Of course, there were no textbooks in the beginnings of language, but linguists have developed certain methods to trace back words even beyond earliest records. Thus we have knowledge not only of the last 1000 years of English. We can even make an assumption about the very roots of the language.
Most languages belong to language families. A language family is a group of related languages that developed from a common historic ancestor, referred to as proto language (proto means 'early' in Greek). The ancestral language is usually not known directly, but it is possible to discover many of its features by applying the comparative method that can demonstrate the family status of many languages.
One scientific way to study the origin of language is to try to prove historical relationships between languages. To find language families, that is, groups of languages descended from a common ancestor, linguists compare languages to find systematic differences or similarities.
This method of analyzing languages is known as the comparative method, and linguists using it are referred to as comparative linguists. Some languages are obviously related to one another, as shown by the presence of systematic differences.