That's a good one. And it shows how kid's naivity can get straight to the essense of some important questions.

These two percentages refer to two different measurements. The first one compares the building blocks (base pairs) of human and chimp DNA and finds that they are ~97% similar. Of course, if we compare human and Neanderthal DNA, as others pointed out, this similarity is even higher (~99%), which shows that humans and Neanderthals are more closely related species than humans and chimps.

Now, for two very very closely related species there might be occasional interbreeding. That is, humans and Neanderthals (when and where they coexisted) occasionaly mated and gave viable offspring. This CANNOT happen between humans and chimps (the "viable offspring" part, not the "mating"), because they are NOT enough closely related. Normally, when two individuals from the same species mate they freely exchange genetic material. So, if two individuals are NOT from the same species (as humans and Neanderthals) and effective mating takes place very rarely, then the exchange of the genetic material is very low. This 2.8% (that I think is closer to 4%) that your son mentioned refers to this exchange of genetic material.

there are two theories: the first says that it's because while we (neanderthal and sapiens) both evolved from ancient monkeys (hence the 98% in common with chimpanzees), we (sapiens) evolved later than neanderthals, so we share only few genes that were inherited from our last common ancestors.

The second theory explain this fact as a proof of a parallel evolution of the two species and possible interbreeding between neanderthals and ancient sapiens, which have led to us.

I hope it was useful

Interesting question that stems from a misunderstanding. You are more than 2.8% related to a Neanderthal-- this is simply the unique portion that is estimated to have evolved in the Neanderthal lineage that was mixed back in when the homo sapien-ancestors of Eurasians interbred with Neanderthals. In other words, it is estimated that 1-4% of the modern genome comes directly from Neanderthals, and the rest is identical and thus shared between Neanderthals and modern sapiens. See http://www.technologyreview.com/biomedicine/25269/ for more on this topic.

I suspect the current estimate for the amount of our DNA shared between modern sapiens and chimps may be a bit lower than 98% as genome sequencing advances, but you can tell your son that we share 95-98% with chimps and 99% with Neanderthals.

This milk is 98% fat-free, that one has 1% fat.

That's a good one. And it shows how kid's naivity can get straight to the essense of some important questions.

These two percentages refer to two different measurements. The first one compares the building blocks (base pairs) of human and chimp DNA and finds that they are ~97% similar. Of course, if we compare human and Neanderthal DNA, as others pointed out, this similarity is even higher (~99%), which shows that humans and Neanderthals are more closely related species than humans and chimps.

Now, for two very very closely related species there might be occasional interbreeding. That is, humans and Neanderthals (when and where they coexisted) occasionaly mated and gave viable offspring. This CANNOT happen between humans and chimps (the "viable offspring" part, not the "mating"), because they are NOT enough closely related. Normally, when two individuals from the same species mate they freely exchange genetic material. So, if two individuals are NOT from the same species (as humans and Neanderthals) and effective mating takes place very rarely, then the exchange of the genetic material is very low. This 2.8% (that I think is closer to 4%) that your son mentioned refers to this exchange of genetic material.

I think Kostas' answer almost covers it. Upon successful breeding the genetic exchange actually is more or less 50%. But that percentage is then diluted in the human population. The mating happened so often that 2-4 of the genetic material in the entire human population stems from Neanderthal, and thus is no longer diluted (although it may be selected). That also means that it is not the same 2-4 percent in different individuals. So we each have larger chunks of DNA (pieces of chromosomes) that stem from Neanderthal. We also have 98% of total sequence identical with chimps (the same was true for Neanderthal). You can see the comparison with chimps from sequencing. You can compare with Neanderthal by looking for stretches of DNA that contain SNPs that normally occur in Neanderthal. A whole stretch of these indicate that the piece of chromosome stems from Neanderthal. That is what 23andme does with the SNP arrays they use.

I think Diana's answer is almost right as well. Except that the "rest" is not completely identical. There really are small differences between a stretch of DNA that is natural in humans and a stretch that was inherited from the Neanderthal genome. That is far less than the 2% difference in sequence with chimps, but you can still detect what comes from where.

Lorraine D Rodriguez-Rivera - I do not think that logically follows. There is not a linear progression of DNA sequence change from chimpanzee through Neanderthal to modern humans. The lineage leading to the common ancestor of Neanderthals and modern humans split from that leading to chimps about six million years ago. Neanderthals and moderns humans did not split until a few hundred thousand years ago, and the great similarity of their genomes is owing to their long shared history. The differences between Neanderthals and modern humans have accumulated, then, during the relatively brief time that they have been separated, not retained from chimpanzees (except for the trivially longer time that modern humans have survived than have Neanderthals).

