Wim E. Crusio

Génétique, Neurogénétique et Comportement, CNRS UPR 9074, Institut de Transgénose, 3b rue de la Férollerie, 45071 Orléans Cedex 02, France.

Address correspondence and proofs to: Dr. W.E. Crusio at the above address. Tel: + 33 2 38 25 79 74 Fax: + 33 2 38 25 79 79, email: crusio@cnrs-orleans.fr.

In the present target article, Prof. Crow combines evidence from an admirable diversity of fields to concoct a provocative theory of "everything". Going from the geographical distribution of the human species to language and psychosis, he combines evidence that ranges from palaeontology to linguistics and neuroscience. Much as I admire his attempt, I think that he unfortunately is aiming too high. By including so many widely divergent topics, his theory has become so fuzzy, that I'm not even sure what its main point is: providing an explanation for psychosis? Explaining lateralisation and cerebral dominance? Explaining the evolution of language? Defining Homo sapiens? Or all of these simultaneously? It seems to me that his theory, although certainly intriguing, has not yet matured enough.

In addition, I have the definite impression that many topics covered in the target article are not really necessary for whatever theory it is that Crow proposes. Because of this, the article contains many debatable points that apparently are not at all crucial for Crow's reasoning. For instance, the part on sexual selection. First of all, the fact that a gene is X-Y homologous is asserted to open the possibility for sexual selection on characters influenced by this gene. This is correct. But autosomal genes may display sex-limited expression, meaning that the same autosomal gene has differential effects in males and females (Killick, 1971). In the extreme case, the character may be expressed in only one sex (for instance milk yield in cattle and egg production in fowl; see Killick, 1971, for examples and references). Therefore, such genes can also be subjected to sexual selection. And the fact that sexual selection may be exerted, does not prove that it actually is exerted. A necessary condition would be that females actually base mate-choice on (a) character(s) that are influenced by such a gene, an aspect that is not addressed in the present target article. In short, whether the gene in question is an X-Y homologous one or not, has nothing whatsoever to do with the possible occurrence of sexual selection. But secondly, and much more importantly, Crow completely fails to make it plausible why sexual selection is even necessary for his theory at all. Would it really make much difference if no sexual selection were present?

In fact, it may even be doubted that the putative right-shift factor is due to an X-Y homologous gene, although much space is devoted to this issue. Crow presents two lines of evidence but both are weak. The first bit of evidence is the existence of a sex difference and a within-family association between sex and handedness. The second one is the fact that individuals with sex chromosome aneuploidies have relative hemispheric impairments or delays. However, both phenomena might equally well be explained by sex-limited expression of an autosomal gene. Furthermore, patients with sex chromosome aneuploidies suffer from many other impairments, too. Surely Crow doesn't want to suggest that all of these phenotypes are being influenced by X-Y homologous genes?

Many other points also remain unclear. For instance, the argument on the relations between psychotic illness and handedness, on the one hand, and the genetic origins of language on the other hand. However, in this commentary I will limit myself to one final point, the so-called speciation event.

Following Crow, Homo sapiens distinguishes itself from its predecessors by its use of creative language. Linguistic capabilities became possible after a mutational event in a region of X-Y homology that made hemispheric specialisation possible. The latter is a necessary condition for language as modern Man knows it. This argumentation has a number of holes in it.

In the first place, I do not see how it can be excluded that already some of the predecessors of H. sapiens also used language. Crow mentions that "archaeological evidence for representational capacity that parallels the possession of language is restricted to the last 50,000 years or so". Of course, much evidence of this type does not conserve easily (for instance, wood carvings) and it can be expected that any earlier signs for this representational capacity would not survive the ages. This is already illustrated by the fact that H. sapiens has existed for at least 137,000-250,000 years. Yet, no evidence for representational capacity from periods older than 50,000 years has been found. Thus, either H. sapiens came into the possession of this representational capacity only 50,000 years ago, or he did have this faculty but no earlier evidence survived until present. If the latter is true for H. sapiens, it may also be true for his predecessors.

Another point concerns the postulated necessity of hemispheric specialisation for linguistic capabilities. Is this not turning causality on its head? The fact that, after hemispheric specialisation, linguistic competence is relegated to one hemisphere predominantly need not imply that this is also a necessary condition. Indeed, the fact that individuals with lower levels of hemispheric specialisation are still capable of speech bespeaks differently. Nevertheless, even if we would accept brain asymmetry as a necessary condition, then it still remains unclear that this is something specific to H. sapiens. In fact, brain asymmetries occur even in rodents (and are sometimes influenced by the Y-chromosome, van Abeelen et al., 1989) and hemispheric specialisation has been found in infrahuman species (Fagot et al., 1997). I find it difficult to accept, therefore, that hemispheric specialisation, leading to speech, distinguishes H. sapiens from H. erectus and other predecessors. It is even not certain at all that language is in fact the faculty accounting for the success of H. sapiens, as Crow suggests at the very beginning of his target article. After all, H. sapiens stayed within a rather small geographical area for 100,000 years or more before starting to conquer other habitats about 50,000 years ago.

Finally, Crow's hypothesis entails a single-gene mutation causing a saltatory speciation event leading to H. sapiens. If I understand him correctly, however, the gene that underwent this mutation is the same as the one postulated by Annett (1995) and McManus (1985) to explain variation in human handedness. In addition, allelic variations in this gene may be related to schizophrenia. As this gene is still variable in the modern human population, we are led to the fascinating possibility that some individuals within this population actually do not belong to the human species, Homo sapiens. Although the hypothesis that a sizeable number of psychotic Neanderthals are amongst us might adequately explain many unfortunate events in recent human history, the foregoing will make it abundantly clear that I am still very sceptical about the possible truth of this proposition.

Preparation of this commentary benefited from support by the Centre National de la Recherche Scientifique (UPR 9074), Ministry for Research and Technology, Région Centre, and Préfecture de la Région Centre. UPR 9074 is affiliated with INSERM and the University of Orléans.