About the borrowing hierarchy
The borrowing hierarchy presented here prioritizes which kinds of words are more likely to be borrowed in a language depending on the intensity of contact. So let's start with verbs. It's suggested on page 320 of Markedness and Language Change that the following borrowing hierarchy exists:
content verbs > modal verbs > existential verbsSo what this is saying is that verbs from general vocabulary (content verbs) are likely to be borrowed first before other verbs that express moods such as "can", "would", etc. If contact is intensive enough we should even observe borrowed verbs denoting existence such as "to be". Later on in the same page of this book, it specifies the hierarchy followed by nouns:
content noun > pronounSo again, we expect ordinary nouns borrowed into a language before loaned pronouns find their way into a language. If we see a borrowed pronoun, it can be a sign of rather intensive language contact, as happened in Old English when the language borrowed the Norse pronoun "they". To add to this, I personally suspect that third person pronouns are more likely to be borrowed from one language to the next than first or second person pronouns and I might be tempted to modify the chain to: content noun > 3p pronoun > 1p/2p pronoun. But I digress. That assertion isn't important to the evidence I'm about to shamelessly suggest to demonstrate intensive contacts between an early stage of Proto-Indo-European (PIE) and Proto-Semitic (PSem) around 5500 BCE.
Evidence for intensive contacts between Proto-IE & Proto-Semitic
As already explained with a footnote, the idea that early Proto-IE speakers and Proto-Semitic speakers were in contact with each other isn't controversial, but debate continues on as to how, where and when they were in contact with each other. The neolithic is really the most sensible time for this to have occurred since it gives widening trade as a motive for this cultural and linguistic exchange. The how is clear then: growing trading networks in the Neolithic and exchange of goods. The relations at this time between the Balkans and Syria are reflected firmly in archaeology which show commonalities in ceramics and material culture. However, Neolithic trade only gives us a window between 6000 and 4000 BCE, so as far as I'm concerned, the precise "when" is too obvious to feign confusion. The matter of where this contact occurred continues to be a senseless debate, egged on by sensationalists like Ivanov and Gamkrelidze who attempt desperately to place Indo-European in Anatolia. It doesn't fly. The least controversial hypothesis is that PIE was centered around modernday Ukraine and there's no reason to rebel against this economical view. However, it must be understood that if this contact started a millenium or more before PIE proper, there's no guarantee that earlier stages of the language existed further to the south as I suggested earlier for the stage of PIE I refer to as Mid IE (MIE).
So let's get straight to the chase and start mapping the possible loans from PSem into PIE and see how intensive this contact might have been:
PSem *šáðrawu 'he causes to scatter (caus.)' (root *[ðrw])modal verbs:
=> PIE *streu- 'to scatter' (via MIE *satréwa-)
PSem *yiθ 'there is'content nouns:
=> PIE *ʔes- 'to be' (via MIE *es-)
PSem *báwiʔu 'it is come (stat.)'
=> (?) PIE *bʰeuh₂- 'to appear, to become' (via MIE *béuxa-)
PSem *θáwru 'bull'pronouns:
=> PIE *táuro- (via early Late IE *tä́urə-)
PSem *gádyu 'kid, young goat'
=> PIE *ǵʰáido- 'goat' (via early Late IE *gä́id̰ə-)
PSem *sábʕatum 'seven'
=> PIE *septḿ̥ (via MIE *séptam)
PSem *šu 'him, himself'So I hope you all enjoy these ideas. I'm still stuck on finding an example for "modal verbs" because PIE morphology expressed mood by way of inflection not seperate words but I'll be continuing to adapt my argument in the future. If I goofed up on the Semitic verbal forms, please let me know and correct me because I had to derive them from their root forms and I'm more familiar with IE and Aegean languages than Proto-Semitic which I've sadly neglected far too long.
=> PIE *swe 'oneself' (via MIE *sʷa).
 Hock/Joseph, Language History, Language Change, and Language Relationship: An Introduction to Historical and Comparative Linguistics (1996), p.513 (see link): "Given the evidence of 'bull' and other pastoral/agricultural words in western Indo-European languages that are likely to be borrowed from Semitic or Afro-Asiatic, one may begin to wonder whether the western Indo-European agricultural words for 'field', 'plough', and 'sow, seed' may likewise owe their origin to Semitic or Afro-Asiatic influence. Archeologists find much evidence indicating that many important aspects of European agriculture, perhaps even all of agriculture, spread from the ancient Near East."
(Mar 19 2008) I decided to throw PSem *báwiʔu and PIE *bʰeuh₂- into the list as a point of discussion. The verb 'to become' is considered an existential verb in the cited book, and hence would be more evidence of the strongest kind of language contact. I add a question mark here because I'm undecided as to which of two competing proposals work best. Nostraticists often link PIE *bʰeuh₂- to Uralic *puxi 'tree' with an underlying meaning of 'to grow up from the ground' and hence later 'to appear' and 'to become' in PIE. This is plausible and I must admit that in order to establish a clear link between the PIE and PSem words, the fact that the laryngeals are incongruent (i.e. a glottal stop in PSem but a uvular fricative in PIE) needs to be addressed. Otherwise however there is an alluring semantic and phonetic link between the two that would postdate the etymology that many Nostraticists provide. So chew on that for a while. ;)