Neanderthals: Expert Q&A

Q: Are Neanderthal genes found throughout the world's people? And are there greater densities among people from certain locations? Sean Karshis, Miami

Ed Green: Comparing sequence variation in Neanderthals to sequence variation that exists in people today, it's clear that there is Neanderthal ancestry in some of them. Specifically, we see that the DNA sequence from people whose ancestors migrated into Eurasia from Africa now contain Neanderthal ancestry. Surprisingly, about the same amount of Neanderthal ancestry is seen in all non-African genomes, even people from places where Neanderthals never lived, like the New World.

Q: The genetic variation in domesticated dogs and even chimps is much greater than in Homo sapiens. How does the variation in those two species compare to the differences between human and Neanderthal DNA? Greater or lesser? James Elkins, Richlands, VA

Green: Two chimpanzees are roughly as different in DNA sequence as a human and a Neanderthal.

Q: Where can I get my DNA tested to see my Neanderthal Quotient? Bruce, Opalka

Green: 23andme offers this.

Q: You said most of modern humans have some genes of Neanderthals. Would you say that today there are more than one human species? John Polo, Miami

Green: The question of what constitutes a species is tricky. The concept of species, itself, was very useful in the past for comparing, contrasting, and classifying the many forms of life on our planet. Today, we know much more about how life forms reproduce and evolve. Interestingly, the concept of species has become less useful and perhaps even not useful at all! To answer your specific question, you would first have to formalize a definition of species. This is quite tricky and there is no uniform consensus on exactly what is meant by a species. Try this yourself. For any succinct definition of species you might come up with, there will be some clear violation of that definition.

Q: What physical or physiological traits that characterize humans today actually have their origin from Neanderthals? Curt Schroeder, Regina, Saskatchewan

Green: There is some evidence that some important immune function genes come from Neanderthal admixture. We are currently working to answer this question more broadly.

Q: Given the three main blood types A, B, and O, is it possible that these exist due to the original genetic admixture of CroMagnons with Neanderthals and separately with Denisovans? George Dietrich, Ontario, Canada

Green: Unlike Neanderthal ancestry, these blood groups are present in people all over the world. Thus, they are unlikely to have derived from Neanderthal or Denisovan ancestry.

Q: I understand Neanderthal genetic material is found only in non-Africans (red hair is a Neanderthal trait.) Racial questions are sensitive, but surely such a variation in the genome must have an effect. What can that effect be? Any thoughts? Marcia Golub, NYC

Green: The most striking feature of human DNA variation is how little of it there is. This includes the extra diversity that is the result of Neanderthal ancestry. The biological impact of this Neanderthal ancestry is not fully understood, but is almost certainly small and may in fact be nonexistent. Before we had the Neanderthal genome, we had sequence variation from many people across the world. Searches for genes that are very different between various groups identified very few specific cases. The handful of such cases, interestingly, turned out not to match the Neanderthal sequence we generated.

Q: It is logical that modern man coming from Africa would interbreed with Neanderthals occupying Europe. That only seems possible if they had common ancestors. What is the evidence of that commonality, or of independent origins, and how far back? John Wolfgram, Michigan Bluff, CA 95631

Green: Neanderthals and modern humans had a common ancestor just like humans and chimpanzee have a common ancestor (and humans and monkeys, humans and horses, humans and fish, etc.) All of these common ancestors lived long ago in the past. Obviously, they are no longer here, but their descendants (us!) are. A reasonable estimate for the common ancestor of humans and Neanderthals would be the individuals whose bones we find and call Homo erectus.

Q: If humans have indeed interbred with our modern ancestors...could the mixing have led to some form of hybrid vigor as seen in different but related species/sub-species of plants? Ryan Caballes, NY

Green: Perhaps Neanderthals and humans are not genetically distinct enough to expect much of a hybrid vigor effect. We might also expect to see much higher frequency of Neanderthal alleles if there was/is indeed an advantage of having an allele of both human and Neanderthal origin.

Q: In science the teacher said the definition of a species is a group of organisms that can interbreed and produce fertile offspring. How would it be possible for Homo sapiens and Homo neanderthalensis to interbreed and produce fertile offspring? Megan, Lancaster, NY

Green: That is one definition of a species. It's a common and fairly useful definition. Strictly by that definition, Homo sapiens and Homo neanderthalensis are not separate species. Also, according to that definition, brown bears and polar bears are not separate species, since we find hybrids of these two!

Q: I read years back that the trait for red hair came from the Neanderthals. True, or no? Linda, Hartman

Green: There is some evidence that Neanderthals had a genetic variant that would have resulted in red hair. There is no evidence that this particular variant is present in humans today. We have our own way of making red hair.

Q: I read increased brain size in Homo sapiens evolution may have created problems in pregnancy/delivery because births would have been difficult. Might this, rather than warfare or competition, have reduced Neanderthals through failed interbreeding? Philip Finn, Illinois

Green: It is interesting and unknown what complications may have arisen in childbirth for the early hybrid offspring of humans and Neanderthals. It seems unlikely that this alone would have caused Neanderthal extinction, though, since Neanderthals would still be able to reproduce with other Neanderthals.

Q: Is there viable Neanderthal DNA somewhere? Junior Waysouth , Walla Walla, WA

Green: Yes – all over. Many people in the world have a few percent of their DNA from Neanderthals. 

Q: Is the interbreeding apparent in the morphology and physiology of humans? For example, Europeans are not lactose intolerant, or wide versus narrow feet. Bruce Williamson, Philadelphia, PA

Green: Perhaps, and we'd love to know for sure. Lactose tolerance, however, is not due to Neanderthals. This trait arose coincident with dairying, about 10,000 years ago, long after the Neanderthals were gone.

Q: I want to thank you and your team for answering questions. Does the genetic evidence of interbreeding match fossil records and archaeological patterns of migration and evolution? Azka, Texas

Green: It matches some interpretations of the fossil record. We see Neanderthal ancestry in people all over Eurasia. The fossil and archeological records suggest that Eurasians are all the descendants of an early group of migrants who left African about 80,000 years ago. Thus, the ancestry signal is consistent with a single origin for Eurasians at that time. One interesting open question is why there was not more, continuous admixture. That is, why don't we see a signal for more admixture in people who today live where Neanderthals lasted longer? The answer to this puzzle is still to be found.