I think it comes from Arthur Rackham myself, but not so much from the way he depicted Elves (since they look more like the christmas germanic goblin versions) but rather from his faeries and the way he drew his heroines especially. Like I'm sure Tolkien was familiar enough with his illustrations of Sigurd and whatnot, but I mean more like what he did for Undine. I think Tolkien probably saw Undine and said, yep, that's the one! And Galadriel is basically that... heheh
When she was swimming the sundering seas, I definitely thought of it.
Sure it's not showing the ears, and Undine is more of a classic naiad, Thetis of the sea or the river nymph, as opposed to sidhe of the mound style, but I just mean for a kind of general visual take.
For the ears I think that comes into it at first more from the faerie realm that way, or that element of the fairy depictions particularly from late 19th century illustration. Going back a fair ways before that I'd imagine, sure, but that's where I think it cracked off into the modern era again. So take a character like Ariel in the Tempest and the way she's usually drawn, or the way Fuseli painted his nightmare versions, or the pointy ears of various demons from medieval era stuff maybe.
Or of course, the original woodland archetype of Sylvanus (the Roman deity I mean, not the D&D one) Pan/Faunus, often depicted with big pointy animal ears, usually deer-like or donkey-like. Also all the kind of Christmas elves, and dwarves and gnomes with that look from out the Germanic/Norse traditions. Cause what we get in D&D is basically that crossed with the tautha de danann concept or the Tolkien style elves right? Filtered through like a hundred years of stylistic choices made by American illustrators like Frazetta too, since he helped cook up that first roto-toon. I mean I suppose it's gotta be at least as much dudes like that, as it is callbacks to Rackham and the celtic revival era stuff, or nibelungenlied ring cycle stuff that prob caught Tolkien's eye from the generation before him.
I also wouldn't underestimate the influence of Star Trek, and the vulcan space Elf-look there. I think the convention is almost certainly way more pulp illustration than anything else though.
Curiously in the days of BG1, I think there was perhaps a push to keep the ears hidden under the locks, and make them look more or less human, just so the limited number of portraits there could be more adaptive. So like Jaheira's portrait in BG1 say, which could work both ways, provided you buy in on the 'Elves lookin' kinda Human' kick. I think the traditionalist look of Alan Lee's artwork is a big influence on the current version. I mean for as traditional as going back to the late 70s is gonna get ya hehe. "Faeries" came out what like 78 or 79? I think you'd be hard pressed to find anything in popular illustration going back much earlier that would totally accord with the more familiar D&D depictions of the 1980s, unless you're looking at Frazetta doing Conan stuff in like late 1960s I guess. Though there you'd be a lot more likely to catch someone just not wearing pants, not necessarily with the pointed ears hehe.
I think for a shape of the ears the Elfshot elf-arrow/elf-bolt is another possible take. Since that's pretty much the shape of their ears right?
Here's a bitsy bit from Etymonline, just on the word...
elf (n.) "one of a race of powerful supernatural beings in Germanic folklore," Old English elf (Mercian, Kentish), Ã¦lf (Northumbrian), ylfe (plural, West Saxon) "sprite, fairy, goblin, incubus," from Proto-Germanic *albiz (source also of Old Saxon alf, Old Norse alfr, German alp "evil spirit, goblin, incubus"), origin unknown; according to Watkins, possibly from PIE *albho- "white." Used figuratively for "mischievous person" from 1550s.
In addition to elf/Ã¦lf (masc.), Old English had parallel form *elfen (fem.), the plural of which was *elfenna, -elfen, from Proto-Germanic *albinjo-. Both words survived into Middle English and were active there, the former as elf (with the vowel of the plural), plural elves, the latter as elven, West Midlands dialect alven (plural elvene).
The Germanic elf originally was dwarfish and malicious (compare elf-lock "knot in hair," Old English Ã¦lfadl "nightmare," Ã¦lfsogoÃ°a "hiccup," thought to be caused by elves); in the Middle Ages they were confused to some degree with faeries; the more noble version begins with Spenser. Nonetheless a popular component in Anglo-Saxon names, many of which survive as modern given names and surnames, such as Ã†lfrÃ¦d "Elf-counsel" (Alfred), Ã†lfwine "Elf-friend" (Alvin), Ã†lfric "Elf-ruler" (Eldridge), also women's names such as Ã†lfflÃ¦d "Elf-beauty." Elf Lock hair tangled, especially by Queen Mab, "which it was not fortunate to disentangle" [according to Robert Nares' glossary of Shakespeare] is from 1592.
ps. oh also, the Lang's Fairy Books, that colored volume Fairy Tale series from the turn of the last century with Grey, Green etc books. Have to mention those!!! The illustrations by HJ Ford, W Heath Robinson and Charles Robinson etc... . There are some definite proto glimpses in there for sure. Some of Howard Pyle's and NC Wyeth's Arthurian era stuff as well probably, though I'm struggling to find any ears there, they're invariably on the little mischievous or devilish creatures, the sprites and the imps and the goblins more than the fey humans. Dulac's stuff has it as well on occasion, for examples of what was en vogue pre Tolkien. Just for some stuff that was sort of percolating around at that time, which had the vibe and probably fed into it. Especially as the faeries become kinda more sexualized and thus human-ized in that way. Basically when you get Nixies that start looking more like this...
rather than the swamp thing hehe.
But then for the ears those are really more like what the Witch has going on in this one.
Like late 1880s to that first decade of the 20th century, in the popular fairy tale tomes of that era. I don't think it starts getting more explicitly what we think of now as Elves till the 1970s though, with that little nouveau fantasy revival that happened then, while D&D was being born.