African Languages!

As you noted, English has lost most of the inflections that were rampant in Proto-Indo-European. Their primary use today is for actual conveyance of meaning, such as singular/plural for nouns (dog/dogs, man/men, thesis/theses, etc.) and tense/mode for verbs (wish/wished, come/came, do/done, etc.), and in a few cases gender (waiter/waitress).

Of course we have stubbornly kept a few inflections that mean nothing, such as do/does, am/is/are, etc.

But in general, the English-speaking community has done a pretty good job of discarding inflections, as our language became more positional than inflected. The high point of this effort can be pinpointed as the Norman Invasion, when French became the official language of government, commerce, and scholarship. The French inflection system was so different from the Anglo-Saxon, and the phonetics of English were changing so quickly under the influence of French, that the inflections first became garbled, and finally enormously simplified.

By the time that the French overlords adopted the language of their subjects (a rare but not singular phenomenon), that language was so changed that Beowulf would not have understood it.

There are almost as many grammatical paradigms as there are language families. Chinese, for example, has no inflections at all. If you need to make it clear that your best friend is male or female, you simply say "man friend" or "woman friend," and btw all of those words are monosyllables. If you need to make it clear that you went to school on the bus after breakfast, you simply say, "I eat early meal ride bus attend school." The word order mirrors the order of the events. And yes, if it's important to make clear that this happened in the past, you simply say "yesterday I eat breakfast," or "last week I eat breakfast." Word order is very rigid in Chinese.

I don't know very much about the Afro-Asiatic family, but judging by the Semitic branch (Hebrew, Arabic, Aramaic, Phoenician, etc.) I'd guess that they, too, use distinct words more than inflections.

Japanese, on the other hand, is highly inflected.

Archeological evidence suggests that the technology of spoken language was invented around 70,000 years ago. This is when we first begin to see evidence of complex, coordinated activities involving multiple people, who could not possibly have been using their hands to communicate at the same time.

This implies that spoken language was invented once, and as the diaspora of H. sapiens spread out of Africa to the other continents, the original language simply continued to evolve, both to discuss new things, activities and ideas, and simply by subtle shifts in pronunciation and grammar. -- Not to mention external influence. Languages in tropical latitudes tend to have a lot of vowels, because breathing helps keep the body cool. Whereas in northern latitudes they have more consonants so people can retain their body heat.

And then there's Hawaiian: For several generations it was shouted between canoes over the noise of the waves, so its phonetic structure has been vastly simplified to avoid misunderstanding. Its only consonants are HKLMNPW and the glottal stop. This is why Samoan "salofa" becomes "aloha" in Hawaii.

Unfortunately, language seems to turn over completely in a few thousand years, so it's impossible to trace relationships back any farther. The oldest relationship that has been identified is between the Na-Dene family of western North America (Navajo, Tlingit, etc.) and the Yenisei language of Siberia, which would clock their separation sometime before 10KYA. Unfortunately, there is no complete agreement among linguists on this. There are so few similarities that the probability of this being due to chance is a little too high.

In any case, if all languages do indeed derive from one ancestor, their myriad structural characteristics imply that inflection, as well as everything else, is fluid.