyenas and ancient relatives of modern cattle.
Then, 130,000 years ago, the climate returned to a warmer interglacial period. Deer and wild horses are becoming more common in recording mitochondrial DNA, and the last traces of Denisovans disappeared from the cave 120,000 years ago. We have no way of knowing if they are dead or if they are just gone. This response, according to Zavala, is probably buried at other sites in the region.
“To better understand these issues, we need to find more sites with Denisovan and Neanderthal remains during this period,” she told Ars. “It is possible that the movement or disappearance of the first Denisovan was due to the climate, but to determine this we would need to identify similar changes in other places with Denisovan remains. More sites from this period are needed to track the movements (and disappearances) of different populations of Neanderthals and Denisovans. “
Who deserves the credit for these scrapers?
These questions are important, in part, because archaeologists want to know who made the stone tools they unearthed in the Denisova Cave and other sites around Eurasia. The oldest tools in Denisova Cave date from a period known as the Early Middle Paleolithic, which spanned approximately 200,000 to 170,000 years ago. The Denisovans had the cave to themselves for most of this time, and the Neanderthals arrived just in time for the last part.
Evidence seems to suggest that the “first and major makers” of the earliest Middle Paleolithic scrapers, notch tools, and cave cores were Denisovans. But since the Neanderthals arrived shortly before 170,000 years ago, they may have made at least some of the late period stone tools. This leaves archaeologists with the question of whether the Neanderthals learned these fabrication techniques from the Denisovans, contributed their own ideas, or developed similar technology independently. Again, the answers are unlikely to come from the Denisova Cave itself.
“What we need are other sites that contain distinctive artifacts and only Denisovan or Neanderthal fossils / DNA deposited next to them, so that we can link each hominid group to an assemblage of ‘specific artifacts,’ Roberts told Ars.
Similar questions remain about bone tools and ornaments dating to a much more recent age of 45,000 years, a period referred to as the Early Upper Paleolithic. Our species arrived in Eurasia between 50,000 and 45,000 years ago, and it is tempting to take credit for the relatively advanced suite of bone tools and jewelry that litter these layers of Denisova cave and d ‘other sites across Europe. But there is evidence to suggest that we likely traded technology with the Neanderthals.
The return of the Denisovans
The Neanderthals living in Denisova Cave about 80,000 years ago probably had no cultural memory of sharing the cave with another group of hominids; after all, the Denisovans had been absent from Denisova Cave for 40,000 years at that time. So it’s interesting to wonder what both sides thought when a group of Denisovans, genetically distinct from the first population to make their home in the cave, appeared at least 80,000 years ago.
“Denisova 11 – the bone fragment of the daughter of a Neanderthal mother and a Denisovan father – testifies to the fact that Denisovans and Neanderthals have become very comfortable at least once!” Roberts told Ars.
Denisovan’s mitochondrial DNA in these layers of the cave floor is similar to that of another Denisovan whose DNA archaeologists recovered from a site 70,000 to 45,000 years old on the Tibetan Plateau. And that could indicate where the second Denisovan population to arrive in Altai originated from. Paleontologists who study animal remains from this period suggest that some large Southeast Asian mammals migrated along the Himalayan foothills in the Altai Mountains.
“These wildlife migrations may have spurred the dispersal of the Denisovans to the region in which their remains were first discovered,” wrote Zavala and his colleagues.
When the worlds collided
This second wave of Denisovans was still around 45,000 years ago when another hominid species, us, started shedding mitochondrial DNA in the Denisova cave floor.
In their samples from a cave chamber, Zavala and his colleagues found mitochondrial DNA sequences from Denisovans, Neanderthals and Homo sapiens in a layer dating from 45,000 to 22,000 years ago. Altogether, this suggests that the Denisovans, as well as the Neanderthals, may still have lived in the Altai Mountains when the first members of our species arrived.
This is not very surprising; the location of the Altai Mountains, including the Denisova Cave, makes it a geographic meeting point for species moving between Africa, Europe and Asia. The cave’s bones and mitochondrial DNA include species of African and East Asian hyenas as well as extinct European hyena species. But it could also mean that the bone tools of the initial Upper Paleolithic may have been a multispecies effort.
Nature, 2021 DOI: 10.1020874 / 2071-0437-2021-53-2-1 (About DOIs).