how did the four main Romance languages develop?

For starters, I'm not sure how you ranked these as the "main Romance languages." By number of native speakers, this is how Wikipedia ranks them:

• Spanish, 330 million (tied with English as the world's #2 language, after #1 Mandarin)
• Portuguese, 178 million, #7 (Hindustani, Arabic and Bengali outrank it)
• French, 68 million, #16
• Italian, 62 million, #20
• Romanian, 24 million, #42
• Catalan, 7 million
• Occitan, 2 million (a dialect continuum that extends from Spain to Italy and includes Provençal)

There are other speech variants within the Romance group, with large numbers of native speakers, which linguists generally define as distinct languages, but may not be treated as such politically and culturally. Examples include Romansh, Sicilian, Galician and Sardinian. They do not qualify as dialects because they are not readily understood by outsiders, even though the native speakers almost invariably speak and understand the dominant language of the larger region because they are required to learn it in school. This is similar to Irish Gaelic, which is clearly not a dialect of English; few Britons can understand it, but there probably isn't one Irish person who isn't fluent in English.

As for how the Romance languages evolved, this happens in every language group. There was once an Indo-European tribe in Scandinavia that spoke a single language, Proto-Germanic. After some of the more adventurous members migrated to the main part of the continent and then headed out in opposite directions, their languages diverged into Old German and Gothic. Old German separated into Anglo-Saxon, Frankish, Dutch, Frisian, and German, while Old Norse separated into Danish, Swedish, Norwegian and Icelandic; Gothic began to evolve but then died out. (This is vastly oversimplified.)

The same thing happened to Latin. At the zenith of the Roman Empire, there was so much formal and informal contact between Rome and the outlying provinces, that the uniformity of the distant dialects was constantly reinforced by fresh communication from the hometown. Language follows the coin, and to a lesser extent the flag, so the influence of Imperial commerce and the Imperial government kept the citizens in the far corners of the Empire bound into a network marked by constant communication.

The same thing is happening in reverse now in many dialect continua. With electronic media propagating news, scholarship and entertainment instantaneously across national boundaries, with live newspeople, scholars and entertainers following only a bit more slowly on jet planes, the differences among American, British, Australian, South African and Indian English, which in my own childhood were great impediments to understanding, are shrinking rapidly. The same is true of Spanish in Latin America.

Education had little to do with the divergence of Latin dialects, since it barely existed in those days. Only a minute fraction of the population could read and write, so for everyone else "schooling" was rudimentary and only available to the wealthy. As the Roman government collapsed and the Legionnaires either found their way home or assimilated into the local populations, the Imperial commerce network crashed with it and there was an awful centuries-long economic depression.

Without contact with people from other regions, dialects naturally diverge due to a variety of forces.

• The substrate of a colonized region's original language, e.g. the influence of the Germanic Frankish language on French. This is why it has umlauted vowels, the gargled German R, and a preference for the present perfect (I have gone) over the preterit (I went).
• A superstrate of the language of a group who migrates in after the colonizing power is gone, e.g. the Slavic influence on Romanian. This is why it still retains the Latin declension of nouns in five cases (alone among the Romance languages), has the Slavic vowels î and ă, and borrows Slavic words like do for "to" and şi for "and."
• The advance of civilization. As life becomes more complex and entire new fields of occupation and interest arise, linguistic paradigms left over from the Stone Age become a hindrance to communication. Latin, like Proto-Germanic and all early Indo-European languages, retained much of the cumbersome Proto-Indo-European grammatical structure, which became increasingly out of step with the communication needs of its speakers. You seem to have some knowledge of Latin, so surely you and your fellow scholars have experimented with trying to carry on a 21st-century conversation in the language. It can be done (it's the official language of Vatican City) but it's a big pain! The resources of the language in which we think can't help but shape our thoughts, and an ancient language is more of a hindrance than a resource for the shaping of modern thoughts.
• Spurious changes. Slang, jargon, unconscious modifications, and just plain silliness can take a language in new directions. C before I and E was hard (pronounced K) in Classical Latin. But it softened over the centuries, and not in a single way. In Italian and Romanian it became the English CH sound, in French and Portuguese it became S, and in Spanish it became the English TH sound. Linguists diligently try to explain these shifts because it's their job, but when it all comes down, it just happened.
• These changes may seem like "simplifications" to us, looking back over 2,000 years, because the intricacies of Latin grammar seem superfluous. But the languages have become more complex in other ways that we don't notice, because they have adapted to the complexities of modern life, just as English has. Look at the new grammatical construct that arose in the 20th century, the noun-adjective compound, such as user-friendly or fuel-efficient. The Romance languages have similar new grammatical features.
• I suggest a different point of view. Why haven't languages like German, Greek and Russian undergone this kind of simplification? They are hamstrung by Stone Age grammatical paradigms. Their verb conjugations and noun declensions are daunting to foreign students, and get in the way of efficient communication. German syntax, in particular, with its Schachtelsätzen (nested clauses) is outrageous.
• As I have often noted, the Chinese, with their unbroken 4,000-year continuity of civilization, have simplified their language to the point that it is a magnificient communication tool. It has no inflections for gender, tense, case, number, no articles, prepositions or other "noise words." Every word in a Chinese sentence carries an important portion of the meaning. As a result, Chinese sentences are shorter than ours and the language can be spoken more slowly, making it easier to understand.

