Weren't the Chimer just an offshoot of the Altmer? The only surviving full Chimer that you can see in the games is Almalexia and yeah she looks just like your average High Elf/Altmer. Golden skin/eyes, tall af.
But yeah it's p cool how the story writers just took your common high fantasy races (elves, drow, orcs, dwarves, etc) and turned them into these 'mer' peoples, each with their own history and evolution. There's a bit too many of them though: Altmer, Dunmer, Chimer, Bosmer, Dwemer, Orsimer, Falmer, etc.
So, deviation of the Mer. At the beginning of time, there was the Aldmer or the first ones. They were the direct descendents of the Ehlnofey, or Earth Bones. Who in turn were descendants of the Aedra. There were 2 varieties of Ehlnofey, Wandering and Old. They also were space aliens. Basically their planet exploded at somepoint and dumped them here, some one at a time, some on a large chunk of their planet they then fortified. Eventually, the wandering ones found the old ones, and they went to war, because the old ones had more power due to remnants of their "planet" or plane, but the wandering ones had more strength and numbers, due to fucking like rabbits out in the wild and struggling to survive. The wandering ones came to be the ancestors of Man, having traveled far and wide, and spread their bones and roots everywhere. Where the Old ones came to be the Aldmer and the mer, cloistered and secure in lands they shaped for themselves. The war that these Ehlnofey took part in carved the continents as they are out. In relative order of importance in modern days, Tamriel, Akavir, Pyandonea, Atmora, and Yokuda.
So having said all that, we'll focus on the old Ehlnofey. They eventually became Aldmer, these vanished in the Merethic era. Also called the Mythic Era, or the Age of Elves by the Nords. Because also at this point, the beast peoples were elves, Betmer. Dunno where they came from. But I'd imagine it's because the Nedes, and Nords tend to call anything not like them Mer. During the Merethic period, Nords arrived, Nedes started to prosper, and in High Rock, there was a tower redescovered and Nedes breeding with Mer, and creating half-breeds eventually known as the Breton. A lot of focus was on these towers Summerset Isle had one, High Rock had one(Adamantine), the Aylieds had one(White Gold.) So the Altmer started to form, staying in Summerset Isle, and doing their best to stay in the form of Chrysalis by the Convention. Essentially the Merethic is where a lot of the REHEHEHEALLY weird stuff happened in myth. But most importantly the devision of Nedes, Nords, Men, and Mer started to form. Because of cultural divisions and the whole changing of races to suit their environment that happens in Tamriel. There was space travel during the Merethic. People dove into the Aetherius, and returned with magical materials that made no sense.
I said all that so you can understand that it's not just a willy nilly we're going to make fantasy races into weird new things. But a rather distinct history, of a world.
Alright onwards to why the elves actually started to deviate. Religion, was involved. See, the Aylied as they died out left a power gap. Which was eventually taken up by the Nords who evolved into Imperials. And as they lived the Aylied blocked a lot of people from traveling through their lands. So them being Cyrodiil meant that for awhile, these places were seperate. The Aldmer having split off and started to form clans even before the Aylieds and the *sighs* Dragons, started cutting off trade. The Dwemer began to appear, they had been a reclusive clan of Aldmer, who started messing with the Aetherius and the remains of the Ehlnofey very early on. And they set up in the Velothi mountains which separate Skyrim and Morrowind in modern days. Forming Freeholds and underground cities and generally doing a LOT of science. The chimer, were a very long lived clan of Aldmer, who had followed their Prophet, Veloth, out of the Aldmeri homelands where they had been since the time of Old Ehlnofey. Trinimac, and his own followers, a warrior cult, attempted to halt the Chimer, and so a god tricked Trinimac, a hero of the Aldmer, and a dogmatic one at that, into it's mouth, and essentially inverted his soul, turning him inside out.
Reality is mutable in TES lore. So when that happened to Trinimac, and when the Daedra, Boethiah, and Mephala, began teaching the Tri-Angled Truth. Forming the Psijic Endeavor, and how to reach Chim to the Chimer and Veloths followers, They did this through Trinimacs voice. Leading the people who followed Trinimac to become Pariah. For their Heretical teachings they were branded with the name Orsimer. Or literally the Pariah folk. Eventually, Saint Veloth, led his people to Resdaynia, or Morrowind. Where they settled and started to follow a practice of worshipping both Daedra, Aedra and the other Ancestors. These Chimer, were still, Aldmer, up until the splitting of the heart and the betrayal cast them as Dunmer.
TL;dr: When they talk about Dwemer, Altmer, Chimer, Bosmer and the Proto-orsimer. They are talking about clans of Aldmeri who split off into distinct units and began to settle differing areas. All descendants of the Old Ehlnofey. The only reason they became so distinct were because the mutability of Mundas allowed them to adapt to their environments, or interfering gods, Like Azura, or Boethiah changed them. They only became completely distinct when they began calling themselves something different from Aldmer, and started calling themselves such.
