Final verdict

A. Ruhlen’s point that comparative reconstruction is not the only way to show that languages have a common ancestor is valid in itself. He observes that linguists posited the Indo-European group long before Proto-Indo-European itself had been worked out by working backward from the languages. The similarities between language families are close enough that his point is likely valid for mega-groups, such as Amerind and Eurasiatic.

B. A question still remains, however, as to how realistic even this approach is for Proto-World. The issues could be resolved as more proto-languages are reconstructed, although work of this kind is done increasingly less by modern linguists, and for reasons we will see later, it may be entirely impossible to reconstruct protolanguages for many families.