Marian Annett (2000) No Homo Speciated on Cerebral Dominance.. Psycoloquy: 11(020) Language sex Chromosomes (2)

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PSYCOLOQUY (ISSN 1055-0143) is sponsored by the American Psychological Association (APA).
Psycoloquy 11(020): No Homo Speciated on Cerebral Dominance.

Commentary on Crow on Language-Sex-Chromosomes

Marian Annett
Department of Psychology,
University of Leicester,
Leicester LE1 7RH


Crow's (2000) thesis that Homo sapiens speciated on a gene for cerebral asymmetry is inconsistent with the right shift (RS) theory of handedness and cerebral dominance (Annett, 1972, 1985). The gene is not a human universal but is absent in nearly one in five people. Asymmetries in the skulls of fossil hominids suggest the gene may have originated long before Homo sapiens. The gene is 'for' speech, not language. The advantages of cerebral lateralisation for speech are probably bought at costs to non-speech functions and risks for psychosis. Individual differences for various skills in human groups may have been influenced by alleles at the RS locus from early stages of human evolution.


cerebral asymmetry, Homo sapiens, human evolution, language evolution, lateralisation, mate recognition, saltation, sex chromosomes, sexual selection, speciation.
1. The target article (Crow 2000) offers a complex scenario for human evolution in which the key player is a single gene for cerebral lateralisation. This gene is said to influence the degree as well as the direction of asymmetry and to have major effects on the development of linguistic ability, critical to the speciation of Homo sapiens. I have already commented on Crow's theory of the origins of language (Annett, 1998a). My purpose here is to show that the thesis is inconsistent with the right shift (RS) theory of handedness and cerebral dominance (Annett, 1972, 1985; 1998a for a recent review). The term 'cerebral dominance' (CD) is used below as shorthand for 'cerebral laterality for speech, language and other functions as observed in the majority of modern humans'. The most cogent reason why Homo sapiens could not have speciated on CD is that the typical pattern is a variable, not a universal. Crow is correct in suggesting that the hypothesised gene (RS + on my theory) is of great potential interest, but the interest depends on individual differences for speech based processing associated with the RS locus, not the 'essence' of Homo sapiens.

2. The RS theory suggests that hand, paw and other preferences depend on accidents of congenital growth in complex nervous systems, from mice to men. Left and right hand preferences are universal in primates but in non-humans the distribution is 50/50 to each side, many animals having mixed preferences. What is different about humans is displacement of the chance distribution in a dextral direction, about 65% consistent right-handers and some 35% with mixed and left preferences. What is the agent of shift? This is likely to be a relative advantage for the left hemisphere, probably induced through a disadvantage to the right hemisphere at a critical phase of early growth. This channels speech learning to the left side, so initiating the typical pattern of cerebral functional specialisation. (The mechanism could also lateralise sign learning in the deaf, Poizner et al., 1989, even if the gene was selected for its effects on speech). The right hemisphere disadvantage weakens the left hand, thus weighting chance differences between the hands in favour of the right.

3. It is widely believed that the hemisphere controlling speech 'ought' to control the preferred hand but this is wrong. The correlation between right handedness and typical CD depends the presence of the RS + gene. The gene determines left cerebral speech (given normal development) and incidentally raises the probability of (but does not determine) dextral preference. In the absence of the gene (RS - - genotypes), hand and brain each lateralise at random. Several independent studies of speech disorders in patients with unilateral cerebral lesions, based on population representative samples, indicate that about 9.25% of modern Europeans have right hemisphere speech (Annett, 1985, 1998b). If these represent half of those who lack the RS + gene then about 18.5%, nearly one person in five, do not carry the agent of typical CD. (See Alexander and Annett, 1996, Annett and Alexander, 1996 for recent expositions and tests of the theory.) Estimates of gene frequency (RS + = .57: RS - = .43) and estimates of the effect of the gene on the handedness distribution, both derived from neurology, predict findings for handedness in families (Annett, 1978, 1999a). The critical point for the present purpose is that, according to this theory, nearly one in five people do not have typical CD. If CD is not universal in Homo sapiens, it is not likely to have been universal in any earlier Homo.

