My DNA contains almost 2% from Neanderthals. How did that happen?
![The Neanderthal woman’s remains were found in Gibraltar. (Image credit: Royal Pavilion & Museums; Brighton & Hove [UK]): https://www.livescience.com/64620-ancient-britons-facial-reconstruction.html Re-created by Oscar Nilsson, a forensic artist based in Sweden.](https://miro.medium.com/v2/resize:fit:1400/1*unLbcoVMl9IItwvqQZlE2w.jpeg)
An essay recently published in The Conversation motivated my writing this post. Here is the link: Neanderthals died out 40,000 years ago, but there has never been more of their DNA on Earth: https://theconversation.com/neanderthals-died-out-40-000-years-ago-but-there-has-never-been-more-of-their-dna-on-earth-189021#comment_2867151
Co-existence and Reproduction: Our “Mysterious” Ancestors
Today we know much more about the evolution of modern humans and our distant ancestors. DNA technology has revealed so-called “ghost” lineages (below) that remain for us to learn more about. For now, lets concentrate on Homo sapiens and Home neandethalenis, the two lineages that we most closely associate. Keep in mind that it is also well established that Homo denisova, another hominid, coexisted during that same time period and all three interbred (https://en.wikipedia.org/wiki/Denisovan): “Denisovans apparently interbred with modern humans, with the highest percentages (roughly 5%) occurring in Melanesians, Aboriginal Australians, and Filipino Negritos” .
In my opinion: It seem obvious to me, based upon the strictest biological definition of “species”, these three lineages existed more like races of the same species. For more about “speciation” have a look here: https://en.wikipedia.org/wiki/Species . We can speculate that they might have evolved from a common ancestor enabling them to be reproductively compatible, i.e. produce viable offspring who themselves were fecund in future generations.
Below we can see how the relationships might have evolved among several lineages of our ancestors.
It is thought that H. sapiens and H. neandethalenis shared common territories more than 40,000 years bp and interbred for ~14,000 years among small populations scattered around Euro-Asia. H. sapiens it is now believed lived in larger local populations, possibly ~ 10x the size that of a typical H. neanderthalensis community.
How are we able to reconstruct our history with Neanderthals and other ancient humans?
Largely through many years of research into the analytical methods used in building lineages using DNA extracted from ancient teeth and bones that were many thousands of years old. The 2022 Noble prize in Physiology or Medicine was recently awarded to Svante Pääbo for his work with ancient humans: https://www.science.org/content/article/nobel-prize-physiology-or-medicine-2022 .
Six discoveries that have shone light on modern human survival
Dr. P. Spikins, Professor of the Archaeology of Human Origins, University of York (UK), recently named six recent discoveries that have had large impacts on our understanding of modern and ancient human histories.
- There are more human Homo species than we ever imagined: to date, it is estimated there are 21 known species.
- Hybrid humans are part of our history: genetics and phenotypic diversity underlay our past adaptations to changing climates and ecological communities.
- We got lucky: often living in small populations, random genetic variants could have resulted in characteristics that were truly under DNA control and others not activated without a certain trigger, e.g. epigenetics.
- Our fate is intertwined with nature: quoting — — “Three major human species Homo erectus, Homo heidelbergensis, and Homo neanderthalensis died out with major shifts in climate such as the Adams event. This was a temporary breakdown of Earth’s magnetic field 42,000 years ago, which coincided with the extinction of the Neanderthals.”
- Kindness is an evolutionary advantage: like many of you reading this, “kindness” is a word that can seem out of context in the world some humans experience today. Archeological and anthropological evidence indicate modern humans have seemingly evolved “altruistic” behaviours that have served us well.
- We’re a sensitive species: quoting — — “Our complex feelings may not always be pleasant to live with, but they are part of key transformations which created large, connected communities.”. Our modern news is filled daily with disasters and human suffering. But invariably, we witness humans helping other humans.
What about all the ancient DNA we are finding in modern human populations?
It is estimated that at least 21 Homo spp have existed, and there are two ancients having the most evidence for sharing overlapping time and space congruent with Homo sapiens. Those two are Neanderthals and the Denisovans (Homo denisovans). There is evidence that all three interbred. However, evidence tells us that at least 99% of modern human DNA is the same as Neanderthal: estimates of ~2–4% of the DNA in Europeans and Asians is found to be Neanderthal. We must also be reminded that chimpanzees and bonobos also share ~99% of the same DNA with us modern humans. Why do we look so different? That’s another story to be told.
Why is there so much Neanderthal DNA in our modern human populations?
The best genetic explanation is this — — “Introgression”. In other words, genetic matings between two or more species followed by successful backcrossing to one, or both. Let’s consider the simplest case: modern humans and Neanderthals. It could look something like this (below).
The graphic shown above tells us that all possible matings must have occurred between Sapiens and Neanderthals. Inbreeding must have also been more common within small hybrid Neanderthal communities. Occasional outcrosses with Sapiens would have maintained enough local genetic diversity for adaptation to changing environments. Keep in mind that the “Hybrid species” (bottom) refers to us — - modern hybrid humans.
Because of the likelihood of close inbreeding, possibly resulting in an increase in deleterious recessive genes within a community, the genetic load may have been expressed fairly early. We can only speculate on the impacts given current knowledge: go to this source for more in-depth genetic analyses about samples taken from a cave in eastern Europe: “ 45,930 and 42,580 years ago from Bacho Kiro Cave, Bulgaria”.
The Introgression figure above shows there to be early matings between SS (pure Sapiens) x NN (pure Neanderthals), and all possible combinations thereafter. We can only presume that all possible genotypes resulting from such diploid crosses were random, e.g. SS (25%), SN (50%), NN (25%): each individual would have a genome containing 23 pairs of chromosomes (2n=46). The SN genotype we now know backcrossed with SS. For example, after only a few generations of inbreeding most SN-genomes were largely Sapien and SN content was reduced to small amounts after a few generations of matings. An example, using 5 generations of inbreeding of Sapiens x Neanderthal within a a hybrid population could result in ~(0.50)⁵~ 3% Neanderthal (SN) content, leaving approximately 97% Sapiens (SS). If that is a good approximation of our history, the Neanderthal DNA is very close to what has been maintained in modern human populations, i.e. 1.5–3.5 %. This makes sense when we remember that modern humans continued passing on Neanderthal DNA to the present.
Having no excessive selection pressures against low levels of Neanderthal content, it now appears to be fairly entrenched in our modern Euro-Asian populations. You can find references to modern deleterious genetic variants in low frequencies that are detected in modern H. sapiens which have been attributed to Neanderthal DNA. Examples have been published in the scientific literature: https://www.newscientist.com/article/2077269-our-neanderthal-genes-linked-to-risk-of-depression-and-addiction/ . Thousands of years ago and in different climates the Neanderthal variants might have had higher fitness.
My personal footnote
After having my DNA analyzed I discovered that my genome contains almost 2% Neanderthal DNA, in line with the evidence reported above. That amount puts me in the upper 75th percentile of humans with Neanderthal heritage. I must say I’m proud that my >40,000 year old ancestors made it this far and I am one of the modern repositories!
Thank you for using your time to read my writing!
You can reach me at this email address: zackflorence2016@gmail.com
