Foxp2 and the evolutionary genetics of language

Abstract

FOXP2 was characterized as "the language gene" after it was discovered in looking for the hereditary cause of a severe disability (not loss) affecting the speech/language of a 30-member London family. Disrupting one of a person's two copies of FOXP2 causes the disability. FOXP2 protein has two amino acid changes which distinguish humans from chimpanzees. There were no other changes in FOXP2 protein going back to the common ancestor of human and mouse. Speculation that those "language gene" mutations were key to the evolution of human language 50,000 years ago, however, is not tenable. Genomic evidence presented here dates the human mutations in FOXP2 to 1.8 or 1.9 million years ago, about the time of the emergence of the genus Homo, consistent with the later finding that Neanderthals had these same two mutations. Evidence from birdsong and from mice with "humanized" Foxp2 point to involvement of FOXP2 and its human mutations with the basal ganglia. The function of the basal ganglia is primarily the facilitation of learning, however, and not specifically vocalization. The origin of language requires major changes in the connectivity of the language areas of the cortex, which then provide inputs to cortio-basal ganglia circuits. It is these special connective structures, not brain size, that enable language. Language is a defining characteristic of the biological species Homo sapiens, dating back some 200,000 years. But Chomskian Universal Grammar is not what is innate about language. No simple recent mutation enabled recursion in language after all the other structures enabling higher cognition, speech, and language were in place. Human language co-evolved with the expanding Homo brain during its two million years of evolution. FOXP2 protein is a transcription factor, regulating the expression of several hundred genes in complex networks. It is essential for non-language related neurodevelopment in all vertebrate species. While a disabled FOXP2 gene interferes with language, many genetic processes which interfere with brain function and with normal language use are not related to the origin or evolution of language in any direct way. The human mutations in FOXP2 had only an indirect effect on the evolution of language.