![]() In order to identify evidence for positive selection, a measure of divergence to diversity is calculated. Evolutionary pressures that distinguish us from our nearest primate cousins can be summarized both in terms of transcriptome and genome, by comparing variation between individuals within a species (human or chimpanzee in this instance) and variation between species. ![]() We can now compare chimpanzee and human genomes, both in expression patterns of associated genes (transcriptomes) and in DNA structure (genomes). ![]() In this article, the evidence for evolutionary changes in terms of gene structure and gene expression in brain will be discussed, followed by a review of the specific characteristics of the X chromosome and its potential role in the development of sexually dimorphic characteristics. Yet, since our paths of evolution diverged, that 1% of genetic difference has changed humans substantially, in terms of our behavioural and cognitive traits – presumably driven to a large extent by positive Darwinian selection. In crude terms, 99% of the two species’ DNA is identical. Recently, the first candidate genes have been discovered.Ĭhimpanzees and humans last shared a common ancestor between 5 and 7 million years ago. Evidence supporting each of these mechanisms is available from both human and animal models. These include the biased expression of genes subject to X-inactivation, haploinsufficiency (in males) for non-inactivated genes with no Y homology, sex-specific brain functions and genomic imprinting of X-linked loci. Mechanisms by which sex-chromosomal effects, due to X-linked genes, influence neural development or function are reviewed. Genes on the sex chromosomes can directly influence sexual dimorphism in cognition and behaviour, independent of the action of sex steroids. Recently, it has become clear that a long-standing theory, implicating X-linked genes in a sexually antagonistic evolutionary role, is probably correct. Playing centre stage, in terms of the proportion of genes involved in brain development and cognitive function, is the X chromosome. Protein evolution is driving an accelerating increase in brain complexity and size. Significant species differences can be observed both at the levels of function (gene expression) and structure (amino acid sequence). These modifications have occurred despite evolutionary constraints upon the diversity of genetic influences, on the development and function of neural tissue. Yet, since the paths of primate evolution diverged, there have been remarkable developments in the behavioural and cognitive attainments of our species, which ultimately reflect subtle differences in gene structure and function. Chimpanzees and humans last shared a common ancestor between 5 and 7 million years ago 99% of the two species’ DNA is identical.
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