There has been a ton of great discoveries (see You Might Also Like below) this summer that enhance our understanding of our evolutionary history, including a study recently released in Nature confirming that two groups of our evolutionary cousins – Neanderthals and Denisovans – likely interbred more frequently than was previously known. The implications of this study have a direct bearing on us: modern humans carry genetic traces of both Neanderthal and Denisovan DNA and unraveling this complicated genetic heritage may allow us to better understand the social landscape of our evolutionary ancestors.
Everything we know about the Denisovans comes from a small body of evidence consisting of a partial finger bone, two teeth, and a single toe recovered from the Denisova Cave in the Altai Mountains in Siberia. From these scarce human remains we’ve sequenced the Denisovan genome, but we have little else to help us understand who they were and how they lived. We haven’t found any tools or artifacts that can be definitively linked to them.
The story we have put together has been largely thanks to genetics. We know they were a distinct ancestral species; they diverged from the Neanderthal line 400,000 years ago. They participated in one (or many) of the waves of migration out-of-Africa but it’s hard to pinpoint exactly when given the evidence we have. Their migrations would have occurred between 300,000 years ago, when Neanderthals began their migration, and 60,000 years ago when modern humans followed. And we know these groups did not keep to themselves: Denisovan DNA can be found in living humans from Asia (less than 1%) and Melanesia (up to 6%). In fact, modern Tibetans have a variation of a gene that regulates blood hemoglobin, which allows them to live in oxygen-thin places at high elevations. This gene can be traced to the Denisovans. European and Asian populations carry 1-2% of Neanderthal DNA and scientists believe that genetic inheritance had a large part to play in the adaptability of modern humans to a wide range of environments as these genes impact everything from blood clotting to depression and addiction to sun sensitivity. How these different groups of hominids interacted remains something that is less understood.
But with the recent news coming out of Denisova Cave, we may have another small piece of the puzzle. Researchers retrieved more than 2,000 bone fragments in 2008, including a tiny remnant of a “long bone” labeled Denisova 11. The thickness of the outer part of the bone fragment suggests that the bone belonged to a female of about 13 years old, while radiocarbon dating tells us she lived 50,000 - 90,000 years ago. What is truly remarkable about this fragment, however, is that the mitochondrial DNA, which is inherited from the mother, resembled DNA from a Neanderthal found in Croatia (but not the Neanderthals previously identified within the Denisova cave). Her yDNA - the DNA inherited from her father - was Denisovan, although he seems to have had a touch of Neanderthal DNA in his heritage as well. She has been nicknamed Denny. From the long bone sample that represents her entire existence, 38.6% of the fragments collected carried alleles matching the Neanderthal genome while 42.3% carried alleles matching the Denisovan genome. She is physical proof that Neanderthals and Denisovans were interacting and interbreeding with each other.
Her genetic similarity with the Croatian Neanderthal gives us clues to a possible migration pattern. The Croatian Neanderthal died about 55,000 years ago, which is a more definite date than the age assigned to Denny, and the Neanderthal recovered from Denisova Cave is about 120,000 years old. Scientists propose that Denny’s mother came from a group of Neanderthals that either traveled east to the Altai Mountains and partly replaced the local Neanderthals or a group the left the Altai mountains and traveled into Europe after she was born. In this scenario, the Neanderthals are roamers which fits with the fossil remains and genetic heritage that we have been able to find to date.
The work to arrive at understanding Denny’s parentage warrants its own recognition. While the media celebrations the conclusion, the work that it takes to arrive at the “Aha!” moment isn’t often acknowledged. This work began in 1984 with the first discovery of a tooth. In 2000 another tooth was found, and in 2008, a collection of 2,000 unidentified bone fragments were analyzed for signs for human proteins. The scientists revealed they had been able to date the fragment to 50,000 - 90,000 years ago using radiocarbon dating in 2016, and began to sequence the mitochondrial DNA to compare the data to other known sequences. This study provided evidence on the parentage of Denisova 11 – although it does not delve too deeply into the possibility that the parents themselves were of mixed Neanderthal-Denisovan heritage. This represents a span of over three decades of work to unravel, unveil and identify a fragment of our evolutionary history.
We don’t yet understand how the Denisovans and Neanderthals and modern humans interacted – whether these were peaceful meetings or not. And scientists say that there is yet unidentified ancient DNA in our lineage that does not belong to either the Denisovans or Neanderthals so there is more for us to learn about our genetic and social history. These findings do tell us that these meetings may have been more frequent than we understand – and the larger story continues to unfold.
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Slon, Viviane et. al. (2018) The genome of the offspring of a Neanderthal mother and a Denisovan father. Nature (561): 113-116.
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