The Genetics of Speech

For centuries, scientists and philosophers have argued that speech, a distinctly human trait, was an essential component for the formation of early societies. Ancient mythologies often suggest that language predates humanity, while biologists have uncovered interesting links that might explain the evolutionary development of human speech. In addition, numerous animals are capable of rudimentary speech, which studies indicate might be more complex than previously thought. Despite this, no known animal species possess the capacity for language seen in humans.

This separation between humans and closely related animals has baffled researchers for years. How can we explain the Human talent for speech? What, if anything, is responsible for the evolution of speech? How does a scientist determine where to even start with such a complicated question? Recent discoveries might finally help to answer some of these questions.

In the beginning…

One aspect that makes studying speech so complicated is the lack of a historical record. Unlike writing, which can last for millennia, the technology to record speech has only recently become available. In addition, while many animals can display basic elements of language, including sign language and symbol-based behaviors in the great apes or vocal communication in whales, these fall far short of the speech capabilities of humans. Thus, it seems that speech may require particular traits that are currently only found in Homo Sapiens.

Early efforts to determine the origins of speech examined the basic human and animal speech organs, including the lips, tongue, and larynx. Interestingly, from an anatomical standpoint most of these organs are highly similar across species. Therefore, many scientists believe that the distinction between species lies not in the physiology of speech organs, but instead in a cognitive difference that sets humans apart.

Unfortunately, the genetic pathways that regulate our brains are still largely a mystery. It turns out that thousands of genes are likely working together to form our brains and control our behaviors, and it is extremely unlikely that a single gene will be able to fully control speech. Instead, scientists will have to determine which genes are involved and how they are interconnected.

Fortunately, scientists have many tools that they can utilize to determine how human genetics might have allowed the development of speech. One such method involved the analysis of a family of individuals with a variety of speech disorders, including improper pronunciation, trouble with word order, and difficulty understanding speech from others. Since this disorder was found in many members of this family scientists hypothesized that it was likely genetic, and examination of the family’s DNA revealed a mutation in a little known gene called FOXP2.
The FOXP2 protein is an important transcription factor, a protein that can enhance or repress the expression of other proteins. Thus, it is likely that the function of FOXP2 is associated with how it controls other genes, and that this large collection of proteins is regulating speech. Interestingly, FOXP2 is very highly conserved between organisms; the Human and Chimp versions feature only 2 dissimilar amino acid, the building blocks that form proteins. It is likely that these small changes must be incredibly important to account for the differences in protein function!

Additional studies have used this approach to identify candidate genes in other speech disorders. Stuttering was found to follow complex inheritance patterns in families, although there was a higher incidence in both members of identical twins than zygotic twins. Studies on adopted children show that exposure to stuttering does not increase the odds that a child will develop a stuffer. Instead, a number of candidate genes have now been identified that might explain the condition.

While these studies are exciting, both from a medical and developmental standpoint, scientists are still in the early stages of understanding speech. Genes like FOXP2 are strongly associated with speech, but additional studies will be required to determine the exact mechanism. Interestingly, preliminary studies on mice show that mutations in the mouse FOXP2 protein can lead to changes in vocalization patterns. Future studies will dive deeper into the function of these genes and will continue to unlock the genetics of speech.