Language is a uniquely human trait, and how it evolved is one of the greatest unanswered questions in neuroscience. Spoken and written language facilitate much of what makes up human culture. Thus, understanding its origins is tantamount to cracking the age old philosophical question, “From whence do we come?”
The biggest clue to date comes from the discovery of FOXP2– a gene that, when mutated, causes aberrations specifically in speech and language. FOXP2 encodes a transcription factor, which is a protein that binds to DNA to regulate gene expression. Transcription factors act like molecular switches that turn genes on and off. FOXP2 transcription factor activity is important during neural development, and it continues to play a role throughout life by facilitating motor learning. Mutations in this gene cause difficulties in articulation, or how clearly words are pronounced, as well as deficits in verbal cognition, such as reading comprehension and vocabulary.
In 2002 a team of researchers studied the evolution of FOXP2. Their data appeared to show evidence for a selective sweep at the FOXP2 gene. In genetics parlance, this means there is low genetic variation near a mutation in DNA, indicating that evolution strongly favored that mutation. In this case, the mutation the researchers identified encoded two amino acid changes in the FOXP2 protein itself. Their data was interpreted as evidence that language must have evolved via these changes in FOXP2.
The 2002 study compared a fragment of the FOXP2 gene across 21 human DNA samples. However, genetic research using a small sample size is prone to bias, meaning the sample may not accurately represent the greater population as a whole. Now a new study published in Cell is challenging the 2002 finding.
An important consideration in designing human evolutionary genetics studies is that there is much lower genetic diversity outside of Africa than within it. Modern humans can be genetically subdivided into seven ancient lineages, or “macro-haplogroups.” These are defined by the DNA in the mitochondria, genetic material outside of a cell’s nucleus. Because the mitochondrial DNA is is inherited directly from the mother it can be used trace back the maternal line and define large groups of people with shared descent. All ancient lineages outside of Africa are descended from just a single macro-haplogroup. To make claims about recent human evolution, a study would need to accurately represent all modern human genetic diversity, meaning they would need to sample all the macro-haplogroups instead of only one.
Would the evidence for a recent selective sweep at the FOXP2 gene hold up in an adequately diverse sample of human FOXP2 genes?
Would the evidence for a recent selective sweep at the FOXP2 gene hold up in an adequately diverse sample of human FOXP2 genes? Atkinson and colleagues put this question to the test. They discovered that the evidence for recent protein-coding mutations in the FOXP2 gene disappears when tested in two data sets that include DNA from people throughout Africa in addition to the rest of the world.
However, the researchers did uncover a new clue for how FOXP2 evolved in humans. They found a region of FOXP2 that has undergone human-specific changes compared to chimpanzees and ancient hominins (the technical term for “cavemen” like neanderthals). Interestingly, they did not find a selective sweep where one specific mutation is positively selected for. Instead, they found different mutations in each sample happening in exactly the same region. This suggests that any mutation that canceled the functionality of this region was uniquely selected for in humans.
The researchers conducted several tests to determine the role of this region. They determined that the region is likely to be an enhancer, a regulatory element that controls transcription of large groups of genes. This means that human-specific changes in this region likely altered the ability of transcription factors and other molecular machinery to bind to it, thus allowing specific alterations in gene expression in the human brain.
This study changes our understanding of the evolution of language by showing that the FOXP2 gene body has not undergone recent positive selection in humans. The version of the FOXP2 protein we carry has existed before the advent of modern humans and was shared with other ancient hominins. However, the researchers did find human-specific selection for mutations that appear to disrupt the functionality of one specific regulatory region within the FOXP2 gene. Although much remains to be revealed about the evolution of speech and language, this study takes us one step closer to understanding the molecular origins of this amazing human ability.
How do you think language evolved? Tell us below in the comments!
Illustrated by Sean Noah.
Lai, C.S., Fisher, S.E., Hurst, J.A., Vargha-Khadem, F., & Monaco, A.P. (2001). A forkhead-domain gene is mutated in a severe speech and language disorder.Nature, 413(6855), 519-23.
Enard, W., Przeworski, M., Fisher, S.E., Lai, C.S., Wiebe, V., Kitano, T., Monaco, A.P., & Pääbo S. (2002). Molecular evolution of FOXP2, a gene involved in speech and language.Nature, 418(6900), 869-72.
Atkinson, E.G., Audesse, A.J., Palacios, J.A., Bobo, D.M., Webb, A.E., Ramachandran, S., & Henn, B.M. (2018). No Evidence for Recent Selection at FOXP2 among Diverse Human Populations.Cell, 174(6), 1424-1435.