Linguists like to put languages into groups. Some of these, as in biology, are groupings based on historical descent (Germanic vs Romance), some of long standing (Indo-European vs Ural Altaic vs Micronesian). Some categorizations show more sensitive to morpho-syntactic form (analytic vs agglutinative) and some are tied to whether they got to where they are spoken by tough guys who rode little horses over long distances (Finno-Ugaric (and Basque?)). There is a tacit agreement that these groupings are significant typologically and hence linguistically significant as well. In what follows, I want to query the ‘hence.’ I would like to offer a line of argument that concludes that typological differences tell us nothing about FL. Or, to put this another way, the structure of FL in no way reflects the typological differences that linguists have uncovered. Or, to put this in reverse, typological distinctions among languages have no FL import. If this is correct, then typology is not a good probe into the structure of FL. And so if your interest is the structure of FL (i.e. if liming the fine structure of FL is how you measure linguistic significance), you might be well advised to study something other than typology.
Before proceeding let me confess that I am not all that confident about the argument that follows. There are several reasons for this. First, I am unsure that the premises are as solid as I would like them to be. As you will see, it relies on some semi-evolutionary speculation (and we all know how great that is, not!). Second, even given the premises, I am unsure that the logic is airtight. However, I think that the argument form is interesting and it rests on widely held minimalist premises (based on a relatively new and, IMO, very important observation regarding the evolutionary stability of FL), so even if the argument fails it might tell us something about these premises. So, with these caveats, cavils, hedges and CYAs out of the way, here is the argument.
Big fact 1: the stability of FL. Chomsky has emphasized this point recently. It is the observation that whatever change (genetic, epi-genetic, angelic) led to the re-wiring of the human brain thus supporting the distinctive species specific nature of human linguistic facility, whatever change that was, it has remained intact and unchanged in the species since its biological entrance. How do we know?
We know because of Big Fact 2: any kid can learn any language and any kid learning any language does so in essentially the same way. Thus, for example, a kid from NYC raised in Papua New Guinea (PNG) will acquire the local argot just like a native (and in the same way, with the same stages, making the same kinds of mistakes etc.). And vice versa for a PNGer in NYC, despite the relative biological isolation of PNGers for a pretty long period of time. If you don’t like this pair, plug in any you would like, say Piraha speakers and German speakers or Hebrew Speakers and Japanese. A child’s biological background seems irrelevant to which Gs it can acquire and how it acquires them. Thus, since humans separated about 100kya (trek out of Africa and all that), FL has remained biologically stable in the species. It has not changed. That’s the big fact of interest.
Now, observe that 100k years is more than enough time for evolution to work its magic. Think of Darwin’s finches. As soon as a niche opened up, these little critters evolved to exploit it. And quickly filling niches is not reserved just for finches. Humans do the same thing. Think of lactase persistence (here). The capacity to usefully digest milk products arose with the spread of cattle domestication (i.e. roughly 5-10kya). So, humans also evolutionarily track novel “environmental” options and change to exploit them at a relatively rapid rate. If 5-10k years is enough for the evolution of the digestive system, then 100k years should be enough for FL to “evolve” should there be something there to evolve. But, as we saw above, this seems to be false. Or, more accurately, Big Fact 2 implies Big Fact 1 and Big Fact 1 denies that FL has evolved in the last 100k years. In sum, it seems that once the change allowing FL to emerge occurred nothing else happened evolution wise to differentially affect this capacity across humans. So far as we can tell, all human FLs are the same.
We can add to this a third “observation,” or, more accurately, something I believe that linguists think is likely to be true though we probably only have anecdotal evidence for it. Let’s call this Big Fact 3 (understanding the slight tendentiousness of the “fact” part): kids can learn multiple first languages simultaneously and do so in the same way despite the languages involved.  So, LADs can acquire English and Hebrew (a Germanic and Semitic language) as easily as German and Swedish (two Germanic languages), or Navajo and French or Basque and Spanish as easily as French and Spanish or… In fact, kids will acquire any two languages no matter how typologically distinct in effectively the same way. In short, typological difference has no discernable impact on the course of acquisition of two first languages. So, not only is there no ethnically-biologically based genetic pre-disposition among FLs for some Gs over others, there is not even a cognitive preference for acquiring Gs of the same type over Gs that are typologically radically different.
