A publication of the Archaeological Institute of America
For the first time, DNA of a premodern human has been recovered. Svante Pääbo of the University of Munich and colleagues in Germany and the United States successfully extracted the DNA from a right humerus (upper arm bone) of a Neandertal. Their findings, presented in the July issue of the journal Cell, provide important information about when Neandertals and modern humans diverged from a common ancestor, the nature of interaction between Neandertals and modern humans, and the ultimate fate of the Neandertals.
The humerus was found by quarry workers in the Feldhofer Cave, near Dusseldorf, Germany, in 1856, along with the top of a cranium, two femurs (upper leg bones), right radius and ulna (lower arm bones), part of the left ilium (pelvis), and fragments of a shoulder blade and ribs. The Neander Valley, in which the cave was located, later gave its name to the early human represented by these and other remains. The Feldhofer fossils are believed to date from between 40,000 and 50,000 years ago. They are now in the Rheinisches Landesmuseum in Bonn, which permitted removal of a 3.5 gram sample from the humerus for analysis.
As an initial test, amino acids from the bone were examined to determine whether or not DNA might be preserved in the sample. Degradation of amino acids and DNA is caused by the same factors, including water and temperature. Thus the condition of amino acids in bone, which is easily determined, can be used as a guide to the condition of any DNA preserved in the sample. The results were encouraging, and the scientists decided to attempt recovering and replicating DNA from the bone. To be certain of the results, each step in the analysis was repeated in Pääbo's lab and the findings were then duplicated independently by Mark Stoneking and Anne Stone at Pennsylvania State University.
The researchers focused on DNA from the mitochondria, organelles within cells, rather than from the nucleus. Mitochondrial DNA is more abundant than nuclear DNA, and is thus more likely to be recovered in sufficient amounts to allow replication. In addition, mitochondrial DNA is transmitted only from the mother so that changes from generation to generation result from mutation alone rather than recombination of the mother and father's DNA. The scientists obtained a sequence of 379 amino acid base pairs by replicating shorter, overlapping segments. They identified 27 differences between the Neandertal DNA and a modern reference DNA sample over the replicated sequence. By contrast, DNA from a random sample of a modern population might vary from the reference DNA in five to eight places.
DNA dating is based on the assumption (debated by geneticists) that mutations occur at a constant rate. The accumulated mutations in DNA can be measured, and the time necessary for them to occur calculated. The amount of difference between Neandertal and human DNA suggests that our common ancestor existed about 550,000 to 690,000 years ago. Although this date must be qualified (it is based on one specimen only, and the DNA clock may or may not be as accurate as we assume), it is in accord with the fossil record. Osteological characteristics of the 300,000-year-old remains from the Sima de los Huesos in northern Spain (see "Faces from the Past," ARCHAEOLOGY, May/June 1997) and the 400,000- to 500,000-year-old jaw from Mauer, Germany, indicate that these humans, generally classified as Homo heidelbergensis, are ancestral to Neandertals. This suggests the split between the ancestors of modern humans and Neandertals had occurred somewhat earlier, about the time indicated by the new DNA date.
The relationship between Neandertals and modern humans, who are thought to have arisen in Africa some 120,000 to 150,000 years ago, and the demise of the Neandertals are intertwined. The two coexisted in Southwest Asia for a long period (see "The Peopling of Eurasia," ARCHAEOLOGY, January/February 1996). Excavations at sites in Israel have yielded remains of modern humans at Skhul and Qafzeh caves dated from as early as 120,000 to 90,000 years ago, and Neandertal remains at Kebara Cave dated from 60,000 years ago and Amud Cave dated from 40,000 to 50,000 years ago. In western Europe, Neandertals persisted until 30,000 years ago and possibly somewhat later. The question arises: To what extent did the two interact in terms of cultural exchange or trade and interbreeding? Were the Neandertals out-competed by modern humans or killed off by them, or were they absorbed into the population and genetically swamped? At Arcy-sur-Cure, in France, stone tools and personal ornaments similar to those associated elsewhere with modern humans have been found with 34,000-year-old Neandertal remains, suggesting trade between the two groups. Despite this evidence for cultural exchange, a study of temporal bones from Arcy-sur-Cure and other sites indicates significant differences between Neandertals and modern humans, suggesting interbreeding did not occur (see "Neandertal News," ARCHAEOLOGY, September/October 1996).
If Neandertals made a significant genetic contribution to modern humans, similarities should exist between DNA of Neandertals and that of people from Europe, where the Neandertals persisted the longest. Pääbo and his colleagues compared the Neandertal DNA to that from five modern populations, but it proved no closer to DNA from modern Europeans than to that from four other groups. While this does not rule out the possibility of Neandertal and modern human mixing, it suggests that the Neandertal genetic contribution to modern gene pools, if any, was small.
Setting aside the particulars of this new study, the fact that it was possible to recover Neandertal DNA is a breakthrough itself and opens many possibilities for similar investigations. Mammoth remains preserved in the Siberian permafrost have yielded DNA from 50,000 to 100,000 years ago, but preservation of DNA more than 100,000 years old is thought to be unlikely. While DNA from a pre-Neandertal form, like the Homo heidelbergensis from Atapuerca, will probably never be recovered, it would be interesting to compare DNA from early Homo sapiens and Neandertals from Southwest Asia where the two coexisted for such a long period. Another intriguing possibility would be to compare later Homo sapiens and the last of the Neandertals, those from 30,000 years ago, found at western European sites like Zafarraya Cave in southern Spain. The fossils from Feldhofer belong somewhere in the middle period of Neandertals. Would DNA from different regions and periods confirm the results obtained from this study, or would they suggest other degrees of interaction? The recent claim that Homo erectus was alive in Southeast Asia as recently as 53,000 to 27,000 years ago (see "Homo erectus Survival," ARCHAEOLOGY, March/April 1997), makes it conceivable that DNA from this species could also be recovered.