The form of the ablative case suffix in Mongolic is a well isogloss separating all modern Mongolic varieties and all Middle Mongol sources save Muqaddimat al–Adab. Middle Mongol reflects *-(a-)ča, while modern languages (and MA) show *-(a-)sa. Thus, irregular deaffrication of *č in this suffix is taken to be a Common Mongolic innovation.
There are no external comparisons that could hint to a further etymology of the ablative suffix. Khitan has an unrelated ablative in <e(n)d.ii>, homographical with dative+genitive. Sometimes a comparison with the Turkic equative (likely an original prolative) *-ča is made, but the semantics are off. Poppe also claims cognacy to Manchu či, but Jurchen data (Kiyose 309) shows that the original form was ti and the similarity probably fortuitous.
I’m running the risk of this becoming a genetics blog, but another interesting paper came out this week: Bronze Age population dynamics and the rise of dairy pastoralism on the eastern Eurasian steppe.