Complex speech in humans

The remarkable range of human speech is a more recent evolutionary development than previously thought, a new study claims.

In a paper that awaits peer-review on pre-print repository bioRxiv, an international team of researchers reveals that the structure of the human vocal tract and related parts of the face, which together deliver optimum conditions for speech production, is unique to modern humans.

The team, led by David Gokhman of the Hebrew University of Jerusalem, includes scientists from genetics powerhouses such as the Broad Institute in Massachusetts, US, and the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany.

They contend that older human species such as Neanderthal and Denisovans would not have enjoyed the full capacity for speech that we do. In fact, the authors state “the evolution of vocalisation apparatus of modern humans is unique among hominins and great apes.”

Interestingly, the team make this claim not based on genetics, but on ‘epigenetics’ – the study of the way that factors outside of genes can control and effect heredity.

The paper examines the epigenetic expression known as DNA methylation. This looks at ‘methyl groups’, which are molecules derived from methane, containing one carbon atom bonded to three hydrogen atoms.

The groups originate in the environment, and bond to DNA. While long considered deleterious, scientists now realise that methylation significantly contributes to both the activation and repression of gene function.

Patterns of methylation can be mapped. Comparing the maps of modern and archaic humans, as well as great apes, led Gokhman and colleagues to conclude that complex speech is a recent development. The scientists state “the molecular mechanisms that underlie the modern human face and voice … arose after the split from Neanderthals and Denisovans”.

The epigenetic nature of the mechanisms might well demonstrate that significant evolutionary change can happen without corresponding change in genes.

Many are still inclined to agree with the sentiment Darwin expressed in The Descent of Man that “language owes its origin to the imitation and modification of various natural sounds, the voices of other animals, and man’s own instinctive cries.” This is because it seems sensible that some capacity for speech would arise first, and the full complexity of symbolic logic and language would follow.

But Darwin’s suggestion is long out of date. Modern research has given rise to multiple competing theories based on more recent aspects of evolutionary theory. Widely accepted, however, is the notion that words are cheap. Literally.

Words don’t require substantial energy investment from an organism, so there is little at stake in using them. This means that it’s just as cheap to lie as to tell the truth, and scientists think this might have been a barrier to the evolution of spoken language.

Researchers now believe that for spoken language to become a successful and stable evolutionary strategy, humans must first have developed both full symbolic culture and extremely high levels of interpersonal trust.

While the last condition may strike us as unlikely in the current era, the paper’s claim that the physical architecture needed for speech is a relatively recent adaptation seems to support this theory.