DNA Lends a Hand

The debate is raging in anthropological circles.  Are anatomically modern humans and Neandertals so closely related as to be considered members of the same species?  Or are they different enough to be considered completely separate species, not able to breed with one another?  According to John H. Relethford’s article, “New Views on Neandertal DNA,” the true answer to these questions may lie somewhere in between these two extremes.

Relethford reports on the extraction of a mitochondrial DNA sequence from a Neandertal specimen from Feldhofer Cave, in Germany, in 1997, and two other subsequent extractions done on samples from Mezmaiskaya Cave, in the Caucaus, in 2000, and Vindija Cave in Croatia, also done in 2000 (9).  The immediate reports from comparing these extractions with modern-day humans’ mitochondrial DNA sequences claimed that Neandertals “were only distant cousins” (Relethford: 9).  Thus, the “‘Neandertal problem,’” the problem of fitting Neandertals into the hominid family tree, was solved for good (Relethford: 9).

Well, not really, says Relethford.  Genetic differences between Neandertals and modern humans are much greater than genetic differences between two modern humans (Relethford: 10).  However, these genetic differences can be attributed to Neandertals being a different subspecies of human (Relethford: 10).  As Relethford points out: “The difference between Neandertal and living human [mitochondrial] DNA is actually less than the difference found between two out of three chimpanzee subspecies” (10).

Mitochondrial DNA sequences like those of Neandertals have not been found among modern day humans.  However, says Relethford, this does not mean that Neandertals did not contribute to current human genes.  It is possible that they did contribute, but, due to genetic drift, their sequences were made extinct (Relethford: 10).  This has happened with modern human populations, such as native Australians, who have ancestors who have mitochondrial DNA that is not now seen among that population (Relethford: 10).

Relethford also asserts that it is possible that “Neandertals were . . . part of a geographically widespread species,” which is an alternative to the more common hypothesis as to why Neandertal mitochondrial DNA is as different from modern humans from Asia and Africa as it is from modern humans from Europe (10).  The more common idea is that all humans had a common ancestor after Neandertals branched off and became a different species.  However, it is also possible that Neandertals contributed their genes in roughly equal portions to each population of modern humans, resulting in the same distribution to each group.

Relethford’s conclusion is, thus, that Neandertal mitochondrial DNA does not answer all the questions about how humans developed, nor the questions about how closely related Neandertals are to modern humans.  By most of the observations he provides, the hypothesis that seems to be the best supported by the evidence is that humans and Neandertals are not genetically close enough to be definitely of the same species, but that the differences are small enough to indicate that Neandertals and humans were probably able to interbreed and produce offspring.  He also provides a couple of plausible reasons for why the Neandertal genetic patterns are not seen in today’s populations.  Over all, Relethford did an excellent job of analyzing the evidence, and provided plausible alternative hypotheses that fit with the genetics, either of the two major theories as to how humans developed (the Out-of-Africa theory or multi-regional development theory) and the fossil record.

Relethford takes the assumption that – because the difference between modern human and Neandertal mitochondrial DNA is larger than the difference now seen between humans must mean that Neandertals were a separate species – and turns it on its ear.  The explanation that the differences of “27-35 substitutions” is lower than is seen between subspecies of chimpanzees (Relethford: 10).  It seems that the evaluation of the genes showing definite separation of Neandertals and humans has been a little hasty, and that Relethford is providing a balance for the rushed information that Neandertals must have been a long-off relation.

The evidence that Neandertals were indeed a separate species hinge partially on the evidence of genetic difference.  However, it does appear that there is great controversy as to what the genetic evidence means.  G. A. Clark informs us that even modern humans’ mitochondrial DNA yields conflicting results as to whether there was synchronized multiple-site development of humans or, rather, that we developed in and radiated out of Africa (5).  There are also debates on the “African Eve” theory (which states that all humans now living had a common ancestor who lived in Africa), which is based on research done with mitochondrial DNA.  Clark also explains that mitochondrial DNA just does not offer support for many theories that deal with human development and origin, such as the lack of genetic evidence of a bottleneck in early human emergence (5).

