“Today Europeans have a small percentage 1 to 3% of their DNA that is a match the genome of Neanderthals. That DNA is spread around in our genomes as tiny little bits. To have spread to all Europeans that intermixing event would have had to have occurred long ago so that Neanderthal DNA introduced into the population would have had time to spread. Each generation that DNA not only would be passed on but would also be mixed in our genomes as we recombine our chromosomes prior to reproduction. So genetic models can backtrack how long it would take since a hybridization event for our genomes to look like they do today. These models suggest it would have been 40 to 60 thousand years ago. Last year, a complete sequence of a 45,000 year old modern human from a Siberian cave was produced. Analysis of that skeleton revealed that this person also had about 3% Neanderthal sequence but significantly that Neanderthal DNA was found in long pieces of intact Neanderthal genome segments rather than broken into thousands of pieces like it is in my genome. This is just what geneticists predict would be found in someone who was much closer in time to the hybridization event. In fact with that genome now they were able to calculate much more precisely how many generations had passed since the introduction of Neanderthal DNA into the lineage that lead to that 45,000 year old man.”
“A comparison of the amount of similarity in mitochondrial DNA between individual humans, Neanderthals and other fossils. Fig. 7. Nature (2013) doi:10.1038/nature12788 A mitochondrial genome sequence of a hominin from Sima de los Huesos. Matthias Meyer et al.”
“Below is a figure showing a similar set of data but with more samples. Samples in grey represent individuals from all people groups on Earth alive today. The length of the lines shows how different they are genetically with African lineages having the most variation. All Neanderthal sequences are more similar to one another than they are to any person alive today. Mitochondrial DNA differences between lineages of modern humans and Neanderthals.”