Modern Humans and Neanderthal Interbreeding
Improved understanding of genetics has provided a great deal of information about the human body, and about human history. One of the most surprising discoveries in the understanding of the human genome is one source of some of that genetic material. A small percentage of modern humans show evidence of one particular type of ancient genetic material; Neanderthal genes. Non-African modern humans have between one and four percent Neanderthal DNA; early modern humans had significantly more Neanderthal DNA, around six to nine percent. It is important to remember that Neanderthals did not evolve in, and never lived in Africa; those individuals with only African origins cannot, therefore, have Neanderthal DNA.
There are two types of DNA in the body; nuclear DNA and mtDNA. MtDNA is found in the mitochrondria, but not the nucleus of the cell. Neanderthal DNA only appears in the nuclear DNA of modern humans, but not in the mtDNA. The reasons for this are not well-understood.
In addition to not understanding how the genetics passed from Neanderthal to human, science and archaeology have provided little information about the specifics of interbreeding. Neanderthals, while quite similar to humans, had some definite differences, including a lack of language. Biology and genetics has provided a few answers.
Individuals born as the result of breeding between a modern human and a Neanderthal were clearly healthy enough to survive to adulthood and breed themselves. If they had not, their genes would not have survived into the modern world. They were not sterile as a result of cross-breeding, nor did they consistently die young. In addition, they must have been relatively accepted by their group; survival would not have been likely without the help of others.
Some of the genes associated with the Neanderthal genome in modern humans have to do with structure and regulation; however, the expression of genes, or phenotypes, do also shed some interesting light. A study of the Neanderthal genome suggests, for instance, that the Neanderthals were, at least in part, fair skinned and red haired.
Modern humans did not only breed with Neanderthals, but also with other early human species, including the Denisovans. These species, now extinct, survive in small ways in our own DNA.