Did Neanderthals and humans interbreed?

Some time between 300 – 600,000 years ago humans diverged from the Neanderthals. However, this wasn’t the end of our genetic interaction. Some of us have a few percent Neanderthal DNA, suggesting out two species interbred at some point after the divergence. But is this necessarily the case? This weeks question comes from Art, who asks

suppose (a) this DNA is a holdover from the common ancestor of humans and Neandertals; and (b) it had nearly disappeared from the human lineage when a splinter group of humans left Africa, but that splinter group happened to retain it.  It seems this would explain the commonality of DNA, even if no interbreeding took place.

Adam, do you think humans and Neandertals interbred?   What is your reasoning?

Although human and Neandertal interbreeding is talked about so frequently this is actually a viable alternative. What Art is talking about is the idea that our Neanderthal DNA is a result of ancient population structure, sometimes referred to as “incomplete lineage sorting” (ILS). It is a real biological phenomenon that could explain how we got Neanderthal DNA without any interbreeding occurring; and was actually considered by many scientists for a while. By chance some human populations might share more alleles with Neanderthals than others. This increased similarity is then confused for evidence of interbreeding by sex-obsessed scientists.

ILS is a surprisingly common occurrence and happened when the human branch split from gorillas and orang-utans. In other words a small amount of your DNA appears more similar to gorilla DNA than chimp, despite the fact we’re more closely related to chimps. The amount of DNA “confused” by ILS is small, typically between 1 – 3%. But then we only have about 4% Neanderthal DNA.

4 alleles (represented by different colours) are present in a common ancestor which splits into 2 branches. One branch then splits again. Because of chance species 2 inherits an allele present in species 3, even though it is more closely related to species 1. This is ILS

One way to test whether ILS has occurred is with linkage disequilibrium. This is when two or more alleles are more likely to be inherited together than would be expected due to random chance. However, sexual reproduction jumbles up the genome, “breaking” linked genes. This means that over time the number of linked genes decreases.

This is useful because under the ILS model we got Neanderthal DNA before our species split 300 – 600,000 years ago. Under the interbreeding model we got it much more recently, within the last 100,000 years. So we can then look at the number of linked alleles shared by us and Neanderthals (which means that they were inherited from a common ancestor). The ILS model predicts we should see fewer linked alleles in this shared DNA, since more time has passed.

Last year a team of scientists investigated whether the amount of linkage disequilibrium was consistent with the ILS model and they found it was not. Instead it indicated that we gained the Neanderthal DNA at some point within the last 100,000 years.  This almost certainly suggests that we interbred with Neanderthals after all.

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23 thoughts on “Did Neanderthals and humans interbreed?

    • But now we have a sex-obsessed chicken and egg paradox. Are scientists sex-obsessed because they’re sex-obsessed humans; or are they detecting the natural bias towards it in humans

      • I’ve been Neandertal obsessed ever since I read William Golding’s book The Inheritors when I was twelve. That was a long time ago and so much has changed. What I find intereasting is that ever since Svane Paabo dropped the bomb so to speak a lot of studies have come out on both sides regarding Neandertal intelligence, diet, technology and the ability to speak. (Our ancestors became smarter because they ate meat. Neandertals were stupid because they only ate meat. They didn’t eat shellfish; yeah they did. They ate stomachs.They were less intelligent because their vision was too good.) My favorite discoveries though, are they invented pitch and string. It almost seems like, now that we know they’re related, they couldn’t possibly be as backward as once thought. Are they reevaluating old research, finding new stuff or getting more funding?

        • My personal favourite is how much people doubted Neanderthal’s use of bone tools, even though their ancestors were perfectly capable of it. To preserve the image of dumb Neanderthals it must be believed they somehow forgot to make bone tools.

          What’s happening these days is a re-evaluation of just about everything. People are becoming more cautious about interpreting stuff as evidence of behavioural modernity and also more cautious about viewing stuff as sign a species was dumb (like the Neanderthals). I think they’re beginning to reach a happy medium where discoveries are treated with appropriate skepticism, yet early signs of modern intelligence are not being rejected out of hand. Of course, there is still huge amounts of debate over everything.

        • The new evidence points to neanderthals being the first modern men. A people that over 200k years ago developed the first industrial process making glue/epoxy by “Sweating” birch bark (pine works well too). This is an important ingredient for “Hafting” tools and weapons. Many so called “Hand Axes” attributed to neanderthals look as though they could have been easily hafted into iconic “Dwarven Axes” necessary for the high wood usage of living in ice age Eurasia. And let’s not forget the massive cc’s of grey matter between the Neanderthal’s ears. This means the possibilities for rises and falls of neanderthal culture and technology over the eons are extraordinary and become more exciting with each new find. For instance, the first form of transportation http://en.wikipedia.org/wiki/File:Archangel_reindeer3.jpg
          If neanderthals did travel like this the only surviving proof would be an extremely rare cave painting. But if their mind is modern they would likely do the same thing everyone else does when living in an area with vast
          herds of reindeer/caribou- hunt them, feed them, domesticate them, ride and harness them, dressed in clothes made from them.

  1. . The evidence for the interbreeding of Neanderthals and Humans seems to be pretty convincing. Besides the genetic link that the post discusses, I believe that scientists have actually found fossil evidence of a human and Neanderthal living together. The really convincing evidence for their interbreeding is the fact that a child’s body was also discovered near the couple. Many scientists believe that the child was the product of the interbreeding of a human and a Neanderthal.

    I think this topic is very interesting, especially when it is considered in relation to the depictions of human evolution in the David H. Koch Hall of Human Origins in the Smithsonian’s Museum of Natural History. The museum offers good information about the history of human origins, but I think it focuses too much on the idea that humans are the superior species. It makes no mention of possible interbreeding between early human species nor does it convey the factual information that is contained in the post. Instead, the exhibit features a linear flow of the development of humans from early cave dwellers to the masters of fire to the creators of stone tools to the dominators of the world. This linear approach that only examines the innovative side of human evolution fails to portray the full story of human evolution. It also glorifies our own evolution, but does not discuss other species in as much detail as it does the innovations of humans. It is only natural that this exhibit would not discuss the possible interbreeding of humans and other “primitive” human species because it would counter the idea that humans are the superior species.

  2. Pingback: Are orang-utans our closest living relatives? | EvoAnth

  3. >Some time between 300 – 600,000 years ago humans diverged from the Neanderthals.
    How is the premise even proved? Have Asians diverged from Europeans?

  4. In nature and laboratories, it is impossible for 202 mitochondrial base pair differences to breed and produce a viable offspring. The Bonobo and Common chimpanzee only have 75-130 differences and they cannot produce a viable offspring–so, why should a theory (peice of paper) be a believable source when we have testable facts that contradict the theory?

        • Adam, I agree that chromosome count is paramount, however, within the bonobo and common chimp C-count of 48, they cannot viably reproduce due to their MtDNA differences. Alas, the fact still remains that a paper is undermining observable, testable, proven data. Not to mention that the 202 differences were a “best case” scenario during the comparison that Paablo/Kerr produced in their paper–their ancient DNA didn’t have “start/stop” codons to prove the comparison points.

          • I’d be curious to see the references for your numbers; as the papers I’m aware of suggest that there are more than 300 mtDNA differences between bonobo groups; let alone between chimps and bonobos.

      • Because it is major genomic speciation. Some rats have 46 chromosomes, but due to their genomic differences we cannot mate. :o) The same shall be stated with all of natural species.

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