Language families
Isolates: Languages With No Known Relatives
What an isolate is, which are best documented, and why 'no relatives' is a statement about evidence.
Language isolates are languages that have no demonstrable genealogical relationship with any other language, according to Britannica. After using the comparative method to demonstrate relationships, these languages have none at the current level of evidence. That does not prove there are no relatives. While no proven genealogical connections have been found for language isolates, the qualifier "not yet" suggests unknown connections may still be uncovered. Language isolate examples include Basque, Zuni, and Cuitlatec, according to Ethnologue and Britannica], among others.
While an isolate being “the last survivor of an ancient family" is certainly possible, it is not part of the label. First, an undiscovered, undiscoverable relative could still exist. Second, an isolate means no determinable relatives without guaranteed transparency. What the isolate label does state is that this language with no demonstrated relatives has stood the test of today's sophisticated comparative analysis. An isolate also does not mean they are old. While some isolates are old, such as Basque, it typically hasn't been determined if the language isolate is the original language, or is itself a relative of an extinct language.
The isolates meaning vs unclassified
The category of language isolates is often confused with unclassified languages. The confusion usually arises from partly complete data. Unclassified languages are simply too poorly preserved or documented to classify, whereas isolates are sufficiently documented.
Unclassified languages might still fall under existing classifications, even if none are known at present. Language isolates, however, have collected enough data for analysis, but no genetic family has been discovered through comparisons. In other words, for a language to be considered an isolate, sufficient linguistic data should be available to determine its relatedness. Additionally, established isolates are known to be eligible for relationships that have not proven themselves yet.
When sufficient documentation becomes available, isolates may be proven to be related. In the same way, unclassified languages are subject to reclassification as new facts are unearthed.
The best-documented examples of isolates
There is no one source with a primary count of documented evidence for exact numbers that is current, though I did find the following on recognizable examples.
Zuni's language isolate page is in the Ethnologue. Encyclopaedia Britannica and numerous others also recognize the indigenous isolate of Cuitlatec. Zuni and Basque are the isolates listed to have more specifically defined resources.
Possibly the most cited as the isolate is also recognized by Pacific Linguist's Simon G. Musgrave's paper Language Families of the World, as "the best-known isolate". Basque's cases have no standing as relatives of each other, making Basque one of the most studied isolates.
I hope this short list of disputes goes some way to showing what current countable evidence gives.
Why linguists urge caution on the isolate labeling?
The trouble with language isolates for linguists is that even though no genealogical connections have been established through analysis, it is still unable to prove that these isolates do not have genetic connections. An isolate is a language that has not been shown to descend from a common ancestor or share a linguistic kinship with other languages.
Caution of the isolate classification arises from simple fact that it merely notes that no relative has been proven.
In other words, the fact that a language isolate has not been found to have any family members doesn't mean that it doesn't have family members at all. It simply means that the evidence of its family relationships, if any, is not available, or else insufficient.
In this regard, while the comparative method can show that languages share certain characteristics or features, it cannot prove that they do not. As such, when linguists say that a language is an isolate, they are really saying that the language doesn't display a specific linear genetic relationship with any other known language. Accordingly, classifying a language as an isolate does not mean that we will never discover any links in the future.
Where this sits in the Register
Written by the Linguasphere desk. Classification data referred to here comes from the Linguasphere Register — see sources and attribution.