I'm influenced easily by commenters. I'm weak. No, it's all good because I've been recently influenced in a good way :) The other day, someone commented on Proto-Indo-European (PIE) phonology, regarding something that might have occured several millenia back in Pre-IE. Since I just finished a rant on Nostratic, I may as well add more to this conjectural fun.
One interesting thing that many have noticed is the few vowels that PIE contains compared to the number of consonants. PIE may have five vowels, however *i and *u can be shown to be merely vocalic reflexes of consonants *y and *w. For example, we can see that the zerograde forms *bʰuh₂- and *likʷ- of roots *bʰeuh₂- "to become" and *leikʷ- "to leave" behave in the same way as words with zerograde roots with resonant consonants like *bʰr̥- and *gʰʷn̥- from *bʰer- "to carry" and *gʰʷen- "to kill". To add, *a is so rare for some reason that some feel it doesn't exist at all (although I think this is an overly extreme position). Only *e and *o are productive in ablaut patterns and as a result it's suspected that it all comes from a centralized vowel system of *ə and *a. My view is that there was indeed an earlier system like this and that *a is rare because most instances of former *a had shifted to *o at a very late date in PIE's history, leaving behind a residue of instances of *a in words like *daḱru "teardrop" and *ǵʰans- "goose". These rare words would serve as witnesses to the earlier stage of the language.
However, if PIE originally did have a centralized vowel system, it implies all sorts of other wonderful things. Such a system, using only a few vowels and many consonants, is also typical of all Abkhaz-Adyghe languages nearby. The Abkhaz-Adyghe (AbAd) group is also known as "North-West Caucasian" or "NWC" because its languages center around the north-west region of the Caucasus Mountains which are flanked by Russia and eastern Turkey. We know that these languages have been there for uncountable ages. If the Nostraticist Allan Bomhard is correct in suggesting that AbAd and a very early form of IE were in contact with each other, is it also possible that their vowel systems evolved together as well?
The interesting thing about the phonological patterns that PIE and AbAd exhibit is that they could have easily evolved from a system with more vowels and less consonants by way of a common process of assimilation. In AbAd, palatalization (the act of pronouncing something with an added "y"-like quality) and labialization (the act of pronouncing something with an added "w"-like quality) are standard features in their sound systems. In PIE, only labialization is a phonemic feature of the language. If we pronounce the sound /u/ (as in "ooooh"), we should notice that we're rounding our lips (i.e. labialization). This can be thought of as a quality that isn't necessarily loyal to /u/ but can also be transferred to neighbouring consonants in a word. So if you say /gu/ (as in the word "googoo"), when do you start rounding your lips? Chances are, you start rounding your lips while you're pronouncing /g/. This might be called anticipatory labialization because you're anticipating the next sound, so subconsciously you begin to round your lips ahead of schedule. This sort of anticipation is a universal tendency for speakers around the world to do.
So what does anticipation have to do with pre-PIE and pre-AbAd sound systems? When you say /gu/, it's practically unavoidable that you're really saying [gʷu] and this is how easily a plain stop can become a labialized stop. Similarly, a sound like /gi/ can easily become /gʲi/ through the same process. This process alone doesn't make a new phoneme however since the English sounds [g] and [gʷ] are still just allophones of a single phoneme /g/. They are said to be non-distinctive or non-contrastive because there is no contrast between /gu/ and /gʷu/ in the language, and the accompanied lip-rounding of /g/ is predictable when neighbouring the vowel /u/ in English.
Nonetheless, assimilation is no doubt the first step in creating the rich consonant systems found in PIE and AbAd. The second step involves the reduction of all previous vowels to a mere handful. Once this happens, labialization and palatalization can automatically become phonemic because the previous triggers for the assimilation (i.e. the original qualities of the neighbouring vowels) become erased due to vowel mergers. As a simple hypothetical example, if a language began with a 3-vowel system (/a/, /i/ and/u/, let's say) and all the vowels were suddenly reduced to /ə/, then we might automatically have a contrast between /gʷə/, /gʲə/ and /gə/ (from earlier /gu/, /gi/ and /ga/). Suddenly we have three consonants (u-coloured /gʷ/, i-coloured /gʲ/ and plain /g/) where there was formerly only one (plain /g/). Since we'd have a contrastive triplet, we could then say validly that the three sounds are contrastive and thus phonemic. This is probably how the sound systems of these two language groups evolved.
Knowing that and considering the close geographical proximity of Proto-IE and Proto-AbAd to each other, it's then tempting to suspect that they evolved together this way because of areal influence. Their similarly evolved sound systems may provide some proof that they were in contact with each other at an early date. For me, I think that this is a serious possibility but it would have to have occured long before PIE itself which is said to be dated to about 4000 BCE. So I would suggest a date of 8000 BCE or so when the reduction of the vowel systems of both proto-languages would have begun to occur. (There's still more to talk about on this subject alone. So many topics so little time! Ugh.)
Others may not be convinced of any of this or my suggested date, and that's fine. This is conjecture afterall. Much more needs to be determined before any of this can be stated with certainty but it's always healthy to ponder every once in a while beyond what we currently know in order to encourage new avenues of research and exciting discoveries in comparative linguistics.
 The idea that the alternation of *e and *o in Proto-Indo-European ablaut (e.g. *pódm̥ "foot" [acc.sg.] / *pedós "of the foot" [gen.sg.]) comes from an earlier ablaut of *ə and *a was proposed as early as 1965 by Pullyblank. See Bomhard/Kerns, The Nostratic Macrofamily: A Study in Distant Linguistic Relationship (1994), p.76.
 Read Reinterpreting the Proto-Indo-European velar series to understand why I think PIE in fact doesn't have palatalized phonemes and how PIE's antiquated orthography can be deceiving.