 

I'll let you in on a big secret if you promise not to tell anybody. Often when I am asked questions like these, I just look up the answer on Wikipedia.

I can't find enough information on Lorrain to decide whether I personally would call it a distinct language or a dialect of French. As I noted earlier, two variants of speech are dialects if the speakers can understand each other, with perhaps only a little effort of getting used to the slight differences. Otherwise they are languages. Of course this rule is not as easy to apply as it sounds.

• People in adjacent communities with closely related languages may have so much interaction with each other that they learn each other's language without realizing it. This is what happened to the Czechs and the Slovaks during the seven or eight decades in which they were combined into a single country. Also, during the existence of the USSR, the Estonians were fed up with watching Russian television so they tuned in the TV stations from across the sea in Finland. Eventually they all learned to understand Finnish, even though the Finns had no such experience and therefore cannot easily understand Estonian.
• There is the phenomenon of a language continuum. Say you have City A, City B, City C... City Z all in a row. The people in City A and City B can understand each other, the people in City B and City C can understand each other... and so forth... the people in City Y and City Z can understand each other. But the people in City A and City Z cannot understand each other. Sometimes we call this one language, sometimes we don't. Dutch and German, for example, form a language continuum: the people who live on both sides of the border can understand each other, but the people in Berlin and Amsterdam cannot.

Anyway, the Wikipedia article says that Lorrain was influenced by the dialects of German spoken in nearby Luxemburg. So it's possible that the average Frenchman could not understand it.

The Hutterites are an Anabaptist sect of Christianity, like the Amish and Mennonites. They originated in Austria in the 16th century, and then migrated to Moravia, where Czech is spoken. They eventually came to America about the same time that the Amish and Mennonites did, and for the same reason: for all of our country's faults, it was an oasis of religious freedom compared to most of the rest of the world. So they had a long time for their dialect to diverge from Standard German.

Although actually what usually happens in cases like this is that the language of the original, larger community evolves, while the smaller, expatriate community is more conservative. I would imagine that Hutterite German is pretty similar to Amish German, very old-fashioned, with a few quirks that evolved over here.

As I have mentioned before, my grandparents were part of a wave of Bohemian immigration (we call them "Czech" today because it's easier to spell and pronounce) in the late 19th century. Eventually there were so many of them in the Midwest that they had their own newspapers, and they even made a few movies. They proudly sent a couple of the best ones back to the old country... and the people there couldn't understand the dialog. It sounded like their ancient ancestors. They had to dub it into 20th century Czech.

 

When I went to the post office in Târgu Mureş (it was spelled Tîrgu in those days) to buy stamps, I asked the clerk if the stamps he showed me were enough postage for zeci grama, ten grams. He said, Do zeci grama, "to ten grams."

It's also Slavic. It's the sound of the Russian letter Ы

I have only seen the origin of cal given as the Latin word caballus, the root of French cheval and English cavalry. However, I always thought it odd that in Romanian it would have lost the B. (And this is not from Wikipedia, I learned that fifty years ago before there were such things as personal computers and the internet.)

And it's also Slavic. It's the sound of the Bulgarian letter Ъ, which is the "U" in their own spelling of their country's name, България. And it's also the pronunciation of unstressed A and O in Russian.

 

I bought a BMW motorcycle at the factory in München and spent eleven weeks riding it around Europe with my Chinese girlfriend. I wanted to visit as many of my Esperanto pen-pals as possible. Since Esperanto was (and probably still is) more widely spoken in Eastern Europe, that's where most of them lived. We went through Czechoslovakia, Hungary, Romania, Bulgaria and Yugoslavia (although two of those countries don't exist any more) plus several Western countries.