To summarize, perhaps: We are 98% or so identical to chimps. ON THE COMPLETELY OTHER HAND, 1-4% of European/Asian/Native American DNA comes from Neanderthals. This does not mean that we (I am one) are 1-4% identical to Neanderthals. Whether this makes these individuals more or less like chimps cannot be determined from these data.

@Ricardo Azpiroz - I agree that we cannot determine this from these percentage values, but we certainly could determine it from the sequences themselves. I am completely confident that Neanderthals and modern humans share approximately the same degree of difference in DNA sequence from chimpanzees. It could not be otherwise, since nearly all of the sequence differences between either of us and chimps accumulated during the very long period of time before the Neanderthals split with modern humans, and since the very small number of differences between Neanderthals and modern humans accumulated only during the very short time since we separated into individual lineages. Those changes in the Neanderthal genome would not have made it more like the chimp genome sequence, but rather less like the chimp genome sequence, just as would be the case for the changes during the period of shared Neanderthal-modern human evolution and just like the changes unique to the modern human lineage.

Sequences of Neanderthals come from highly-degraded DNA samples, and then we do not know for sure if 2-4% of the sequences are share by Neanderthals and modern humans. This does not means that Neanderthals are our ancestors. It is probable that both come from a common ancestor. All conclusions that we have until now are hypothesis.

Correct - but an hypothesis remains so when "not proven experimentally" - obviously we cannot repeat evolution to prove it ... but I believe evolution to be true (although "believe" might be the wrong phrasing here): look at the body-plan of all our ancestors including those we don't like (e.g. rats) or at the biochemistry of mitochondria and the ubiquitous use of ATP to make us tick and think (and sometimes thick) !

I happen to visit Neandertal in Germany last friday - Mr. H.N. leaning over the reeling in the museum, dressed in a suit but with a stone knife in his hand, looks frightening close to some hollywood movie actors ! Why he is dressed up and Miss H.N. is shown naked, is another Q somehow related to Hollywood !

Interesting discussion with good answers, but was not really fully satisfied, so I will chip in.

It is correct to say that these two percentages refer to two different measurements and that, if we just compare human and Neanderthal DNA, as we do with human and chimp DNA, they will be more than 99% because humans and Neanderthals are more closely related.

However, I have two disagreements with the most popular answers:

1) First, I do not fully agree with the interpretation of the 2.8% (or 4%) of neanderthal DNA. That is a controversial calculation, but essentially tries to be a comparison with other humans that did not mate to neanderthals, for example africans. And what this measures is what makes neanderthals unique compared to ancient humans, creating at least 3 populations: Humans (100% humans), Neanderthals (100% neanderthals) and humans that mated with neanderthals, that have up to 4% of the neanderthal specific DNA inside their own genome.

2) Finally, if there is interbreeding, I do not think we can consider neanderthals and humans two different species. If humans and Neanderthals mated and gave viable offspring, they should be consider variants of the same species. See for example this:

http://eenews.net/public/Greenwire/2012/08/16/1

or this for a recent discussion of what is a species:

http://blogs.plos.org/everyone/2012/07/25/what-is-a-species/?__utma=261502610.92003466.1337166889.1346224071.1346226881.120&__utmb=261502610.5.9.1346226887041&__utmc=261502610&__utmx=-&__utmz=261502610.1346226881.120.75.utmcsr=en.wikipedia.org|utmccn=(referral)|utmcmd=referral|utmcct=/wiki/Drosophila_synthetica&__utmv=-&__utmk=229807007

Eduardo, in general you are right. Interbreeding usual indicates that two “kinds of animals” under consideration are actually one species. However, this is only true if offspring resulting from interbreeding is fertile. For example, horses are able to have offspring with donkeys (closely related), but their offspring is infertile (not closely enough).

Accordingly, humans and neanderthals are on first sight one species, because they were closely enough related to produce fertile offspring. Then, Europeans are descendants of that (fertile) offspring resulting from interbreeding between neanderthals and humans. Neanderthal DNA can also be found in “male-typical” DNA passed on from fathers to sons (Y-Chromosome). However, neanderthal DNA is not found in DNA passed on only from mothers to their offspring (mtDNA). That is, there is no “female-typical” neanderthal DNA present is the genome of Europeans.

As far as I know, there is no final conclusion in the literature how to interpret this result.

Either, only male neanderthals actually mated with female modern humans and never the other way around (kind of a Hollywood-scenario with neanderthals raiding the “peaceful” homo sapiens sapiens and kidnapping female modern humans).

Or, “casual” interbreeding between neanderthals and modern humans occurred. It resulted not only in male but female offspring as well. However, in this second case, the female offspring would be infertile (or had at least massive adaptive disadvantages) having no chance to pass on mtDNA to any female offspring.

