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Ref ID: 32394
Ref Type: Journal Article
Authors: Sallares, R.
Title: Review of "The history and geography of human genes by L. L. Cavalli-Sforza, P. Menozzi and A. Piazza
Date: 1997
Source: Ancient Biomolecules
Abstract: The History and Geography of Human Genes by L. L. Cavalli-Sforza, P. Menozzi and A. Piazza[*] Department of Biochemistry and Applied Molecular Biology, UMIST, Manchester M60 1QD, UK This is an outstanding book. The data collection ceased in 1986, ten years ago and this is a long time in modern science. Nevertheless it is safe to say that this book will be a standard work of reference for many years to come, especially since many of the populations which are most important for understanding human genetic diversity are rapidly disappearing and the opportunity to study them will soon be lost forever. The authors start from the premise that the major breakthrough in the study of human variation was the introduction of genetic markers, since the methods traditionally employed in physical anthropology, such as craniometry, are subject to too much short-term environmental influence to be of value for long-term evolutionary and historical studies. A huge database of modern gene frequencies, derived from protein electrophoresis and immunological data for blood groups, has been assembled and used to reconstruct phylogenetic trees, by the average linkage method, and principal component maps, with the results being tested for sampling errors by the bootstrap method. The criteria for selecting populations for study are carefully discussed, as in the case of the Icelandic population. In general mixed and urban populations were excluded, as well as all those not yet in place at the end of the fifteenth century AD. The discussion of the methods of gene-frequency analysis is extremely interesting. The maximum parsimony method favoured in cladistics is rejected on the grounds that it does not contain any testable scientific hypotheses and does not locate the root of phylogenetic trees. Population genetic distances based on genefrequency differences are mainly determined by genetic drift, while comparisons of species depend on mutations, which are independent of genetic drift. Consequently population evolution is fundamentally different from species evolution. The possible influence of natural selection on gene-frequency data is also considered carefully. Although malaria has clearly had an enormous influence on human populations in tropical and subtropical regions of the Old World, neutral interpretations seem satisfactory for much of the data, under the assumption of constant evolutionary rates. The results of the phylogenetic analysis of the gene-frequency data are systematically compared to the available data for the history of human languages in a bid to reconcile the two trees. The book closes with Darwin's prediction that human population evolution and language development would be found to run parallel to each other. This prediction has been amply vindicated by this book, despite some cases of language replacement without gene replacement (e.g. Turkey and Hungary) and of gene flow which are given due consideration. A remarkable correlation is reported between the genetic data and the linguistic superfamilies Austric and Nostratic/Eurasiatic. It is clear that the main obstacle to further progress in this field does not lie in genetics, but in the lack of agreement among linguists about so many problems in language history, above all the relationships of native American languages. One of the most interesting and doubtless most controversial conclusions of the book is that the genetic data supports Greenberg's classification of native American languages into just three families (Amerind, NaDe:he and Eskimo) brought to the western hemisphere by three separate migrations. The importance of genetic drift is very clear particularly in the discussion of Amerindian populations. Although the restricted number of mitochondrial DNA (mtDNA) lineages may suggest a population bottleneck during the initial migrations, the gene frequency data suggests instead that many (e.g.) HLA alleles were present at the beginning and later lost owing to genetic drift, because many Amerindian tribes had extremely small founding populations. A mass of archaeological evidence is also introduced into the equation to try to relate population evolution and language evolution to what is known about the (pre)history of human societies. The multidisciplinary approach of the authors is praiseworthy and essential for such an enterprise. Since the authors are not archaeologists themselves, it is inevitable that some errors will be found, especially considering that the sources of information used are sometimes rather old. The Cambridge Encyclopaedia of Archaeology is getting dated now. For the sake of illustration in a short review we can consider some of the statements that are made about Egypt and North Africa. The Persian king who conquered Egypt was Cambyses, not Darius (p. 218). On p. 217 it is stated that the Phoenicians are believed to have circumnavigated Africa. While it is true that Herodotus says this, not many modern historians believe him. On p. 219 an old source (Murdock, 1959) is cited for the Hilalian invasions in North Africa in the eleventh century AD, whose reality is rejected by a modern revisionist school of historians. The statement (p. 162) that the only animals domesticated in Africa were the cat, guineafowl and ass gives a false impression of the level of interest of the ancient Egyptians in animal domestication. There is evidence that they attempted to domesticate nearly all the wild animals they encountered, with success as regards mongooses and ostriches, but no success in relation to hyaenas, African buffalo and oryx, for example (Boessneck, J. [1988] Die Tierwelt des Alten Agypten, Munich). We are told that the Roman conquests are a clear example of a spread by an elite with very little, if any, demic component (p. 265). Few Roman historians would accept that. The Romans (with their Italian allies) founded many colonies in all the lands they conquered, although it may well be the case that the genes of the indigenous inhabitants prevailed in the long run. Having said all this, these criticisms of the historical and archaeological data do not detract from the value of