The human homolog of Foxp2 codes for a protein – a so-called transcription factor – that regulates the activity of hundreds of genes expressed in various mammalian cell types. Individuals who carry only one functional copy of the gene instead of the usual two experience specific difficulties in learning to speak and in language comprehension. "Genetic mutations that occurred during the 6 million years since our lineage diverged from that of chimpanzees have resulted in localized alterations in two regions of the Foxp2 protein. That is quite striking when one considers that the normal mouse version differs from that found in chimps by only a single mutation, although these two species are separated by over 100 million years of evolution. The question is how the human variant of this transcription factor contributes to the process of language acquisition," says Enard.
Enard and his coworkers had previously shown that the alterations in the human gene for Foxp2 specifically affect certain regions of the brain. When the two human-specific substitutions were introduced into the mouse version of the gene, he and his team observed anatomical changes exclusively in two neuronal circuits in the basal ganglia of the mouse cortex, which are involved in the control of motor function. "These circuits play a crucial role in the acquisition of habitual behaviors and other cognitive and motor capabilities," Enard explains.
Conscious and unconscious learning processes
In their latest work with the same mouse model, Enard and his collaborators found that, under certain conditions, the human version of Foxp2 actually enhances learning. "We have shown for the first time that the evolved alterations in the human gene have an effect on learning ability. The human version modifies the balance between declarative and motor neuron circuits in the brain. As a result, the mice take less time to associate a given stimulus with the appropriate response, and hence learn more rapidly," says Enard.
Learning to speak clearly requires interactions between conscious "declarative" knowledge and the unconscious effects of repetitive stimulation of particular patterns of neural activity. "As we learn, the underlying neuronal processes become automated, they are converted into routine procedures, enabling us to learn faster," Enard explains. Using various tests, the researchers demonstrated that the human-specific mutations enhance cooperative interactions between the two affected circuits in the basal ganglia of the mouse brain. "The human variant of the Foxp2 gene modulates the associative and sensorimotor nerve connections formed, as well as levels of the neurotransmitter dopamine in the basal ganglia, during the learning process. The increased ability to switch between conscious and unconscious forms of learning may play a role in the acquisition of language," Enard concludes.
Foxp2 is the only gene so far that has been shown to be directly associated with the evolution of language, and studies of Foxp2 function promise to throw new light on the evolution of the human brain. The mutation that first revealed the link with language was discovered in a kindred, many of whose members displayed severe speech difficulties, primarily as a consequence of defective control of the muscles of the larynx, the lips and the face.
EurekAlert. 2014. “Language evolution: Quicker on the uptake”. EurekAlert. Posted: September 18, 2014. Available online: http://www.eurekalert.org/pub_releases/2014-09/lm-leq091814.php