From The Descent of Man:
"The formation of different languages and of distinct species and the proofs that both have developed through a gradual process are clearly the same."
Almost the same: there's nothing like convergence in linguistics. IE, there is no external force driving language change, it's all inherited change.
I don't have the book in front of me (so I'm paraphrasing), but Merritt Ruhlen in The Origin of Language : Tracing the Evolution of the Mother Tongue uses a biological concept, "out-group comparison", in his logic. He says there are [a few] thousand species of mammal, all but six (?) (platypus and various echidnas) give live birth. Did Proto-Mammal lay eggs or have live birth? All biologists agree that Proto-Mammal laid eggs, because, 1) some mammals do, and 2) their closest non-mammal relatives, the reptiles, all do.
He employs this logic to argue that if a word is found in, say, Finnish and Samoyed, but no where else in Finno-Ugric, then, if it's also found in Yukaghir, (and isn't a loanword), it was part of the proto-Finnno-Ugric language, even thought it's not in Hungarian or Mansi, etc.
Ruhlen is arguing for the monogeneisis of language; in another book, On the Origin of Languages: Studies in Linguistic Taxonomy (BTW, much more technical), he gives a list of 20-some words that seem to be widespread throughout the world's languages.
The most famous example is "tik" meaning "finger"; it's the root behind "digit", "decimal", and perhaps "toe"; it's found in Africa, Asia, Europe and the Americas.
I just googled an interesting discussion of this: Babel and the Ancient Single Language of the Human Race" by G. R. Morton, as in Morton's Demon!
Ping to PH, VR and Junior; Check Glenn Morton's home page. All I've seen previously is his "demon" essay.
The original differentiation of human language from other species was evolutionary, but not sure if I totally agree with your point.
I think it was Noam Chomsky who said that the babbling of babies contain every single sound used by all the languages of the world. As the child learns any specific language, the other sounds fade away because they are not used or needed.
However, human language change itself, including the various dialects might be due entirely to external forces. It is geographic dispersion that creates different languages to become distinct, but like you said, contain some sounds or words that might be similar to those of near by geographic locations. For example, the romance languages of Europe are similar.
Oh, well, there's always Google...
...eventually. I have 3000 pages or so of reading new stuff given to me as Christmas presents and from other Freepers since Christmas, and I've only read about 200 of it...
Cheers!