We draw language trees as a convention, to help us see relationships. Yet these can lead us astray, allowing us to imagine neat parallel descents, though such tidiness is rare. They are fun, I admit. But the idea that languages split smuggles in the idea that one group went east and one west, and never the twain shall meet. At larger scales we can call it that, because they will be in separate territories. But that isn't quite what happened.
Imagine on a much smaller scale a tribe of a few thousand, speaking mutually intelligibly across many clans in an area. If they interact with other tribes for purposes of trade, that will not distribute equally across the clans. Some will interact a great deal with a tribe whose language is closely related, while on the other ends of the extent there will be clans interacting with those whose language is from another family entirely. Some of the clans will have no outside interaction at all.
Fast forward a hundred years, and the boundaries will be different. Some clans will have moved off, willingly or no. Others will have been wiped out. Others will have become more important, controlling more resources. Let us say that one of the clans becomes very dominant, and its version of the language grows in prestige, though each clan keeps some distinctives. The bordering tribe of unrelated language conquers the whole territory, wiping out some sub-dialects entirely and changing the dominant version considerably, though not eliminating it. Yet two hundred years later, some descendant of the tribe on the other side of the range, the one with the related language, reconquers the territory. Their language is very intelligible to those clans that have had more contact, who over time may even adopt the "new" tongue. Those on the far end of the range get by with more difficulty understanding the new regime, though they can still understand the other clans from their own tribe. For now.
One of the clans that moved off has become very powerful and comes back; protracted wars between those powers with similar languages force some clans up into the hills, where they isolate. Eventually one side prevails and goes into centuries of expansion, sometimes wiping out languages, sometimes being content to simply exact tribute from a people while letting them keep their language, sometimes being influenced by the speech of those they conquer. Groups split off and go further afield, some losing all contact with the central language area, others keeping up trade relations for centuries.
What do we say has happened to the original shared tribal language? Does it still exist at all? If you tried to diagram it, you could get only an approximation of what has occurred.