the whole book in the opinion of this reviewer. The authors in fact display a much better understanding of the importance of human population expansions and movements in the past than many modern archaeologists. The views of the indigenist school in Anglo-American archaeology tell us much more about the current preoccupation with political correctness than about what happened in the past. There is only space here to mention briefly a few of the book's other main conclusions. As regards the evolution of modern humans, the opinion is expressed that the first fission in the phylogenetic tree was between Africans and non-Africans, and that the date for this event derived from classical genetic markers is roughly congruent with that estimated from mtDNA, even though it is rightly acknowledged that most human polymorphisms are much older than the data of spread from Africa. The authors note that their data alone cannot tell us whether the first migration was from Africa to Asia or vice versa, but after taking into account other types of evidence they come down in favour of the 'out-of-Africa' theory, without however completely excluding the possibility of some regional hybridization. One advantage of the fact that the book's database was completed ten years ago is that it enables reviewers to assess how the conclusions match up to the results of more recent work, principally DNA sequencing. In this case most of the latest work certainly supports this book. Despite the criticisms that were made originally of the statistics of the 'out-of-Africa' theory, re-analyses of the mtDNA data have reinforced it (Penny, D. et al. [1995] Mol. Biol. Evol. 12, 863882). Recent research on the Y chromosome provides an independent confirmation of it. Moreover specific African haplogroups that could be ancestral to Europeans and Asians have now been identified (Chen, Y.-S. et al. [1995] Am. J. Hum. Genet. 57, 133-149). The main criticism made in the book of the multiregional hypothesis of Weidenreich and Wolpoff, namely that it is impossible to see how there could have been sufficient gene flow between, say, South Africa and China to maintain evolution in parallel over a period of a million years, certainly fits the preconceptions of this reviewer. Europeans are a relatively homogeneous group who are difficult to place between Africans and Asians. Lapps, Sardinians, Greeks, Yugoslavians, Basques, Icelanders and Finns are the outliers. Recent research on mtDNA sequence variation among the Lapps has shown that their mtDNA gene pool is indeed distinct from that of other European populations (Sajantila, A. et al. [1995] Genome Res. 5, 4252). However the mtDNA of the Basques is much more similar to that of other European populations than would be expected from the divergences in nuclear gene frequencies such as RH- or from the distinctiveness of their language (Bertranpetit, J. et al. [1995] Ann. Hum. Genet. 59, 63-81). The same is true of the Finns. Consequently it is now being proposed that while the timescale for nuclear gene frequency evolution is comparable to that for language evolution, the timescale for mtDNA sequence evolution is significantly longer. The data from Sardinia furnish another illustration of the importance of genetic drift. The fourth principal component map suggests a potential area of gene expansion centred on Greece, but including parts of the south Balkans never historically occupied by the Greeks. This is rather mysterious, but two possible explanations can be suggested. Speakers of languages belonging to the so-called 'Anatolian' group (Hittite etc.) probably existed in Greece in prehistory before Greek developed, providing a prehellenic ethnic stratum (the Pelasgians of classical authors) which could well be the source for the components of the data in question. We should also not forget the movements of the Slavs and Albanians into Greece in the mediaeval period. The authors reaffirm the 'wave-of-advance' model for the spread of farming and the genes of the early Neolithic farmers across Europe from the Near East. This model is quite acceptable, although not all archaeologists will be happy with the authors' dismissal of the arguments put forward by Zvelebil for the survival of Mesolithic cultures in outlying regions of Europe. However, the extension of the model proposed by Renfrew to explain the dispersal of the IndoEuropean languages is very dubious. In the opinion of this reviewer the historical, archaeological and linguistic evidence indicates overwhelmingly that the Indo-European diaspora was a late event which took place during the Bronze Age and continued in many areas even into the Iron Age in the first millennium BC. The authors rightly observe that the principal component maps provide support for population movements from both Anatolia and the Ukraine. However the gene frequency data will never tell us which languages were spoken by the peoples in question. The concentration of these comments on Europe reflects the reviewer's own interests, but the scope of the book covers the entire world. There are many interesting ideas about Australia, New Guinea, China, Japan, India and the Americas which cannot be considered here due to lack of space. However the message from every part of the world is ultimately the same: "The present genetic picture of the aboriginal world is determined largely by the history of Palaeolithic and Neolithic people, when the greatest relative changes in population numbers took place" (p. 299). How do ancient biomolecules fit into this picture? The authors note the work on ancient DNA from the human remains found in Windover in Florida, but are cautious, probably rightly so, about the prospect of obtaining information from ancient biomolecules which could be used in this type of research. They are aware that attempts to investigate ABO blood group polymorphisms in ancient material have been largely unsatisfactory. Nevertheless many, many hypotheses are proposed in the book which at least in principle could be tested with evidence from ancient biomolecules, principally DNA. The History and Geography of Human Genes sets an enormous challenge to all those working in the field of ancient DNA. Consequently it is fitting that it should be the first book to be reviewed in this new journal. ~~~~~~~~ By ROBERT SALLARES[+]
Date Created: 7/21/2003
Volume: 1
Number: 2
Page Start: 179
Page End: 182