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Name: Laurie
Status: educator
Grade: 9-12
Location: Outside U.S.
Country: Australia
Date: Fall 2012



Question:
It has been stated for some time that humans share 97% of their DNA with chimps. More recently there have been claims that they share up to 5% with Neanderthals.....it doesn't add up. What explanation can explain this difference?


Replies:
Laurie,

This is a good question based on inaccuracies in science reporting. When scientists say we share such a large proportion of our genes with chimpanzees, they are right - the vast majority of our genes are nearly identically duplicated in chimpanzees. One would imagine, then, that since Neanderthals were so much more closely related to humans, they would share even more genetic similarities with humans. This is 100% correct. Most estimates have humans and Neanderthals sharing well over 99% of their DNA. This and other studies suggest that Neanderthals and prehistoric humans diverged evolutionarily approximately half a million years ago.

The confusion comes from reports of research done recently on the genetics of humans of non-African descent. The question they were trying to answer was whether Neanderthals and humans had ever interbred. By fossil evidence, it is clear that Neanderthals and human ancestors coexisted at the same time, and thus researchers were interested in whether Neanderthals had contributed at all to our genetic heritage after the point of divergence some half a million years ago.

Researchers looked at the genetics of modern humans of both African and non-African descent and performed an analysis looking for what is called 'linkage disequilibrium'. This is a method by which they see how genes are distributed in a population. If the genes are not distributed in the population as expected, it is possible to infer an additional factor that may account for the unexpected distribution. Researchers who have performed the analysis have explained these results by positing that Neanderthals did indeed interbreed with non-African humans in the distant past (probably at least 50,000 years ago), and thus Neanderthal DNA contributed a small proportion to the DNA of modern humans. It is estimated from some studies that 1-5% of the DNA of modern humans of non-African descent may have come from Neanderthals.

So, to summarize, we easily share over 99% of our DNA with Neanderthals, but in addition somewhere between 1 out of 100 and 1 out of 20 ancestors of non-African humans actually WERE Neanderthals. This is still an open area of scientific research, however, so we may learn more as further studies are done.

S. Unterman Ph.D.


I'm going to stick my neck out here and attempt an explanation. If anyone has a better explanation, I will stand corrected. I believe what this means is that 97% (or more depending on the source) of our DNA matches up with chimpanzees. A lot of our DNA matches with other organisms as well, but the parts that match may be spread out over different chromosomes. But with chimps, much of our DNA matches and lines up together on the same chromosomes in each. But there is 3% that doesn't match with them. That would be considered DNA sequence that is found only in humans (and chimps would have DNA that is only found in chimps). Of that, we share 5% with Neanderthals, but not all of us! Besides being considered a human species, there has been considerable debate as to whether Neanderthals and Homo sapiens (us) ever bred together. There are very few Neanderthal fossil specimens and the DNA that they have is degraded. The best sample to take in that case is mitochondrial DNA (found in the energy producing mitochondria which have their own DNA) and is only passed through the maternal line. It is more plentiful in each cell and therefore easier to study when there isn't a lot of sample. There didn't seem to be any crossover in the sequences studied from mitochondria. But now the nuclear DNA has been studied and some very interesting evidence has been discovered. Humans started migrating out of Africa and different groups parted and went separate ways where they continued to evolve in isolation. Somewhere along the line they seem to have mated and produced offspring because those of us that can trace our ancestry back to Europe (at least out of Africa) can show traces of some sequences that are shared with Neanderthals. But there doesn't seem to be any Neanderthal DNA sharing with those of us who trace our ancestry back to Africa. I think that is a very cool finding! And keep your eyes open, because I'm sure there will be more to follow.

vanhoeck





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