A comparative that now stands on its own
In present-day English, later is the ordinary comparative of late: a later train, a later date, arriving later than planned. As an adverb it also works without an explicit comparison, meaning afterward or at some unspecified future time. In informal American speech it can even serve as a goodbye, clipped from see you later. None of those uses is mysterious once you know that the word is simply late plus the comparative ending. The history is older, though, and the first senses were about slowness rather than the clock.
The origin of the word later is not a borrowing from French or Latin. It is native English, built on a Germanic adjective whose oldest meaning was sluggish rather than subsequent.
Inherited comparatives, then a modern-looking form
The adverb continues Middle English later and latere, themselves from Old English lator. The adjective continues the same Middle English forms from Old English lætra. Both were already comparatives of late, Old English læt. The Middle English Dictionary records a cluster of spellings — latre, lattre, lattere, leter, leatere — and notes that forms with a double t usually kept a short vowel. That variation is the seed of the later split between later and latter.
How far back the modern spelling should be counted is one of the few real disagreements among the handbooks. Dictionaries that work from Old and Middle English treat the word as an inherited comparative already in use before the Norman Conquest, with the time sense belonging to a subsequent period and afterward attested around 1200. The Oxford English Dictionary, looking at the modern lemma, describes it as probably formed inside English from late plus the suffix -er, and cites early-sixteenth-century evidence, a 1527 translation by Laurence Andrewe, as its earliest quotation for that entry. Another account holds that the present word is in part a fifteenth-century remaking, shaped after the pronunciation of latter had already gone its own way. The safest picture is that English has had a comparative of late since Old English, that the afterward sense is firmly medieval, and that the present form may have been reinforced by ordinary comparative formation as latter peeled off.
Independent dating of the adverb as subsequently or afterward in the thirteenth century lines up with the Middle English record rather than with a sixteenth-century coinage.
From sluggish to after the expected time
The parent adjective late is native Germanic. Old English læt meant first of all slow, sluggish, slack, or negligent, and only then coming after the usual or expected time. That is the same semantic step still felt in a latecomer: not merely subsequent, but behind schedule. The Old English adverb late meant slowly.
Behind læt stands Proto-Germanic lataz, reconstructed with the senses slow, sluggish, lazy, and late. Close relatives include Old Norse latr, Dutch laat, German lass in the sense idle or weary, and Gothic lats. English later itself has near matches in Dutch later, West Frisian letter, Saterland Frisian leeter, and German Low German later. Those West Germanic comparatives show that adding a comparative ending to this adjective is an old family habit, not a modern English trick.
Further back, sources do not all write the same Indo-European formula. One common reconstruction is a root meaning slow or weary, itself an extension of a root meaning to let go or slacken. That family is also used to explain English let and, by a different route, Latin lassus (weary), a Greek verb meaning to be weary, and Lithuanian lėnas (slow, quiet). Another reconstruction starts from a root meaning to be tired, still with Latin lassus as a cognate. The disagreement is about the exact formula, not about the Germanic word’s meaning: slowness and weariness, not calendar time.
Morphology is straightforward. The English comparative suffix -er is the same ending that built older, longer, and earlier. In Old English the adverbial comparative was lator and the adjectival comparative lætra. Later speakers kept adding -er to late as the old inflected comparatives leveled out. That is why the word looks so regular today even though its ancestry is older than the regular pattern that now produces it.
When later and latter diverged
The time sense is the one that survived as the main meaning. Around 1200, later could already mean belonging to a subsequent period and, as an adverb, at a subsequent time. Middle English also kept the older more slowly or more tardily, and even more rarely. Those sluggish senses faded as late itself shifted from slow toward not on time and near the end of a stretch.
A second comparative, latter, is essentially a Middle English variant of the same word, with an unlengthened vowel. Spellings with a double t become common in the later fourteenth century. The two forms then specialized. Later kept the open comparative of time: later this week, a later chapter. Latter took over the sense the second of two already named, recorded for later itself by the 1550s but now almost entirely latter’s job. Set phrases such as latter days still show the older nearer-the-end meaning.
The twentieth century added a social use rather than an etymological one. From 1954, American colloquial English uses later as a farewell, shortened from I will see you later. The adverb later on, despite how natural it sounds, is historically redundant: later already meant afterward.
One leftover from the split
If you want a single fact that explains why English has both later and latter, it is the Middle English vowel. The same comparative of late produced a long-vowel form that became later and a short-vowel, double-t form that became latter. Dutch never made that split: later there is still one word for both afterward and the comparative of laat. English kept two spellings and, eventually, two jobs.