The evolution of language
July 15, 2010 Leave a comment
Language has families, too
We think of English as being a language distinct from, say, Hindi, but they both belong to the Indo-European family of languages. Family members include French, Spanish, German, and Walloon, spoken in a tiny area of France and by descendents of settlers in Green Bay, Wisc. Language, like genes, is passed on from generation to generation, and through time undergoes changes and additions, just as genomes do.
In the 1950s, a linguist named Morris Swadesh developed a field known as lexicostatistics, which studies linguistic family trees through quantitative analysis of a core group of words. These 100 to 200 words, known as Swadesh lists, identify the commonalities in any language, and according to Swadesh’s ideas, were thus less likely to change over time. To trace language lineages, Swadesh compared these groups for similarities and words—cognates—that appeared to have a common ancestor. The more cognates two languages shared, the more closely related they were presumed to be.
A fun word to learn: Glottochronology
In a study published in Nature, authors Russell Gray and Quentin Atkinson take the Swadesh approach into the realm of glottochronology, a way of determining the timing of divergence of languages. Based on their computations, they determined that the root of the Indo-European family tree traces back almost 9000 years ago to an area of what is now Turkey, where farmers grew crops and spoke Hittite. As they migrated, they took their languages with them. Gray and Atkinson’s findings counter another proposed origin of the Indo-European family tree, which posited that invading horsemen from the steppes of Asia brought their language with them as they prosecuted their warlike endeavors 6000 years ago.
What does this have to do with biology?
It is both inspired by and inspiring to biological evolutionary study. Swadesh developed his ideas at about the time that Watson and Crick were making their startling elucidation of DNA structure and copying mechanism. In their analyses, Gray and Atkinson used tools very similar to those biologists use to develop phylogenies to explore how species are related. Instead of gene or amino acid sequences, the authors used a series of letters that formed words; instead of identifying mutations, they identified changes in letters or syllables; and instead of orthologues—genes derived from a common ancestor—they identified cognates to elucidate relationships.
But Swadesh preceded biologists in one sense, in that he proposed using his analyses of Swadesh lists to determine time lapses as language developed and diverged; in other words, to use any divergences in cognates as a sort of clock to measure the time over which changes occur. One of the basic assumptions of this approach is that the changes will occur at a relatively constant rate when averaged out over time, giving a reasonably accurate assessment of how far back a relationship can be traced. This idea was only later taken up in biology as the idea of the “molecular clock,” in which biologists use the presumption of a constant rate of change in some gene or amino acid sequences to infer the timing of genetic divergence.
Problems with either approach
There are, of course, problems with either approach, and again, the problems are remarkably similar, whether the field is biology or linguistics. One common problem in building phylogenies is determining which changes occur because of environmental similarities (convergence), instead of relatedness; convergence is also an issue in the linguistic analysis, where words may appear to be cognates when they really are not. Another question that arises in biology is whether or not the genes being examined are suitable markers of evolutionary change; again, the same problem arise in linguistics—are words in the Swadesh lists, for example, suitable choices for comparison among languages? In spite of these inherent problems, Gray and Atkinson’s work has opened up fresh avenues of discovery and debate and brings biology and language closer to one another than ever.