1. A Scroll Takes Time to Unroll, but a Bound Book Opens Anywhere
In the early first century, books were mostly rolls of papyrus. By late antiquity, most books were codices: sheets bound together at one edge, "not very different from the books we still use today," as the British Library puts it [1]. To read the middle of a scroll, you unwind everything before it. In a codex you simply open the book there.
Why people switched is still debated. The British Library notes that the evidence is thin and comes mostly from Egypt, and that literary works in Greek were still written mostly on rolls until the fourth century [1]. So it would be too simple to say that readers wanted to find things faster. Still, once books could be opened in the middle, the idea of "go to a particular spot" had a natural home. A number was not needed yet, though. That is the puzzle: the shape of the book arrived first, and the numbers came much later.
2. Before Readers Had Page Numbers, Binders Had Their Own Marks
Handmade books did have a problem with order. Pages were written on folded sheets, and a bundle of these folded sheets, called a quire, was often eight leaves. Someone had to put the quires in the right sequence when binding the book. Medieval binders used a trick called a catchword: the opening word of the next quire, written at the bottom of the last page of the current quire [4]. If that catchword did not match the first word of the quire that followed, something was out of order.
These marks were tools for the person making the book, not for the person reading it. A reader who wanted a friend to look at "the paragraph near the middle" had no number to give. Even where leaves were numbered, the numbers were written by hand, and they were rare until the end of the Middle Ages [3]. Because each handwritten copy of a book was a little different, a number in one copy would not reliably point to the same text in another copy.
3. The Oldest Surviving Printed Page Numbers Are in a Small Book from Cologne
A printed book makes numbers far more useful, because copies from the same edition generally have the same text in the same place. The oldest surviving book we know of with printed numbers was printed in Cologne by Arnold Ther Hoernen and dated 1470. It is a short Latin sermon of 11 leaves, with the numbers 1 to 11 printed in the right margin, about halfway down the page [2][3].
Notice the words "oldest surviving." Nobody can say that no earlier printed book had numbers, only that none that survives and is catalogued does. The year comes from a note printed in the book itself and recorded in the catalogue [2].
Also notice what the numbers count. They number leaves, meaning one number for each leaf, which has a front and a back. This is called foliation. Counting each side separately, as we do today, is called pagination. Both are loosely called "page numbers," but they are not the same system.
4. What Made Printers Number Every Page: Finding Words in an Index
Printed numbers did not catch on right away. Even after 1470, leaf numbers did not become standard in the 1400s, and printing page numbers only became a common habit in the second quarter of the sixteenth century [3]. What changed?
One answer is the index. An index is a list that says where in a book a word or topic appears, and an index is only useful if there is a number to point to. A Japanese study of early printed books is reported to say that the Venetian printer Aldus Manutius numbered each side of the page in 1499 in connection with an index, and that the Basel printer Froben used the same style of numbering in 1515 [5]. I could not read the full text of that study while preparing this page, so treat the 1499 and 1515 details, and any claim of being "first," as the study's account rather than something I checked independently.
The pattern is still clear. The numbers did not appear the moment the codex did. Page numbers let readers cite a page and find a place [6], and printing made matching copies and indexes possible, so it seems likely that these things are connected. That link is my inference, not a finding from the sources.
5. Try It on a Book You Own: Find Where the Numbers Start and Look Up a Word
Pick a book from your shelf. Check where the page number is printed: at the bottom center, or in an outer corner? Count how far into the book you have to go before the first number appears. Many books leave the first few pages unnumbered, and some use small Roman numerals (i, ii, iii) for the opening pages and start the Arabic numbers 1, 2, 3 later [6].
Then, if the book has an index at the back, look up a word that interests you and turn to the page it lists. You have just used the combination that printers began putting together around 1499. If you want to go further, the library catalogue entry for the 1470 sermon and the studies listed below are good places to start.