In my last post, I was noticing the link between Etruscan genitives in "give" constructions which mark the recipient of a gift and clauses conveying "having", as per John's Newman's Give: A cognitive linguistic study (1996). On that note, there are some extraneous connections that come to my mind in other ancient languages I know of.
I've reasoned for a while now that the source of Indo-European's thematic genitives in *-osyo like *h₁éḱwosyo 'of the horse' is quite simple: the athematic genitive *-ós plus endingless relative pronoun *yo-. This construction would have first developed in Pre-IE (specifically Late IE) as *-asya, replacing former accented genitive *-ás, when Acrostatic Regularization risked making the nominative and genitive identical in the thematic paradigm. The addition of *ya (the original endingless form of the relative pronoun used for nominative, locative and inanimate accusative cases) helped disambiguate and reinforce thematic genitives. This resultant construction, instead of conveying the direct but potentially ambiguous phrase "of X", used the circumlocution "(with) which [is] of X".
With Newman's insights, we might even reinterpret "which [is] of X" as "which X [has]" since a lack of "to have" in Proto-Indo-European encourages a speaker to use the verb "to be" plus a genitive noun to express the possessor. The distinct but semantically equivalent phrases we take for granted in English like "the horse's speed", "the speed [which is] of the horse" and "the speed [which] the horse has" all become a little blurry in such languages.
Then I wonder further. I've already noticed that there's no rational motivation to reconstruct a distinct dative case in pre-IE, if not in IE itself. The dative in *-ei must have only later originated from the pre-existing locative ending in *-i and/or from analogy with *h₁ei- 'to go (to)'. So in pre-IE or IE, without an available dative form, what case is left to express the recipient in phrases using the verb *deh₃- 'to give'?
 Francisco Adrados, On the origins of the Indo-European dative-locative singular endings published in Languages and cultures: Studies in honor of Edgar C. Polomé (1988), p.29 (see link).