For a more sophisticated explanation see:

http://journals.sfu.ca/hypot/index.php/main/article/download/215/pdf_1

The two different scenarios have effect on the hypothesis from the beginning, whether neanderthals and modern humans are one species. The presence of neanderthal DNA in European DNA is an indicator but not (necessarily) an evidence that humans and neanderthals are one species.

Sven, thanks for your response. I think it is not surprising that there is no mtDNA from Neanderthals if you consider that their contribution is small in modern humans (4% of the 1% they differed from us).

Best,

Edu

Just one thought, if males were the only fertile offsprings resulting from interbreeding, could that explain the contrasting results between the Y-Chromosome and myDNA analyses? Differential fertility between male and female offsprings is well document in bird hybrids of the family Fringillidae (that of the canary, http://castrolon.foroactivo.net/t1293-hibridos-su-fertilidad). By the way, wikipedia has a nice entry on hybrids with examples occurring in nature (http://en.wikipedia.org/wiki/Hybrid_(biology)

The presence of neanderthal DNA in european DNA is not evidence that humans and neanderthals interbred. This is not surprising because both (and Denisova too) have a common ancestor. What remains diçfficult at present time is to experimentally demonstrate the two main hypothesis :

There are two main hypothesis concerned with the human genome:

- when sapiens and neanderthal populations met 50 to 70 000 years ago, some crosses occurred (probably few), mixing genes from both origins.

- sapiens and neanderthal do not hybridize, but only possess genes from their ancestor. But since 500-600 000 years, their genomes evolved in different manners, producing the so-called “sapiens” (ie genes coming from the ancestor that evolved in the sapiens way) and “neanderthal” genes. That was the hypothesis during a long time, similarly to what occurs much earlier between men and chimpanzee (whose genomes still harbor highly significant similarities). The main difference is the fact that the evolutionary time since these phylum separate is so long that human and chimpanzee could not bred.

How is it possible to produce simple direct evidence supporting either hypothesis ?

1) If the hybridization hypothesis is correct, offsprings possessed “sapiens” genes, “neanderthal” genes, and hybrid genes (ie genes partly sapiens and partly neanderthal). The lack of any hybridization hypothesis suggests that nuclear genes are coming from their common ancestor and evolved in both the “neanderthal” and “sapiens” ways.

2) At present time we do not have any direct molecular evidence at the DNA level, of a male progeny obtained through the hybridization: mother neanderthal x father sapiens ie:  possessing a neanderthal mtDNA and a nuclear DNA marker “sapiens specific” from the Y chromosome sapiens (in its Y part that do not recombine). I think there is no “female-typical” neanderthal mtDNA present is the genome of Europeans.

3) Similarly, there is no simple molecular evidence (DNA) of a male progeny produced through the hybridization between a sapiens mother x and a male neanderthal, ie possessing a neanderthal Y chromosome and a sapiens mtDNA. As yous says “we have not yet found neanderthal Y chromosomes among modern human genomes”.

As far as there are no published data concerned with the hypothesis 1, 2 and 3, hybridization between sapiens and neanderthal remains a (very) interesting hypothesis, quite complex. Nevertheless, data accumulate and now the holders of the lack of hybridization hypothesis either suggest that very few hybridization occurred or explain that various types of populations possessing significantly different genomes get out of Africa. We need urgently either consistent Y DNA data, and populations data. I am sure that it will be possible in the near future to experimentally either hypothesis.

I admit and realize that even after - or by - reading more of the popularizing articles and re-reading the above old comments and the new one, the numbers are still not clear for me - let alone for non-scientists: "HS & HN genomes are +99% identical but up to 4% of HS is of HN origine". How can that be - was the original question from the 12 year old son that started this exchange. If one goes back to the original articles, the matter becomes not less strange: between 1 and 4% of our DNA is from Neanderthal origine, varying on the individual analyzed and the find-site in Europe. 

The simplest explanation: the variability in the DNA of both HS & HN is too high so that no firm conclusions are possible - and never will. Consequently only hypotheses remain - which brings us back to the other original questions: evolution is and will be hypothetical as experimental proof will always be lacking, and moreover we indeed do not descend from the neanderthals but from their predescors - which are parent to both HS and HN. 

Was their interbreeding of HN and HS? There most certainly has been - but again the matter is hypothesis and impossible to proof experimentally. 

 I would like to detail (unfortunately by using my limited English) what should be a friendly discussion to explain what was done in the last years in order to improve our knowledge of the world conquest by Homo sapiens.