If thess “facts” are indeed facts, the conclusion seems obvious: to the degree that we understand FL as that cognitive-neural feature of humans that underlies our capacity to acquire Gs then it is the same across all humans (in the same sense that hearts or kidneys are, (i.e. abstracting from normal variation)) and this implies that it has not evolved despite apparently sufficient time for doing so.
This raises an obvious question: why not? Why does the process of language acquisition not care about typological differences? Or, if typological differences run deep then why have they had no impact on the FLs of people who have lived in distinct linguistic eco-niches?
Here’s one obvious answer: typological differences are irrelevant to FL. However big these differences may seem to linguists, FL sees these typologically “different” languages as all of a piece. In other words, from the point of view of FL, typological variation is just surface fluff.
Same thing, said differently: there is a difference between variation and typology. Variation is a fact, languages appear on the surface to have different properties. Typology is a mid-level theoretical construct. It is the supposition that variation comes in family types, or, that variation is (at least in part) grammatically principled. The argument above does not question the fact of variation. It calls into question whether this variation is in any FL sense principled, whether the mid level construct is FL significant. It argues it isn’t.
Let me put this last point more positively. Variation establishes a puzzle for linguists in that kids acquire Gs that result in different surface features. So, FL plus a learning theory must be able to accommodate variation. However, if the above is on the right track, then this is not because typological cleavages reflect structural fault lines (or G-attractors) in FL or the learning theory. How exactly FL and learning theories yield distinctive Gs is currently unknown. We have good cases studies of how experience can fix different Gs with different surface properties but I think it is fair to say that there is still lots more fundamental work to be done. Nonetheless, even without knowing how this happens, the argument above suggests that it does not happen in virtue of a typologically differentiated FL.
Let me end with one last observation. Say that the above is correct, it seems to me that a likely corollary is that FL has no internal parameters. What I mean is that FL does not determine a finite space of possible Gs, as GB envisioned. Why not?
Well say that acquisition consisted in fixing the values of a finite series of FL internal open parameters. Then why wouldn’t evolution have fixed the FL of speakers of typologically isolated languages so that the relevant typological parameters were no longer open. On the assumption that “closing” such a parameter would yield an acquisition advantage (fixing parameters would reduce the size of the parameter space, so the more fixed parameters the better as this would simplify the acquisition problem), why wouldn’t evolution take advantage of the eco-niche to speed up G acquisition? Thus, why wouldn’t humans be like finches with FLs quickly specializing to their typological eco-niches? Doesn’t this suggest that parameters are not internal properties of FL?
I am pretty sure that readers will find much to disagree with here. That’s great. I think that the line of reasoning above is reasonable and hangs together. Moreover, if correct, I believe that it is important for pretty obvious reasons. But, there are sure to be counter-arguments and other ways of understanding the “facts.” Can’t wait.
 The example provided by Bill Idsardi. Thanks.
 By two “first” languages I intend to signal the fact that this is a different process from second language acquisition. Form the little I know about this process, there is no strcit upper bound on how many first languages one can simultaneously acquire, though I am willing to bet that past 3 or 4 the process gets pretty hairy.
 It also strongly casts doubt on the idea that FL itself is the product of an evolutionary process. If it is, the question becomes why did it stop when it did and not continue after humans separated? Why no apparent changes in the last 100k years?
 Charles Yang has a forthcoming book on this topic (which I heartily recommend) and Jeff Lidz has done some exemplary work showing how to think of FL and learning theory together to deliver integrated accounts of real time language acquisition. I am sure that there is other work of this kind. Feel free to mention them in comments.