Latest Genetic Evidence Indicates No Interbreeding between Neanderthals and Humans
In the fall of 2006 geneticists from the University of Chicago created quite a stir when they announced evidence that a version of the human brain size gene, (microcephalin) came from Neanderthals as a result of interbreeding between these two species.
Microcephalin plays an important role in brain size development in humans. Mutations to this gene lead to microcephaly, a condition characterized by a severe reduction in brain volume. Evolutionary biologists think that changes in this gene through time led to the emergence of unusually large human brain size as hominids evolved into modern humans.
This prompted geneticists from the University of Chicago to examine the origin of this gene from an evolutionary perspective.
There are two categories of microcephalin genes, referred to as D and non-D haplotypes. The D haplotypes are found in over 70 percent of the world's population, the non-D types in the remaining 30 percent. Based on their characterization, the researchers concluded that the D haplotypes originated about 37,000 years ago and that the non-D haplotypes emerged about 1.1 million years ago. They estimated that the two haplotypes diverged from one another about 1.7 million years ago.
They speculated that the non-D haplotypes uniquely emerged in the lineage that led to modern humans and the D haplotypes originated in the Neanderthal lineage. At 37,000 years ago, when Neanderthals and humans coexisted in Europe, interbreeding is thought to have introduced the D haplotypes into the human gene pool. The D haplotypes then rapidly spread among human people groups until it reached a frequency of about 70 percent.
Even though this evolutionary scenario seems compelling, it's important to note that:
- It is speculative
- It represents indirect evidence for interbreeding, based on an inference
New findings reported at a recent Biology of Genomes meeting (Science, May 18, 2007, p. 967) stand in sharp contrast to the evolutionary scenario proposed by University of Chicago researchers. Through a direct analysis of the nuclear DNA sequences of the Neanderthal genome, researchers from Harvard University and the National Human Genome Research Institute conclusively discovered no indication whatsoever that humans and Neanderthals interbred. Analysis of human and Neanderthal Y-chromosome sequences by researchers from the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, produced no evidence for gene flow between humans and Neanderthals. Earlier studies of Neanderthal mitochondrial DNA yielded a similar conclusion as well. Direct comparisons between Neanderthal DNA indicate that these two species did not interbreed.
This has important implications for studies like the one done by the University of Chicago geneticists.
Other researchers have interpreted unusual regions in the human genome with an apparently ancient date of origin as evidence for interbreeding between modern humans (who appeared on the scene recently) and archaic hominids. (For example, see here and here.) Again, it’s assumed these older genetic locations were introduced into the human gene pool as a result of interbreeding between humans and Neanderthals or humans and Homo erectus.
The latest results on human-Neanderthal interbreeding, however, invalidate these studies and strongly suggest that the methodologies and interpretative approaches used to conclude that humans and archaic hominids interbred must be flawed.
For a detailed discussion of how Neanderthals fit into RTB's human origins model, see Who was Adam?