Sexual Selection, Timing and an X–Y Homologous Gene: Did Homo Sapiens Speciate on the Y Chromosome?

This chapter provides a theory of the speciation of modern Homo sapiens, that a single gene played a critical role in the transition from a precursor species. The theory is founded upon the following: firstly, the premise that hemispheric asymmetry is the defining feature of the human brain and the only plausible correlate of language; secondly, an argument for a specific candidate region (the Xq21.3/Yp11.2 region of homology) based upon the reciprocal deficits associated with the sex chromosome aneuploidies, and the course of chromosomal change in hominid evolution; and thirdly, a particular evolutionary mechanism (sexual selection acting on an X-Y-linked gene) to account for species-specific modification of what initially was a saltational change. These postulates relate to the case of modern Homo sapiens. On the basis of the recent literature, the discussion argues that the third premise has general significance as a mechanism of speciation.

Bihemispheric Language: How the Two Hemispheres Collaborate in the Processing of Language

Speech production in most people is strongly lateralized to the left hemisphere (LH), but language understanding is generally a bilateral activity. At every level of linguistic processing that has been investigated experimentally, the right hemisphere (RH) has been found to make characteristic contributions, from the processing of the affective aspects of intonation, through the appreciation of word connotations, the decoding of the meaning of metaphors and figures of speech, to the understanding of the overall coherency of verbal humour, paragraphs and short stories. If both hemispheres are indeed engaged in linguistic decoding and both processes are required to achieve a normal level of understanding, a central question concerns how the separate language functions on the left and right are integrated. This chapter reviews relevant studies on the hemispheric contributions to language processing and the role of interhemispheric communications in cognition.

From Protolanguage to Language

This chapter discusses the singularity of human language. Although evolution is normally conceived of as a gradual process, it can produce an appearance of catastrophism where functions change or where gradual changes in two or more components impinge on one another. The fossil and archaeological records argue strongly for some such development in the case of human language. The discussion argues that language as people know it requires the conjunction of three things: an event structure derived from reciprocal altruism; the capacity to use unstructured symbolic units (protolanguage); and sufficient ‘spare’ neurones to maintain the coherence of internally generated messages in brains designed by evolution to attend primarily to the environment. These developments co-occurred only in the human species, accounting for the uniqueness of human language.

Preferential Sex Linkage of Sexually Selected Genes: Evidence and a New Explanation

This chapter discusses evidence and theory on sex chromosomal linkage of sexually selected traits that may be the key to a functional separation of sexual and natural selection. It reviews the evidence showing that the X chromosome has a disproportional share concerning the inheritance of sexually selected traits in animals with heterogametic males, and suggests a new explanation that relates this X bias with female choice of heterozygotic males. With numeric simulations, it shows that female choice of heterozygotic males is usually disadvantageous. Because this disadvantage cannot occur when females prefer X-linked male traits, preferential X linkage of sexually selected traits can be expected. As an alternative to fluctuating selection on sex-limited traits, the disadvantage of heterozygotic choice may thus explain the X bias observed for sexually selected traits.

Is the Neural Basis of Vocalisation Different in Non-Human Primates and Homo Sapiens?

From an evolutionary perspective, the voice was a prerequisite for the emergence of speech. Speech, the most advanced mode of vocal communication, became possible only after gradual transformations of the sound-producing system and its central nervous control, in co-evolution with the transformations of the auditory system, had taken place. The discussion suggests that the last step in the evolution of the phonatory system in the brain was the outgrowing and augmenting of the fine fibre portion of the pyramidal tract synapsing directly with the motor nuclei for the vocal cords and the tongue, so that the direct and voluntary control of vocal behaviour became possible. It holds that the answer to the question raised in the title is ‘yes’. The neural basis is in fact quite different. The chapter also explains this difference and its consequences for the evolution of language.

Grades and Transitions in Human Evolution

This chapter assesses the number of grades that have appeared in the course of human evolution. It identifies three grades. The first is characterised by a species mean body mass of under 50 kg; a species mean stature of less than 150 cm; facultative bipedalism; relatively large teeth and jaws; a moderate size brain relative to body mass; and a relatively short period of maturation. The second grade is characterised by a species mean body mass of more than 50 kg; a species mean stature in excess of 150 cm; obligate bipedalism; relatively small teeth and jaws; a moderate size brain relative to body mass; and a relatively short period of maturation. The third grade is similar to the second in terms of body mass, stature, locomotor behaviour and masticatory system size; but exhibits a considerably higher level of encephalisation. It also exhibits delayed maturation.

Introduction

This book reflects an attempt to cross the interdisciplinary boundaries between archaeology, palaeontology, neuropsychology, neuroanatomy, genetics and evolutionary theory to approach the problem of the origins of modern Homo sapiens. It argues that scrutiny of the origin of our species casts speciation, and maybe evolutionary theory as it now stands, in a critical light. This chapter considers man's place in evolutionary theory, F. M. Muller's critique of Darwinian theory, whether language required a macro-mutation, and the status of saltations in evolutionary theory.

What Can the Y Chromosome Tell Us about the Origin of Modern Humans?

This chapter outlines the peculiar genetic history and population characteristics of the Y chromosome, including the interaction with the X. The small size of the Y and its sex-limited transmission make it at first sight an unlikely vehicle for the determining characteristic of the species. Human and ape Y lineages are generally believed to have split about 5–7 million years ago, while extant human Y lineages trace back to a common ancestor that probably lived between 40 and 200 thousand years ago. Between these dates, two substantial segments of DNA on the Y chromosome were duplicated on the Y: the Yq pseudoautosomal region and the Xq/Yp homology region. The former does not contain any good candidate speciation genes but the latter may. The Xq-Yp transposition probably occurred soon after the ape-human split and, at the same time or subsequently, was divided in two by an inversion.

Archaeology and the Origins of Modern Humans: European and African Perspectives

This chapter outlines the archaeological evidence for the relative recency and abruptness of appearance of artefacts associated with the creativity of modern humans. It compares the archaeological evidence associated with the appearance of anatomically modern humans in Europe and Africa. In Europe, there is a rapid appearance of new behavioural elements that are often seen to represent a ‘revolution’ in behavioural and perhaps cognitive terms, centred on c.43–35,000 years before present (BP). In Africa, new behavioural elements seem to appear in a more gradual, mosaic fashion but show many of the distinctive features of European Upper Palaeolithic culture by at least 70–80,000 (BP), including seemingly explicit evidence for fully symbolic expression. The central problem remains that of assessing how far these well-documented changes in the archaeological record reflect not only major shifts in behavioural patterns, but also underlying shifts in the cognitive capacities for behaviour, including increasing complexity in the structure of language.

Do the Hominid-Specific Regions of X–Y Homology Contain Candidate Genes Potentially Involved in a Critical Event Linked to Speciation?

It has been postulated that the critical events leading to major differences between humans and the great apes are associated with major changes on sex chromosomes. Regions of homology between the human sex chromosomes have arisen at different points during mammalian evolution. The two largest blocks are specific to hominids, having appeared at some time after the divergence of humans and chimpanzees. These are the second pairing region found at the telomeres of the sex chromosome long arms and a region of homology between Xq21.3 (X chromosome long arm) and Yp11 (Y chromosome short arm). Questions arise as to whether these regions of the sex chromosomes contain functional genes and these genes might be candidates for the differences in cognitive function that distinguish modern humans from their ancestors. Furthermore, divergence between functional sequences on the X and the Y may lead to a more subtle sexually dimorphic variation.