About 5 to 7 million years ago, human diverged from the common ancestor of human and great apes with a rapid brain enlargement. Brain enlargement with improved cognitive skills is one of the most significant changes during primate evolution, especially during the origin of human. Anthropological and genetic studies have reconstructed the phylogenetic relationships among primates species, and among the living nonhuman primates, chimpanzee is the closest relative of human. However, biologically humans and chimpanzees are fundamentally different. Human conquered the world by acquiring highly developed cognitive skills during evolution while chimapanzees are restricted in several regions in Africa. DNA sequence comparison have confirmed that the two genomes only differ by 1.2% - corresponding to a total of about 40 million single base pair substitutions, together with some insertions, deletions and rearrangements. The observation of sequence similarity between humans and chimps is intriguing and have the following implications: 1) Only a small part of the genes in the genomes are responsible for the biological divergence between humans and chimps, especially the difference in cognition; 2) The difference at gene expression level may play an pivotal role in the functional divergence between humans and chimps. In this study, we aim to identify genes under strong Darwinian positive selection in the human lineage, which potentially contributed to the formation of human cognition. Our result showed that only 1.38% genes show strong positive selection. Further study on a G protein coupled receptor, MrgX2 demonstrated a rapid amino acid substitution in primtes, especially the lineage leading to the origin of humans. We observed four fixed amino acid substitutions in humans that were caused by strong Darwinian positive selection, implying that MrgX2 has undergone adaptive changes during human evolution. These adaptive changes may contribute to the adaptation of human nervous system during human origin. The prefrontal cortex (PFC), is critical for several specific cognitive skills, e. g. the working memory. The central nervous system will experience reconstruction during brain development, and this process is associated with adjustment of gene expression in the brain. Using DNA microarray technology, we tend to dissect the gene expression profiles of PFC at three crucial developmental stages in rhesus monkeys.
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