Arizona Republic, Page B5, Friday August 8, 1997
Reviewing the results of Neanderthal DNA tests By G.A. Clark
In an article contributed to the New York Times and printed in The Arizona Republic ("The Cradle of Humanity," Opinions, July 29)
British paleoanthropologists Chris Stringer and Robin McKie report on the claims of a team of molecular biologists, led by Svante Paabo of the University of Munich, that they have sequenced a tiny fragment of mitochondrial DNA from the upper arm bone of the original Neanderthal skeleton, recovered from a limestone quarry in Germany in 1856. While the research was apparently meticulously done (it is very difficult to recover DNA from ancient bone), what it means in respect of modem human origins, and the Neanderthals' relationship to ourselves, remains very much open to question.
The analysis of mitochondrial DNA has become important in recent years for inferring evolutionary relationships among populations and species because mitochondria, unlike nuclear DNA, are inherited only from the mother and do not recombine. This means that they can be used as a kind of molecular clock since, sequences remain the same from one generation to the next except when mutations occur to change them. Under the problematic assumptions that mutations accumulate at a constant rate, that the rate can be established and "calibrated' and that these mutations are adaptively neutral (that is, they do not affect the reproductive potential of the organisms involved), the number of accumulated mutations between two mitochondrial DNA sequences is a measure of the degree of evolutionary divergence between them. After three months of painstaking work, the Paabo team isolated a sequence of 379 base pairs from the control region in the DNA of mitochondria, the cells' energy-producing organelles, which have their own tiny genome. Then they compared the Neanderthal sequence with 986 sequences from living humans and found on average, about three times as many differences (26) between the Neanderthal and modem human sequences as between those of pairs of modem humans (8). Using a chimp human divergence date of 4 million to 6 million years ago to "calibrate" the clock, they came up with a rate at which the control region accumulated mutations over time. This in turn allowed them to determine that the sequence ancestral to both modem and Neanderthal mitochondria began to diverge between 690,000 and 550,000 years ago, compared to less than 200,000 years ago for the sequence ancestral to modem humans. In order words, they contend that the last common ancestor of Neanderthals and modems is about three times older than the last common ancestor of all people alive today.
While an astounding technical breakthrough, and an important result, the work of the Paabo team is evaluated uncritically by Stringer and McKie in their article, essentially because it tends to support their biases and preconceptions about modern human origins. Other interpretations are possible. It should be noted first that molecular clock models are full of questionable assumptions. Leaving aside differences of opinion about the rate of base pair substitutions, how to calibrate a molecular clock, and whether or not mitochondrial DNA mutations are neutral, the fact that the Neanderthal sequence (and it should be kept in mind that there is - to date - only one) differs from those of modern humans does not resolve the question of whether or not moderns and Neanderthals were different species. This is the primary conclusion of the research according to Stringer and Mickie. Continuity advocates (people who think that Neanderthals contributed genetically to people of western Eurasian ancestry) argue that differences were only subspecific.
In support of this notion, Harvard geneticist Maryellen Ruvolo points out that the genetic variation between the modern and the Neanderthal sequences is within the range of other single species of primates, and that there is in fact no genetic yardstick which might allow us to define a species. A more convincing test of the implications of the Paabo data would be to sequence mitochondrial DNA from an unambiguously modern early European or better yet from alleged archaic and modem human fossils from the Israeli cave sites of Skhul and Qafzeh (supposedly modern) and Kebara and Tabun (supposedly Neanderthal). If the Israeli fossils all show differences with modern humans of the same order of magnitude of those between Neanderthal and moderns, that would be compelling evidence that all Upper Pleistocene hominids diverge from moderns by about the amount (evidence for continuity), and that the distinction between Neanderthals and moderns in the Levant at least is utterly without foundation.
That researchers cannot distinguish a Neanderthal from a modern might seem surprising to some readers, but there is absolutely no consensus on what these terms mean. Arizona State University researcher Catherine Willermet re-examined the criteria used to classify and describe Neanderthal and early modem human skulls by continuity and replacement advocates, and found that only 11 percent of the variables were common to both groups of workers. In other words, continuity and replacement advocates are using different sets of variables to assess differences and similarities, and are defining differently those variables held in common.
Without even discussing the archaeological evidence, which unequivocally supports continuity in adaptation between Neanderthals and moderns wherever they are found, it should be obvious that a single DNA sequence from a single, undated skeleton can be interpreted in many different ways. If we are to come up with a satisfactory explanation for our origins, that explanation must reconcile pattern searches in archaeology and human paleontology, as well as in molecular biology. Those who would argue - like Stringer and Mickie - that Neanderthals became extinct without issue must show how it could have occurred without leaving traces of disjunction in the archaeological record and in the fossils themselves.
G.A. Clark is a professor of anthropology at Arizona State University and, with Catherine Willermet, the editor of 'Conceptual Issues in Modem Human Origins Research' (Aldine de Gruytar), which was published fall 1997.