Thus, it may be that Relethford’s claims as to what the extracted Neandertal DNA means are not as solid as they appear to be.  And I think that it is fair to treat any sort of new conjecture about the relationship between modern humans and Neandertals with a healthy dose of skepticism.  However, Relethford’s projections seem to be appropriate for both the genetic materials and the archaeological evidence, which is extremely important.  If the linking hypothesis does not line up with one or the other, then that hypothesis must be strongly scrutinized to see if it completely goes against pre-existing knowledge.  If it does, then one has to weigh which makes more sense – the new idea, or the (usually better-supported) body of already-known information.

This is why it seems that Relethford’s explanations are plausible.  Not only do they fit with the evidence given from the extracted Neandertal mitochondrial DNA, but they also seem to fit the archaeological evidence that has been dug up over the past century.

For example, Erik Trinkaus, a paleoarchaeologist of Washington University in St. Louis who is working in Portugal, made an amazing discovery (Hybrid: 20).  While digging in a cave-like formation north of Lisbon, Trinkaus found the remains of a four-year-old child (Hybrid: 20).  Examination of the skeleton revealed that the child had traits that are common to both Neandertals and modern humans:  “‘The first thing I noticed was his chin,’ says Trinkaus . . . When he examined his measurements of the boy’s limbs, the proportions were a ‘dead-ringer’ for Neandertal skeletons” (Hybrid: 20).

Although interbreeding is not the only solution to this anatomical problem, it is certainly a very plausible one.  Rejecting that Neandertals and modern humans were close enough to have produced offspring leaves the option that the natural variation of humans was the reason for the oddly-proportioned child in the Portuguese cave.  This is a valid theory.  However, it seems careless to count out the valid argument of interbreeding due to the genetic evidence.  It appears that Neandertals and modern humans are closely-enough related to have produced livable offspring, and that it is possible for Neandertals to have contributed genetic material to modern humans, although those who carried it may have later been victims of genetic drift.

Neandertal remains are primarily found in Europe and the Middle East.  If Neandertals were contributors to the current gene pool for humans, one would think that modern humans from Europe and the Middle East would have mitochondrial DNA that is more similar to that of Neandertals than people who are from Africa or Australia, for instance.  Yet it appears that modern human mitochondrial DNA is fairly uniform in its distance from Neandertals’, living location notwithstanding.  However, Relethford offers two conceivable reasons for the odd genetic data (10).

One possible solution is that Neandertals did interbreed with and provide DNA for modern humans, but that the genes don’t vary from place to place because Neandertals were distributed more evenly throughout Africa, Europe, and Asia than was previously thought.  This is a little doubtful, because it seems odd that archaeologists would not have found any Neandertal remains in places where they were doing digs for early modern humans both in Africa and in eastern Asia, especially since scientists have been aware of Neandertals for over one hundred years.  However, it is possible that an archaeologist will one day be surprised by a Neandertal in the place of what he thought was going to be a modern human.

The second explanation is more believable.  Relethford states that the contribution may well have been primarily to European and Middle Eastern populations of modern humans (10).  However, the genes that they contributed may have been either selected against or the populations may have undergone bottlenecks that wiped out any people carrying Neandertal genes.  This seems to be more plausible than the undiscovered Neandertal fossils in Africa and Asia.

Thus, Relethford’s conjectures appear, for the most part, to be level and consistent with current evidence.  Though many take the same evidence and draw the conclusion that Neandertals could not have possibly been closely related to modern humans, Relethford has provided scientifically sound reasons why these people would be hasty in doing so.  The truth is that we do not have all the facts on this issue, and we certainly never will in the near future.  Thus, it is better to keep possibilities open while contemplating these very difficult questions.  Otherwise, it is possible to get boxed in and to not be receptive to original ideas that might very well be the truth.

Works Cited

Clark, G. A.  “Neandertal Archaeology and Its Implications.”  General Anthropology, vol. 7, no. 2, Spring 2001.  Pp 4-6.

“Hybrid Humans?”  Archaeology, July/August 1999. P. 20.

Relethford, John H.  “New Views on Neandertal DNA.” General Anthropology.  Pp. 9-10.

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