4. The RS + gene may have emerged much earlier than Homo sapiens because skull asymmetries were present in fossil hominids. The typical counter-clockwise torque, mentioned in Crow's target article, depends on the left posterior and right anterior petalias, relative protrusions of the skull on the left side at the back and the right side at the front of the skull. It is not proven that the torque implies typical CD but LeMay (1977) found that its frequency was reduced in left-handers, and also in people with left-handed relatives, as expected if the typical pattern is associated with the RS+ gene, and its absence with chance asymmetry (RS - - genotype). Petalias occur in apes but the joint presence of both posterior and anterior asymmetries is not clear (LeMay, 1985). Holloway and LaCoste-Lareymondie (1982) found distributions in apes consistent with random asymmetry, like their handedness, but the typical human torque was present in 7/9 Homo neanderthalis, 11/12 Homo erectus and 2/3 earlier hominds.

5. Evidence for Australopithecus is less certain, but most of some 16 skulls were said to have asymmetries consistent with the modern pattern (Tobias, 1996, p 82). In all of these types there were group biases, but also individual differences, as in modern man. Holloway (1974, 1996) described latex foam endocasts of fossil hominid brains. A small brain hominid (KNM-ER 1470) dated to some 3 myr bp, showed cortical asymmetry and also a putative Broca's speech area. The urge to talk and lateralisations to facilitate speech could have been present early in hominid evolution. Reports that chimpanzees resemble humans in having a larger planum temporale on the left side (Gannon et al., 1998; Hopkins et al., 1998) raise the question whether this area is specific for speech, or part of an earlier primate communicative system (see Rizzolatti and Arbib, 1998).

6. My third argument with Crow's thesis is that CD is not for high level language but for speech. I reached this conclusion after personally examining a population sample of over 100 children with hemiplegic cerebral palsy, assessing the severity of impairment of the good and bad hands with my peg moving task (Annett, 1973). Left- and right- sided cases, with normal hand function on one side and therefore presumed unilateral brain damage, were equal for verbal and nonverbal intelligence, as measured by the WISC, but differed for history of speech disorders. Comparison of the isolated hemispheres of split-brain patients showed that the critical difference for verbal skills was that only the left side could detect rhymes (Levy, Trevarthen and Sperry, 1972). It is the left hemisphere, in the typical brain, that knows how words sound. Weakness in processing speech sounds (poor phonology) in children (Annett, Eglinton and Smythe, 1996) and undergraduates (Annett, 1999) is associated with reduced bias to right hand preference and skill. This is expected if risks to phonological processing are associated with the RS - - genotype. Efficient speech acquisition in infancy, the ability to manipulate speech sounds in the head, and articulate expression in adulthood are all likely to have advantages for survival and reproduction, independently of any associated language skills.

7. It is speech rather than language that distinguishes humans from other primates. Chimpanzee linguistic and symbolic skills are matters of controversy but they can probably reach the level of a 2 year old human child (Deacon, 1997). Heroic efforts to teach chimpanzees to talk failed because they do not have the structural mechanisms of the human tongue, larynx and relevant neural systems. The evolution of speech involved a redesign of the vocal tract, with descent of the larynx, and potentially fatal consequences if material in the mouth is inhaled (Lieberman, 1998). This hazardous arrangement that puzzled Darwin gives a measure of the value of speech from an early stage of human evolution because the changes to the vocal tract must have taken some time. A gene for CD may have assisted speech from an early stage also, but risks associated with the gene have restricted spread in the population.

8. The gene frequencies inferred from the dysphasia literature, as mentioned above, imply that some 49% of people are heterozygotes (RS + - genotypes), about the maximum possible for a single genetic locus. This suggests there may be a balanced polymorphism with heterozygote advantage (BP+HA) for the RS locus (Annett, 1985, 1995 for review and commentary). The great benefits of the RS + gene (present in single or double dose in some 81%) would be for early speech learning and efficient speech based processing in thought and communication, as outlined above. If these benefits are bought through right hemisphere disadvantage, there might be risks to other functions and perhaps to overall intelligence. Tool-making, spatial thinking and physical prowess in combat might all be impaired by cerebral imbalance. Expression of the RS + gene is probably weaker in males than females in order to moderate these risks for males. The small sex differences for handedness are consistent with sex modification of expression, rather than sex chromosomal location of the gene.

