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Brain Growth
Spoken language is a characteristically human trait and is presumed to be the prerequisite for the evolution of cultures. Speech entails the fine-scale control of the larynx (voice box) and mouth. Individuals who are heterozygous for mutations at the FOXP2 locus have severe problems with language articulation and with forming sentences (Vargha-Khadem et al. 1995; Lai et al. 2001). This observation has provided genetic anthropologists with an interesting gene to study. Enard and colleagues (2002b) have shown that, although the FOXP2 gene is conserved throughout most of mammalian evolution, it has a unique form in humans, having accumulated at least two amino acid-changing mutations just since our divergence from the common ancestor of humans and chimpanzees. These differences are significant, since human and chimpanzee forms of the Foxp2 protein differentially regulate more than 100 genes (Konopka et al. 2009).
In the mouse, the Foxp2 gene is expressed in the developing brain, but its major site of expression is the lung (Shu et al. 2001). In humans, FOXP2 is predominantly expressed in those brain regions that coordinate speech (i.e., the caudate nucleus and inferior olive nuclei); these sites are abnormal in patients with FOXP2 deficiency (Lai et al. 2003). In the cortical regions regulating language and speech, the human-specific FoxP2 appears to promote the expression of these specific transcripts during brain development (Lambert et al. 2011). In birds, the Foxp2 protein is associated with song learning, and the experimental downregulation of Foxp2 expression in certain areas of the brain prevents young male birds from imitating their species-specific song (Teramitsu and White 2006; Haesler et al. 2007). Although it is not certain that FOXP2 is the most critical gene for human language acquisition, it seems to be very important for allowing the orofacial movements and grammar characteristic of human speech.
Literature Cited
Enard, W. and 7 others. 2002b. Molecular evolution of FOXP2, a gene involved in speech and language. Nature 418: 869–872.
Haesler, S., C. Rochefort, B. Georgi, P. Licznerski, P. Osten and C. Scharff. 2007. Incomplete and inaccurate vocal imitation after knockdown of FoxP2 in songbird basal ganglia nucleus area X. PLoS Biology 5: e321.
Konopka, G. and 9 others. 2009. Human-specific transcriptional regulation of CNS development genes by FOXP2. Nature 462: 213–217.
Lai, C. S., S. E. Fisher, J. A. Hurst, F. Vargha-Khadem and A. P. Monaco. 2001. A forkhead-domain gene is mutated in a severe speech and language disorder. Nature 413: 519–523.
Lai, C. S., D. Gerrelli, A. P. Monaco, S. E. Fisher and A. J. Copp. 2003. FOXP2 expression during brain development coincides with adult sites of pathology in a severe speech and language disorder. Brain 126: 2455–2462.
Lambert, N. and 11 others. 2011. Genes expressed in specific areas of the human fetal cerebral cortex display distinct patterns of evolution. PLoS One 6: e17753.
Shu, W., H. Yang, L. Zhang, M. M. Lu and E. E. Morrisey. 2001. Characterization of a new subfamily of winged-helix/forkhead (Fox) genes that are expressed in the lung and act as transcriptional repressors. J. Biol. Chem. 276: 27488–27497.
Teramitsu, I. and S. A. White. 2006. FoxP2 regulation during undirected singing in adult songbirds. J. Neuroscience 26: 7390–7294.
Vargha-Khadem, F., K. Watkins, K. Alcock, P. Fletcher and R. Passingham. 1995. Praxic and nonverbal cognitive deficits in a large family with a genetically transmitted speech and language disorder. Proc. Natl. Acad. Sci. USA 92: 930–933.