Human DNA Contamination Explains Away Genetic Evidence for Human-Neanderthal Interbreeding
You love her
She loves him
He loves somebody else
You just can’t win
-J. Geils Band, Love Stinks
My father had his share of idiosyncrasies. Even though he was a no-nonsense guy, he loved soap operas.
My dad was a nuclear physicist and a college professor. When I was a little kid, we lived on the campus of West Virginia Institute of Technology, where my father taught and served as the chairman of the physics department. He was well-known for instilling fear in the hearts of engineering students who took his classes. Yet, I remember him coming home every day at 2:00 p.m. for an hour to watch Days of Our Lives. He was totally caught up in the make-believe lives of the characters that resided in Salem.
It’s easy to get hooked on the intrigue of a soap opera, no matter how educated and sophisticated you are. I have been closely following a soap opera, of sorts, myself. Instead of being aired on network TV, this drama has unfolded in the pages of the scientific literature. Did Neanderthals and modern humans interbreed? I can hardly wait for the next episode of scientific discoveries to find out.
To date the wait has been well worth it. True to form for a good soap opera, a new study published in PLoS Genetics adds an interesting plot twist to the story.
Until this most recent work, it appeared that Neanderthals and humans had an on-again, off-again romance. Some studies suggest that the two creatures interbred; others indicate that they had nothing to do with each other.
The primary evidence for interbreeding between humans and Neanderthals comes from the fossil record. (See here, here, and here.) Though controversial, a few hominid finds appear to possess a mosaic of human and Neanderthal features, leading to the conclusion that they were Neanderthal-human hybrids. There is also some indirect genetic evidence for interbreeding as well.
On the other hand, analysis of mitochondrial DNA isolated from Neanderthals provides direct evidence that these two species did not hook up.
Preliminary results from studies of Neanderthal nuclear DNA have been much less clear cut, however (Science, November 16, 2007, Vol. 314, p. 1068-1071). Analysis of nuclear DNA is potentially much more informative than mitochondrial DNA. One analysis of Neanderthal nuclear DNA sequences, conducted by a team from the Joint Genome Institute in the US, found no evidence for interbreeding. Another analysis, performed by researchers from the Max Plank Institute in Germany, however, noted some human genetic signatures in the Neanderthal genome, indicating that humans contributed to the Neanderthal gene pool.
To help understand why these two studies yielded disparate results, a team from the University of California, San Francisco, reanalyzed data from both studies. In principle, these two studies should have produced the same result, since both research teams were working with the same samples taken from a single Neanderthal specimen.
Reevaluation of the data from the two studies indicates that one of the research teams—the one that detected interbreeding—inadvertently introduced human DNA as a contaminant into the Neanderthal DNA, explaining why it appears that humans interbred with these hominids.
Alas, in a somewhat anticlimactic fashion, the sordid affair between Neanderthals and humans comes to an end. The scientific evidence increasingly indicates that Neanderthals and humans had nothing to do with one another.
We were friends
But now it’s the end of our love song…
-Dave Mason, We Just Disagree
For more on the relationship between humans and Neanderthals see Who Was Adam?