In the class, I mentioned that all human infants come into this world with what can be thought of as a "universal grammar" that they can "tune" to the local language that they are hearing around them. In otherwords, language is not wholly learned outside--contrary to the common wisdom (the story of how "universal grammar" came about is also a fascinating one--see the mail that I sent to the class last year http://rakaposhi.eas.asu.edu/f06-cse471-mailarchive/msg00090.html )
One question is whether there is really something special about the set of "human languages" as a whole that is different from any other languages.
In particular, if the human baby were to be given away to martians (or other aliens that regularly visit Area 51 and certain phoenix suburbs), would the baby be able to master the martian language? Conversely, if the human babies were to get together--without any intervention from adults, and were to make a brand new language, would it be closer to all the other human languages than it is to any other language?
We can answer the latter very much in affirmative thanks to the fascinating real life story of Nicaraguan Sign Language. A bunch of nicaraguan deaf kids who were ignored by their war-torn society and over a period of time developed a new sign language all their own from scratch. And it is *very* similar to other human languages (see the URL http:/ /en.wikipedia.org/wiki/Nicaraguan_Sign_Language ).
We don't yet have a clear and convincing evidence that babies can't learn martian and other alien languages, but we do know that human kids brought up without human contact are unable to develop language (see http://en.wikipedia.org/wiki/Feral_child )--in otherwords, the underlying universal grammar is able to identify and adapt only to "human" languages!
In our zeal to accentuate differences, we fail to note that in the spectrum of possible languages, all human languages form a really tight cluster--and would be seen so by a martian visiting earth..