9. A recent and surprising contribution to the BP+HA hypothesis was the idea that the RS+ allele might be liable to mutate to a form that loses its directional coding (becomes agnosic for right and left) and handicaps either hemisphere at random. If this occurred in people carrying another copy of the normal gene, both hemispheres would be impaired in 50% of cases. Damage to the speech cortex on both sides of the brain could offer a model for schizophrenia. Annett (1997) showed that the hypothesis of an agnosic RS + gene, together with the genetic model as already applied to the neurology of dysphasia and to family handedness, could account for the frequency of schizophrenia in the relatives of schizophrenics. Homozygosity for the agnosic allele, expected in about 4 in 10,000 of the population, should be considered as a possible cause of autism. In order to limit these risks associated with the presence of the RS + gene, there must be RS - - genotypes who lack whatever facilitates speech learning in the majority.

10. Thus speech delays and disorders, dyslexias and psychoses may all be part of a BP+HA for the RS locus (Annett, 1995, 1997). The 'perfect' medieval knight was expected to excel in verse-making and combat. Advantages for the former must be balanced with the needs of the latter. There are relatively high proportions of left-handers in many sports, including fencing, boxing, tennis, bowling in cricket and pitching in baseball, consistent with the hypothesis that RS + + genotypes are less prevalent among the most talented.

11. The critical species characteristic of Homo sapiens compared with earlier types of Homo is likely to be some feature of potential intelligence, giving enhanced powers of representation of linguistic and other symbols. It is impossible to know when hominids began to talk, of course, and scientific caution leads some to suppose that speech and language emerged suddenly with modern man (Noble and Davidson, 1986; Tattersall, 2000). However, the discovery of a hyoid bone, part of the re-designed vocal tract, with Neanderthal remains (Arensburg et al. 1989) shows that at least one other branch of the human family could talk.

12. In my view, scientific caution requires us to suppose that the many levels of intelligence, between the 2 year old equivalent in of the chimpanzee and an intelligent adult (Piaget, 1950), must have evolved through several stages in various types of hominid ancestor. The development and use of intelligence at all these stages would have been aided by an efficient speech system. Vocalisation, in song and speech would promote the use of words as symbols. Early propositional thought would have to rely on manipulating words 'in the head', before systems of writing were invented. Bearing in mind that reading has not been expected as the norm in western societies for much more than 100 years, it is evident that the main vehicle for the transmission of cultural information has always been oral, in stories, verse and song. Speech and language are intimately related, but to understand individual differences for CD they must be distinguished.

13. With regard to the argument for "hemispheric indecision" -- the poor intellectual abilities of individuals with equal hand skill for square marking, it should be recalled that the major agent of handedness asymmetry, on the RS theory, is random accidents of early growth. Individuals around the point of symmetry might include some in whom the RS + gene was poorly expressed because of adverse influences on growth that also depressed intellectual abilities. There might be some potential schizophrenics with weakened cerebral function on both sides. There could also be some RS - - genotypes, possibly slow developers with superior intellectual potential. Einstein's brain lacked typical asymmetries (Witelson et al., 1999).

14. In summary, the hypothesis that the speciation of Homo sapiens depended on a gene for cerebral asymmetry is neither necessary nor probable. The RS theory suggests that there is a gene that promotes typical CD but it is absent in nearly one-fifth of the population. A similar pattern of individual differences for skull asymmetries, present in most but not all individuals, was evident in hominids from about 3 myr bp. Instead of hypothesising that a gene for CD originated with Homo sapiens some 100,000 years ago it is more plausible to suggest that the gene emerged in early stages of human evolution when an aid to the development and use of speech would be valuable in ways that still apply today. However, the costs to practical skills associated with a double dose of right hemisphere disadvantage, and the risks of psychosis in the presence of an agnosic mutant, restricted the spread of the gene in the population. This theory offers an explanation of individual differences in modern and ancient humans. Crow's thesis suggests that a gene for cerebral specialisation could explain everything, but this would allow the gene to predict nothing.


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