without committee meetings, dictionaries, or even literacy

Imagine that you had a time machine.

If you are like me, there would be many times and places that you would like to visit. In most of them, however, no one spoke English. If you could not afford the Six-Month-Immersion Trip to, say, ancient Egypt, you would have to limit yourself to a time and place where you could speak the language. Consider, perhaps, a trip to England. How far back in time could you go and still be understood? Say we go to London in the year 1400 CE.

As you emerge from the time machine, a good first line to speak, something reassuring and recognizable, might be the opening line of the Lord's Prayer. The first line in a conservative, old-fashioned version of Modern Standard English would be, "Our Father, who is in heaven, blessed be your name." In the English of 1400, as spoken by Chaucer, you would say, "Oure fadir that art in heuenes, halwid be thy name".

Now turn the dial back another four hundred years to 1000 CE, and in Old English, or Anglo-Saxon, you would say, "Reader ure thu the eart on heofonum, si thin nama gehalgod." A chat with Alfred the Great would be out of the question.

Most normal spoken languages over the course of a thousand years undergo enough change that speakers at either end of the millennium, attempting a conversation, would have difficulty understanding each other. Languages like Church Latin or Old Indic (the oldest form of Sanskrit), frozen in ritual, would be your only hope for effective communication with people who lived more than a thousand years ago.

Icelandic is a frequently cited example of a spoken language that has changed little in a thousand years, but it is spoken on an island isolated in the North Atlantic by people whose attitude to their old sagas and poetry has been one approaching religious reverence. Most languages undergo significantly more changes than Icelandic over far fewer than a thousand years for two reasons: first, no two people speak the same language exactly alike; and, second, most people meet a lot more people who speak differently than do the Icelanders.

A language that borrows many words and phrases from another language changes more rapidly than one with a low borrowing rate. Icelandic has one of the lowest borrowing rates in the world. If we are exposed to a number of different ways of speaking, our own way of speaking is likely to change more rapidly. Fortunately, however, although the speed of language change is quite variable, the structure and sequence of language change is not.

Language change is not random; it flows in the direction of accents and phrases admired and emulated by large numbers of people. Once a target accent is selected, the structure of the sound changes that moves the speaker away from his own speech to the target is governed by rules.

The same rules apparently exist in all our minds, mouths, and ears. Linguists just noticed them first. If rules define how a given innovation in pronunciation affects the old speech system — if sound shifts are predictable — then we should be able to play them backward, in effect, to hear earlier language states. That is more or less how Proto-Indo-European was reconstructed.

Most surprising about sound change is its regularity, its conformation to rules no one knows consciously. In early Medieval French there probably was a time when tsent'm 'hundred' was heard as just a dialectical pronunciation of the Latin word kentum 'hundred'.

The differences in sound between the two were allophones, or different sounds that did not create different meanings. But because of other changes in how Latin was spoken, [ts-] began to be heard as a different sound, a phoneme distinct from [k-] that could change the meaning of a word.

At that point people had to decide whether kentum was pronounced with a [k-] or a [ts-]. When French speakers decided to use [ts-], they did so not just for the word kentum but in every word where Latin had the sound k- before a front vowel like -e-. And once this happened, ts- became confused with initial s-} and people had to decide again whether tsentum was pronounced with a [ts-] or [s-]. They chose [s-].

This sequence of shifts dropped below the level of consciousness and spread like a virus through all pre-French words with analogous sequences of sounds. Latin cera 'wax', pronounced [kera], became French cire, pronounced [seer]; and Latin civitas 'community', pronounced [kivitas], became French cite, pronounced [seetay].

Other sound changes happened, too, but they all followed the same unspoken and unconscious rules—the sound shifts were not idiosyncratic or confined to certain words; rather, they spread systematically to all similar sounds in the language. Peoples' ears were very discriminating in identifying words that fit or did not fit the analogy. In words where the Latin k- was followed by a hack vowel like -o it remained a k-, as in Latin costa> French cote.

Sound changes are rule-governed probably because all humans instinctively search for order in language. This must be a hard-wired part of all human brains. We do it without committee meetings, dictionaries, or even literacy, and we are not conscious of what we are doing (unless we are linguists).

Human language is defined by its rules. Rules govern sentence construction (syntax), and the relationship between the sounds of words (phonology and morphology) and their meaning. Learning these rules changes our awareness from that of an infant to a functioning member of the human tribe. Because language is central to human evolution, culture, and social identity, each member of the tribe is biologically equipped to cooperate in converting novel changes into regular parts of the language system.

David W. Anthony, The Horse, The Wheel and Language; Princeton University Press, 2007; pp. 22-23