Now to explain the three isoglosses I have on display above. I've been getting the impression for a while that Indo-Aegean (IAeg) and Altaic-Gilyak (AG) must have remained particularly close after diffusion of the Proto-Steppe community because I can think of at least two sure features that they share with each other that couldn't have been inherited from the parent language. One is the wholesale softening of word-final *-t to *-s as seen in the changes on animate plural marker *-it (n.b. further erosion of word-final *-s causes in turn Proto-Altaic *-r₂) and the other is an occasional correspondence of *a in IAeg and AG with Boreal *u in certain key words. I attribute this curious development to an original mid-central schwa *ə which could sit equally in accented positions as well as unaccented ones.
Upon revisiting these ideas, I've just realized an interesting minimal triplet in Proto-Steppe that serves as a simple but effective argument to justify the necessity of at least four reconstructed vowels at this stage:
The first becomes the source of the Indo-European ablative *-ód and Uralic partitive *-ta. The PIE form originated by agglutinating the postposition to the nominal stem in IAeg (thus *-ata), followed by Penultimate Accent Shift in Old IE which took the fixed accent off the initial (*-áta), then Syncope (*-ád̰) and finally Vowel Shift, yielding PIE *-ód with regular rules. The second and last examples show a vital difference between them since *tu becomes *tʷa (> PIE *twe, n.b. vocalism secondarily affected by *me < Proto-Steppe *mi 'I') while *tə becomes IAeg *ta without labialization of the preceding stop (>PIE *to-). This is explained if there was an unrounded vowel distinct from both low central unrounded *a and high back rounded *u, namely the mid central unrounded schwa which fits so nicely into an otherwise common 3-vowel system. Forms that suggest to some long-range linguists the apparent existence of a proximal demonstrative **ti on the Proto-Steppe level are, I figure, caused by later analogical derivation out of inherited *tə since this proximal demonstrative is only evidenced in Boreal and AG while IAeg seems to preserve only *ta 'that' (> PIE *to- and Aegean *ta) with a distinct proximal counterpart *ka (> PIE *ḱo- and Aegean *ka). I take the IAeg evidence to show an original word *ka 'this' in Proto-Steppe since, if this is not so, the source of the IAeg form would remain much more obscure than that of Boreal and AG's *ti vis-a-vis the securely inherited deictic *tə. The Altaic forms with word-initial sibilant in place of expected *t- are surely caused by pre-Altaic palatalization before high front vowels as has also apparently occurred in its second person pronominal forms.
- *ta 'from'
- *tu 'you (sg.)'
- *tə 'that (near you)'
All these speculative ideas while interesting and worthy of discussion are however, of course, subject to some range of interpretation. Debate remains open.
 See, for example, page 2 of Frederik Kortlandt's article Indo-Uralic and Altaic [pdf].