One way or another many human beings carry a percentage of their DNA inherited from Neanderthal man. This has been suspected for some time (not only in comedy routines); now there is genetic data to back it up. An international team of scientists coordinated at the Max Planck Institute of Evolutionary Anthropology (Leipzig, Germany) and led by a pioneer of paleogenetics Svante Pääbo, has published in the journal Science their completed first draft for the genome sequence of Homo neanderthalensis – Neanderthal man, accompanied by an extensive analysis and interpretation.
[You’ll see articles with the spelling Neanderthal or Neandertal. The name is derived from the place where the first bones were found in 1856, a cave in the Neander Valley (in the German Federal State of North Rhine-Westphalia). In the German of the time this was called the Neanderthal, Thal meaning valley. By convention, the official scientific name is also Homo neanderthalensis. However, in 1901 the Germans changed a number of spellings, including changing Thal to Tal. Some writers now use the spelling Neandertal man.]
Keep in mind that Neanderthal man is extinct. All that we know of this human species is derived from bits of bones and a few artifacts. The four years of work and astonishing technical innovation that went into the assembly, processing, and analysis of the genome represent a major scientific achievement. Unfortunately, in true modern media fashion it is largely misrepresented by sex-laced headlines and will be buried by the disasters (floods, oil spill, economic perils) occurring during early May of 2010.
Nevertheless, the news that we (we being many people with European or Asian heritage) share 1-4% of our genes with Neanderthals has widespread implications not only for genetics, but also for the study of human evolution, and most broadly for the genetic underpinnings of what makes us, Homo sapiens, human.
A Careful Investigation
With so little Neanderthal material to use for DNA testing – the primary contribution came from three small female bones, about 40,000 years old, found in Croatia – and the very real possibility of contamination from handling by modern humans, the research team had to jump through many technical and procedural hoops to arrive at a reasonable composite genome. The article in Science provides a brief account of the measures taken, as does this article in Ars Technica: Genome hints humans, Neanderthals rolled in prehistoric hay. There is still more work to be done on the genome (the published version is considered a first draft) and it is quite likely that some of the techniques and data may be challenged. In short, even the methodology used, while impressive, will be subjected to vigorous skepticism. This will be even more the case for some of the conclusions drawn by the researchers.
In studies conducted by Eric Green and colleagues (National Human Genome Research Institute, USA), concluded that human and Neanderthal genomes differ by about 12.7%, which puts the age of divergence at about 825,000 years. Because even modern humans show considerable differences in genome, the team also compared the Neanderthal genome to a composite genome of 5 contemporary humans. Here the differences were in the range of 8.2%-10.3%. This changed the estimated timeline to divergence occurring between 270,000 and 440,000 years ago.
The researchers also wanted to what specific genes were different between Homo neanderthalensis and Homo sapiens. Not many, as it turned out. Only 78 out of 20,000 protein producing genes had changed since the split with the Neanderthals. Counting other kinds of genetic information (microRNA, promoters, etc.), they found about 350-550 changes. Surprisingly few changes.
Sapiens encounters neanderthalensis
In the comparison of genomes between modern-day humans and Neanderthals, it turned out that people of European and Asian descent have about 1%-4% of their genes in common with Neanderthals. African humans, on the other hand, share very few genes with Neanderthals. Why? It could be contamination of the samples, but while still a possibility; tests show it to be unlikely. The conclusion drawn was that a wave of Home sapiens proceeding from Africa encountered Neanderthals in the Middle East about 50,000 to 80,000 years ago – and they made whoopee – interbred, producing the hybrid of genes we see today. This result contradicted earlier results from testing of mitochondrial DNA (DNA found only on the female side), which seemed to show there was no interbreeding.
Actually, as described by Christine Wilcox at Science Blogs Ancient Sex Scandals: Did We Get It On With Neandertals? there are three plausible scenarios for the current mix of genes:
1. Neanderthals and non-Africans interbred.
2. Neanderthals and humans already shared genes in Africa (coming from a similar gene pool but in different eras). African humans that survived were not from this gene pool.
3. Through natural selection genes remaining among Neanderthals in the north are a better match for genes surviving in humans also living in the north, than for humans living in Africa.
The latter two scenarios don’t require mating between species. Of course, all three are hypotheses – statements of possibility that must be tested, compared against facts, and survive logical criticism – a typical situation for science.
That’s not to say that the hybridization hypothesis is without merit. Indeed, the authors did a damned good job presenting their case, and their reasoning is sound. The point I’m making is that sex isn’t the only option. And if hybridization did occur to the extent they predict, we’re likely to find more hints at its existence. Analyzing the DNA of some of the suspected hybrid fossils, for example, might settle it once and for all. Or we may never know if the gene variants that are similar between Eurasians and Neandertals are due to sex, selection, or substructure. Time may have simply destroyed too much of the evidence for us to be sure.
[Source: Science Blogs]
The sensationalism of possible interbreeding will pass (along with the sexy headlines). What will endure will be ongoing search and discovery based on comparing the genome of Neanderthal, the variants of modern man, (hopefully) genomic studies of other hominids of the past, and our nearest relatives among the apes. By studying the similarities and differences there is a wealth of information (mostly by inference) about not only the genetic evolution of humankind, but also about the nature of our DNA.
Already, research by Prof. Gregory Hannon and colleagues at Cold Spring Harbor Laboratory (USA) has shown that Neanderthal and modern man differ in only 88 protein producing gene sequences – a number they reduced from the previously estimated 1000 differences by using a sample of 50 varied modern humans. They speculate that, with luck, more Neanderthal samples will also be used and the number could be reduced even further. This result is astonishing to the anthropologists. At the level of DNA, the difference between most of us and Neanderthals is minimal.
In short, Hannon says, “the news, so far, is not about how we differ from Neandertals, but how we are so nearly identical, in terms of proteins.” In addition to following up on the functional associations of the 88 proteins identified in the current study, Hannon says new research is likely to address other portions of the genome – particularly those segments responsible for regulating what genes do. In effect, the search for what distinguishes us from our nearest hominid ancestors will shift to from differences in sequence to differences in function.
Once again the question arises: How do we explain that so little difference in DNA results in such massive differences in species? The answers, or at least a new set of questions, probably lie in the study of those things that affect gene expression – epigenetics, proteomics, gene regulation. The completing of genome sequencing for Neanderthal man shows how some of these questions may be formed, and that’s where the fascination of science really begins.