While I can experience raw emotion and react accordingly at the basic level of fight or flee … and viscerally can experience grief, sorrow, worry, optimism, joy, and love, without needing to put my emotions into words … I find I cannot do any deep thinking, requiring logical decision-making or reasoned evaluation of information, without mentally using language. Basically I think in English using English words … although interestingly if the problem requires using French language to resolve, I then find I’m automatically thinking in French. But whether in English of French or whatever, I’m always thinking in precise words and expressions, and usually (or so I believe) grammatically correct sentences too. Basically I think in coherent sentences in a recognised language exactly as though I were having a discussion with myself in my head.

So …

When considering our early human ancestors: did language require a level of “intelligence”, before it was possible, or were language and intelligence actually more of a positive feedback loop, with language (which is ostensibly for communication with others) nevertheless enabling more complex internal thoughts and ideas, such as logical reasoning, within the  individual?  And with this way of thinking (internal communication if you like) one would already have ones ideas in a form that was ready (rehearsed?)  for communication with the rest of ones group, no?

The intelligence must have come first; that and the social relationships that made speech so advantageous, after all a mutation that was too advanced would be no advantage and would disappear. And not only a feed back loop, I would think, but strong selective pressure of every kind would lead to rapid (in evolutionary terms) refinements.

This article makes a plausible case for language co-evolving with tool making about 1.75 m. years BP, both activating the same brain areas using  similar neural networks It does make sense since, as you observed, it's near impossible to make any kind of complex analysis or plan without being able to manipulate ideas with the symbolic function of words to organise thoughts.
http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0072693

However, was it necessary for the vocal apparatus to have evolved to near its current state for speech to develop? This review of a paper suggests not.  http://www.babelsdawn.com/babels_dawn/2010/10/the-evolution-of-the-vocal-tract.html

It's pretty well mainstream now that Neanderthals could speak and there have been some recreations but unfortunately the larynx does not survive fossilisation well in the earlier hominims so it's all still conjecture.

Meles meles wrote:

I’m always thinking in precise words and expressions, and usually (or so I believe) grammatically correct sentences too. Basically I think in coherent sentences in a recognised language exactly as though I were having a discussion with myself in my head.

This is key although grammatically correct expression can be hindered when (or if) the data for that expression is faulty to start with. A misunderstanding of vocabulary would be a case in point.  

For instance, I must have been in my 40s before I realised that calamine and camomile were 2 different words. In fact I didn’t even know that calamine was a word. I suppose the reason for this was that the only time I ever came across the word ‘calamine’ was in the context of pink calamine lotion which I remember being applied to my face, shoulders, back, arms and legs after getting sunburned as a young child. At that time, I would have been too young (and sore) to have to been bothered reading the label on the bottle with any precision as to the spelling of the word. I certainly never had to write it down.

Camomile (chamomile), on the other hand, being another name for the daisy and its famous herbal tea, is a word I have come across many times in life. The confusion wasn't helped by the fact that there is also a chamomile lotion for topical use. Further factors leading to the obscurity of calamine as a word (both for me personally and perhaps generally) would be that the prevention of sunburn (i.e. the use of sunblock and/or avoiding the sun) rather than the treating of sunburn, has become the default approach and in my own lifetime too. Furthermore, in the British Isles the use of calamine lotion for the treatment of childhood ailments such as nappy rash, chickenpox and insect bites has greatly diminished over the last 40 years being widely replaced by the use of zinc oxide creams. Added to this is the fact that in geological circles calamine is generally considered an imprecise word for zinc and is now avoided thus making it archaic.

If I was able to go decades, therefore, with such misapprehension, then one wonders how our ancient ancestors (without recourse to reading, writing, dictionaries, radio, television and computers etc) managed to express complex ideas to one another, when their thoughts could have been similarly based on misunderstood vocabulary. I'd imagine that the development of language and intelligence went hand in glove, with each enhancing (and sometimes retarding) the other.