Sumer was, by many measures, the first great civilization. So naturally it underwent conquest and reconquest. Various peoples, including several Semitic tribes such as the Akkadians, moved in and out, turning the place into an American-style Melting Pot. Eventually the soil turned salty (as often befalls farmland in a dry climate) so it was no longer an agricultural powerhouse, and nearby tribes became more prominent. Akkadian replaced Sumerian as the dominant language.

It's not clear that the Sumerians actually went anywhere, but rather may simply have been absorbed ignominiously into the Melting Pot after losing their leadership status. There's a certain poetic justice in this, since we don't know where they came from either.

The Dacians and the Thracians were both Indo-European tribes. What little genetic evidence we have from the Sumerians does not look like Indo-European DNA. Their language was most definitely not Indo-European; no relation has been found to any known language family.

But the original Roman name was Florentia. Just as Londinium became "London" in English, "Londres" in French and "Londra" in Italian, the name of Florentia underwent the phonetic evolution of each language in which it was spoken. It's "Florence" in French and we adopted their name, just as we have for many European place names such as "Prague" for "Praha" and "Rome" for "Roma."

 

This is an urban legend. King Pedro lived two hundred years before the phoneme developed--and it's not even certain that he lisped. Furthermore, even without that historical knowledge, surely you've wondered why only Z and soft C became TH, whereas S survived intact? Most urban legends survive only because people don't apply rudimentary logical analysis to them!

Medieval Spanish had the largest array of sibilants of all the Romance languages: (in English transcription) S, Z, SH, ZH (as in "leisure"), CH, J, TS and DZ. As the paradigm merged and simplified, one of the sounds just took a wrong turn and ended up as English TH.

There is a certain symmetry in this. Alone among the Romance languages, Spanish has a set of dental consonants. N and T are not pronounced with the tongue against the roof of the mouth as in English, but rather as in Russian and most (all?) Slavic languages, with the tongue either touching the back of the upper teeth or else sticking out between the two rows of teeth. D is also pronounced in this position, either as a stop (cuando) or as a fricative (lado, which has the sound of English "lather"). So for Z to join the ranks of these dental consonants as a voiceless fricative is hardly remarkable.

Not just the vocabulary--in fact not even primarily the vocabulary. It's the grammar and syntax that have undergone extensive change.

The most notable of these simplifications has been the leveling of noun declensions. Latin declined nouns by adding an astounding array of suffixes in five cases: nominative, genitive, dative, accusative and vocative. And there were separate sets for singular and plural and for masculine, feminine and neuter genders, and just as an extra bonus there were multiple patterns so you couldn't automatically guess how to decline an unfamiliar noun. This archaic structure has been leveled in all of the Romance languages except Romanian.

This grammatical complexity facilitated a syntactic freedom that the Romans took advantage of. Scriptor devorabat canem, canem devorabat scriptor, devorabat canem scriptor, etc. No matter how you arrange the words in this sentence, the case endings on the nouns always make it clear that it was the journalist who ate the dog and not vice versa. Apparently the modern French, Spaniards, Italians, etc., don't feel a great yearning for the freedom to turn their sentences inside-out, so they're happy to standardize on a subject-verb-object syntax just like ours. (Yoda would have been a great orator in Latin.)

Other examples include the loss of the neuter gender. Except for Romanian (again), the modern Romance languages only have masculine and feminine. Verbs, while retaining the Latin conjugation system that annoys us anglophones, have nonetheless been greatly simplified. No more pluperfect, for example.

At the same time, the Romance languages have acquired new characteristics to accommodate the advance of civilization since the Iron Age. One of these, frankly, amazes me: the creation of definite articles. Speakers of the Indo-European and Semitic languages seem to feel some sort of psychological pressure to invent a word for "the." It's not universal in Indo-European; most of the Slavic languages don't have it and I have no idea whether it exists in the Indo-Iranian branch, Albanian, etc. But it has been added to the Germanic and Romance languages, and the Greeks already had it in ancient times. I call it a "noise word" because I think it adds just about zero meaning to a sentence. Listen to a Russian talking without it because he can't figure out where it goes; it sounds strange but causes no misunderstanding. There's something about modern civilization that makes people feel like a definite article is a handy thing.

As I have said before, you should most definitely learn Latin before you start proselytizing for its renaissance. Then go spend a couple of weeks in Vatican City where you can immerse yourself in it and converse in it 24/7. Then come back and tell us what a great language it would be for the 21st century!