When did the Spanish language lose the long vowel sounds that Latin had?

Question

It is known that the Latin language had both long and short vowel sounds. Nonetheless, the Spanish language does not have this differentiation and only has short vowels... or at least it seems so. When I was studying Japanese and learning how to transcribe Spanish words into Japanese, the teacher told us that it was OK to simulate the stressed vowels with a long vowel in Japanese. So, words such as "Sevilla" could be transcribed with a long i to simulate the stress in that vowel. Now I wonder if we really utter the stressed vowels a bit longer than the non-stressed ones.

But the real question is: in the evolution from Latin language to the current Spanish language, when did we lose the long vowels? Were the long vowels lost along the evolution of Vulgar Latin (as it seems from the Wikipedia article) or did they somehow reach the Old Spanish language?

Bonus: are there any reminiscences of the long vowel sounds in the current Spanish language?

Answer 1

There are indeed short and long vowel sounds in Spanish, but they are not phonemic, i. e. they are not contrastive. Vowels tend to become longer when stressed; this is very common across languages. Wikipedia says "stressed syllables can be up to 50% longer in duration than non-stressed syllables" in Spanish.

Since Japanese has a vowel quantity contrast but does not have phonemic stress, using one to suggest the other sounds right: it amounts, in fact, to forcing the same correlation between length and stress that is found in Spanish.

The process whereby vowel quantity was lost in Vulgar Latin must have taken quite a long time, but as far as I know Old Spanish already had no trace of vowel quantity. Wikipedia says "Length confusions seem to have begun in unstressed vowels, but they were soon generalized." This is to be expected, since unstressed syllables are usually most prone to loss of phonemic distinctions (neutralization).

In Classical Latin, actual length was not the only difference between so-called long and short vowels: as it happens in many other languages, long vowels were also tense and short vowels were also lax. Mistakes noted by grammarians show that this was so: people tended to mix up short e and i and short o and u (long and short a could not be distinguished by tenseness; /a/ lost its quantity contrast when the rest of the vowels did).

In the end the tense-lax contrast won the game and people started distinguishing vowels only by tenseness and only in stressed syllables. Later the system was simplified even more by conflating (in some branches of Romance) lax i with tense e and lax u with tense o.

The above is how Italian and Portuguese got their seven vowels. Spanish went further by diphthongizing the lax vowels in stressed syllables (/e/→/ie/, /o/→/ue/) and merging them with the tense ones elsewhere, ending up with the current five.

Right now phonemic vowel length exists in a few Romance languages, but in every case it is a new development.