Page joined The Yardbirds in '66 on bass after Paul Samwell-Smith left. Pretty soon he switched from bass to second lead with Jeff Beck. When Jeff left the group (October '66) "The Yardbirds" were falling apart. Page wanted to form a supergroup - himself and Jeff Beck on guitars, with Keith Moon and John Entwistle. For singers they looked at the likes of Steve's Winwood and Marriott, but the group didn't happen, though Page, Beck and Moon did record "Beck's Bolero" in '66, along with John Paul Jones, who expressed an interest in taking it further.
"The Yardbirds" last gig was July '68, but contractual obligations for concerts in Scandinavia meant Page along with bass Chris Dreja, hooked up with Plant and Bonham to play them. Dreja bailed and John Paul Jones came on board to fill his slot.
This line up did a session for PJ Proby and finished the Scandanavian tour as "The New Yardbirds" - and this was the name they used when I first saw them at the Marquee late in '68 - shortly afterwards somebody prevented them from using the "Yardbirds" part of the name, and they came up with "Lead Zeppelin" - which Peter G shortened to "Led Zeppelin" so the Americans would pronounce it right.....
The rest (as they say) is history.