Our Neanderthal DNA: Who were Neanderthals and Denisovans?

Simply put, Neanderthals and Denisovans are our ancient relatives. Behind this brief description there is an incredible story—one that describes how a different species of human could have evolved had history gone differently. Theirs is a story of separation, adaptation, and reunification. Like The Sopranos and the spinning top from Inception, the end of the Neanderthals and Denisovan story is shrouded in mystery and still talked about to this day. So, who are the Neanderthals and Denisovans?
Let’s rewind time, back to about 800,000 years ago. There, you may encounter a creature walking out from under the trees—but it wouldn’t look quite like any human you’ve ever seen. The ancestors of humans have been evolving for hundreds of thousands of years, and at this time their population fractured1. How and why it happened is unclear, but somehow a faction from one of these early human populations broke away from the rest and journeyed out on their own. It is this separation that allowed the distinct evolution of Neanderthals, Denisovans, and even modern humans1.

Anatomically modern man

What do we mean when we say “anatomically modern man”? As the name suggests, “anatomically modern” refers to the many physical features that these early humans shared with us today—features like cranium shape and size, the presence of a chin, and other nuanced details in bone structure. It’s important to make this distinction, because in many ways, Neanderthals and Denisovans could be considered human as well!

Archeological data indicates that these archaic humans evolved into Neanderthals and Denisovans in the Eurasian continent over hundreds of thousands of years before the anatomically modern man—our most immediate ancestors—left Africa. At that time, Europe was predominantly occupied by the Neanderthals. These creatures’ remains were first identified in the Neander Valley in Germany, which is why they’ve been given the name Homo neanderthalensis, or just Neanderthal for short. Mirroring the Neanderthals in Asia were the Denisovans, whose remains were first found in the Siberian Denisova Cave—hence their name1,2.
Genetic and archaeological analysis suggests that Neanderthals and Denisovans were better adapted to the Eurasian climate compared to their African counterparts. Eurasia is a cold environment that receives less sunlight relative to locations near the equator. Over thousands of years living in this environment, Neanderthals evolved a short, stocky build with variations in hair and skin pigmentation which may have been advantageous in such an environment1,3,4. Genetic research using the remains of Neanderthals shows that they likely had a shifted circadian rhythm relative to humans living in Africa, and that they may have been able to produce more brown fat, which plays an important role in body temperature regulation. All of this information suggests that the DNA of Neanderthals was changing to help them better adapt to their northern climate1,3,4.
At this point, it’s fair to ask why they disappeared—after all, they were adapting so well. In truth, the answer is not entirely clear. On one end of the spectrum, scientists suggest that the migration of the anatomically modern man into the Eurasian continent caused conflict and competition, ultimately resulting in Neanderthal and Denisovan extinction. On the other end of the spectrum is the idea that the Neanderthals and Denisovans fused cultures with modern humans and never truly disappeared1,5. It’s hard to accurately say what happened, but the human genome gives us an interesting glimpse into the past.

Ancient humans in Africa

Research shows that people of recent African heritage often do not have identifiable Neanderthal or Denisovan DNA. However, that doesn’t mean people of African heritage do not have _archaic DNA_ (that is, DNA sequences passed from ancient humans). Recent investigations have found evidence that some unknown human group contributed archaic DNA—about 2%—to some African populations. This is an area of ongoing research.6

Approximately 50,000 years ago, anatomically modern man migrated out of Africa and into Eurasia1,3,5. Human movements are not usually unidirectional, so it’s likely that there was some back and forth over the years. However, DNA sequencing projects involving people of recent African heritage show very little—if any—DNA from Neanderthals and Denisovans. That’s in contrast to people of European descent, whose DNA typically consists of about 1-4% Neanderthal DNA. Similarly, people of South Eastern Asia and Australia can sometimes have as much as 6% of their DNA coming from Denisovans3. This is a crucial insight because it tells us that Neanderthals and Denisovans were able to produce children with anatomically modern man, and that they did so outside of Africa.
It also tells us that assimilation of these cultures may have helped modern humans survive in these foreign climates. For example, some variants in the TBX15 gene increase a person’s ability to produce brown fat and is more commonly found in native peoples from Greenland—a very cold environment3. The Bajau Sea Nomads are another example: These people appear to have DNA from Denisovans that may contribute to their exceptional lung capacity7. There is even some evidence that Neanderthals and Denisovans had genetic variants that gave them a stronger immune system, and that these variants were integrated into our genome3. All of this suggests, but does not conclusively show, that anatomically modern man benefited from integrating Neanderthal and Denisovan DNA because it may have helped them quickly adapt to new environments.
Whatever happened to them, it’s clear that the Neanderthals and Denisovans influenced the course of human history. They were our extremely well-adapted relatives who may still be influencing people’s lives today. Scientists are actively working to understand how DNA from these ancient times may affect a person’s traits.

1Sánchez-Quinto, Federico, and Carles Lalueza-Fox. “Almost 20 Years of Neanderthal Palaeogenetics: Adaptation, Admixture, Diversity, Demography and Extinction.” Philosophical Transactions of the Royal Society B: Biological Sciences 370.1660 (2015): 20130374. PMC. Web. 14 May 2018.
2Rogers, Alan R., Ryan J. Bohlender, and Chad D. Huff. “Early History of Neanderthals and Denisovans.” Proceedings of the National Academy of Sciences of the United States of America 114.37 (2017): 9859–9863. PMC. Web. 15 May 2018.
3Marciniak, Stephanie, and George H. Perry. “Harnessing ancient genomes to study the history of human adaptation.” Nature Reviews Genetics, vol. 18, no. 11, Nov. 2017, pp. 659–674., doi:10.1038/nrg.2017.65. Web. 15 May 2018.
4Dannemann, Michael, and Janet Kelso. “The Contribution of Neanderthals to Phenotypic Variation in Modern Humans.” The American Journal of Human Genetics, vol. 101, no. 4, 5 Oct. 2017, pp. 578–589., doi:10.1016/j.ajhg.2017.09.010. Web. 15 May 2018.
5Hublin, Jean-Jacques. “The Last Neanderthal.” Proceedings of the National Academy of Sciences of the United States of America 114.40 (2017): 10520–10522. PMC. Web. 15 May 2018.
6Hammer, Michael F. et al. “Genetic Evidence for Archaic Admixture in Africa.” Proceedings of the National Academy of Sciences of the United States of America 108.37 (2011): 15123–15128. PMC. Web. 5 June 2018.
7Llardo, Melissa A. et al. “Physiological and Genetic Adaptations to diving in Sea Nomads.” Cell vol 173, no. 3, April 2018, pp. 569-580., doi:10.1016/j.cell.2018.03.054. Web. 05 June 2018.