After discussing Tibeto-Burman prefixes, and touching upon the *s- prefix in Tibeto-Burman, I sort of ran into the Indo-European s-mobile again.
For those unknown to the s-mobile, it is a unexplained element *s that seems to optionally appear in front of words. There are several words even in English today that form an s-mobile pair. For example melt besides smelt with some differentiation in meaning but not much.
I have been pondering about the origin of this s-mobile for some time now, it is something that will occasionally bug the mind of any Indo-Europeanist, only to leave them confused and dissatisfied without a proper answer. I too, do not have a proper answer, but I have a little theory that I'd like to explore.
Several ideas have been explorer in the past. Some say it is simply an irregular shift of *s/_C > ø. I don't like irregular shifts that are as wide-spread as this shift is, since it is found in every single branch of Indo-European. If there was a shift that never became particularly popular, I'd at least like to see one of the branches that got rid of it completely.
Another explanation that has been explored by some, is that it might be the s-prefix as found in Semitic languages, which often has a causative meaning. This explanation would be very nice if all s-prefixed verbs could be explained as Semitic loanwords. The problem with this theory is though, that the s-mobile seems to appear in front of nouns as well.
As an example we find the Dutch stier 'bull' and Old English stêor OHG stior. In Old Norse we find the s-less form þjôrra. It won't come as a surprise to anyone that this word is related to Lat. taurus and Gr. ταῦρος. This seems to be a very early loan from Semitic. Arabic has ṯawr. It can't have gone in the other direction because Semitic had access to a t so there would be no reason to replace Indo-European *t with *ṯ. It should be noted that this s-mobile appears in Indo-European, but is absolutely impossible to find in Semitic. This fact led me to think that it must have been some sort of productive suffix in Indo-European.
If I were trying to connect Indo-European with Sino-Tibetan I wouldn't have hesitated to say that the PIE *s is the animal prefix in PST *s. But since such a claim would make me look like a maniac, I will not even go into that, but the resemblance is just a funny coincidence which I wanted to mention.
Now then, we have the strange situation of an element *s that can appear both before verbs and nouns. I asked myself what kind of element can appear before verbs and nouns in Indo-European. And then I realised, that, although only very productive in Graeco-Aryan, Indo-European has a lot of prefixes that can be place before verbs and nouns. Elements such as Skt. pra-, su-, a-, apa- etc. which all have direct reflexes in Greek as well.
So what *s is a prefix like this as well? The s-mobile sometimes seems to give a somewhat intensifying meaning to verbs (although I should really once look into the semantics of that, but this is what is often claimed).
I was wondering if maybe, the *s is a strongly reduced form of the prefix *h₁su- which yields su- in Sanskrit and εὐ- in Greek. Semantically it would make sense. smelting is then 'well-melting'. And a stier would be a 'good-bull'. If this reduced form really does come from a reduced form of *h₁su- it would explain why this prefix is not found in any other languages but Graeco-Aryan languages, since the *s would be that form. Why both forms occurs in Graeco-Aryan though, remains unexplained.
Although the idea is pretty nifty, I'm still very hesitant about this hypothesis. Can pretonic *u really reduce to schwa and then disappear completely? Do we have precedent of this? And then there's the laryngeal. Although the reflex of it wont be commonly found in languages, you would expect indirect evidence of lengthened vowels before an s-mobile in Vedic Sanskrit. As far as I am aware, this does not exist, but it is definitely worth looking into. If I can indeed find indirect evidence in Vedic, I'll be a lot more confident about this hypothesis.