For me the statement "between 1 and 4 % of our DNA is from Neanderthal" is a misunderstanding. In that meaning, we are also able to write that we have DNA from chimpanzee. The fact that most of our nuclear DNA is identical to the DNA from neanderthal should be explained in two ways:

- first this common DNA wad inherited from the common ancestor of both sapiens and neanderthal.

- second a few neanderthal genes were transmitted to sapiens through hybridization.

Thus, most of our genes came from the common ancestor of neanderthal and sapiens, meaning that both genomes are highly similar (for example you should say sapiens possess 98/99% Neanderthal genes, or neanderthal possess 98/99% sapiens genes). If the hybridization hypothesis is correct, it will be better to write "a very limited part of sapiens DNA was inherited from neanderthal through hybridization”.

The old hypothesis, postulated that the sapiens and neanderthal genomes may be 98/99% identical because both inherited their genes from their common ancestor (as explained by Green et al (2010) :”Neanderthal fall into the variation of present day-humans for many regions of the genome”. Moreover, the sapiens genome possess specific DNA sequences and the neanderthal genome possess specific DNA sequences, because they evolved independently during about 400/500 000 years.

It is the fact that neanderthal DNA is closest to what was called “non-african” (in fact the sapiens coming from Africa that colonize Eurasia) than to the African populations tested (Yoruba, San) that suggested the hypothesis of neanderthal DNA transmitted to sapiens through hybridization (but it is only a parsimonious explanation). What is additionally postulated by the hybridization hypothesis is the fact that neanderthal and sapiens were susceptible to mate, but through a very limited gene flow from neanderthal to human (without a reciprocal gene flow because only hybrid progenies possessing a sapiens cytoplasm –ie mtDNA- were noticed). Even if the interbreeding hypothesis is true, the amount of neanderthal DNA introduced into sapiens remains limited, first because of the very high amount of DNA inherited from their common ancestor, and second, as explained by Sankararaman et al. (2014) “We have shown that interbreeding of Neanderthals and modern humans introduced alleles onto the modern human genetic background that were not tolerated, which probably resulted in part from their contributing to male hybrid sterility”. 

“A parsimonious explanation for these observations is that neandertals exchanged genes with the ancestors of non-africans” (Green et al., 2007). It is of interest to consider that Green et al. (2007) also indicated : “Scenario 4 represents old substructure in Africa that persisted from the origin of neandertals until the ancestors of non-africans left Africa. This scenario is also compatible with the current data”. So, another hypothesis is that the so-called non-african (in fact the sapiens coming from Africa) consist in small groups of migrants, now absent from Africa (or not observed), that were genetically closest to Neanderthal than present day Africans (remember that neanderthal was coming from Africa). We also have to consider that two populations migrate out of Africa, the first one to the eastern part of Eurasia and the second one to the western part. Data from the hybridization hypothesis complicate the old hypothesis. It is necessary for people migrating east to have a genome closest to denisova and neanderthal, and for people migrating west to have a genome closest to neanderthal.

Science is a game during which you built hypothesis, and then you perform experiments to test them. This is what was done for human evolution with molecular tools.  I am confident that the high quality researches performed both on fossil DNA, and on analysis of subgroups of migrants, will help to confirm either hypothesis. Things have completely change since the time (1970) I was a young student teaching human evolution in a high school. 

Yves Henry

cher Yves, thank you for this elaborate reply in more than limited English! Nevertheless, it leaves my two conclusions about evolution intact: (i) it is an hypothesis because "not experimentally proven" and (ii) it is considered "fact" because not unproven & concordant with all data (cfr my old reply dd 20 aug 2012) ... and that is true since Darwin, extra confirmed from the 60's when you and I started in science!  

I agree with you philosophic point of view, fred.  I only have tried to explain the fact that very often science omit one of the likely hypothesis and just say "have a look on my thesis" without testing others.

Be very friendly with a plant breeder, interested in human evolution since more than 40 years. Reading a lot of things about wheat and barley invariably drive your attention on human starting to cultivate (probably rye first) and then breed. These sapiens people introduce cultivation in western Europe before envading north america much later.

all my best,

Yve

PS : Probably you know that the "statue de la liberté" was created by Auguste BARTHOLDI, but only a few people know the internal structure was designed by Gustave EIFFEL.

Just read the discussion about the 1-4% DNA inherited from Neandertals or from the commun ancester. The wonderful question of tha 12 y.o. boy was actually very interesting and the divergent discussion that it generated too.

I have a question too... so, if the ancestre hypothesis is true, shouldn't all the Homo sapiens offspring, all leaving humans, have 1-4% Neanderthal DNA what ever where they are coming from? Said in a different manner... Doesn't the fact that the 1-4% of Neanderthal DNA is "concentrated" in Europeans/Asians, but not in Africans, is very strong correlation toward the interbreeding hypothesis?