Others have performed and complied wonderfully detailed histories of the Explorer line, which I will summarize here.
The story of the Explorer began with two earlier bubblebacks, the references 6098 and 6150 (housing the calibre A296), which were utilized by adventurers and mountaineers at the beginning of the 1950’s. They did not bear the name Explorer, however, nor did they have the arabic 3-6-9 on the dial that later became synonymous with the model line. Only in 1953, when the subsequent references 6298 and 6350 were introduced, were they fitted with the typical “explorer” dial; the Mercedes hands were also added slightly later.
When Sir Edmund Hillary and Sherpa Tenzing Norgay climbed Mount Everest on May 29, 1953, Hillary wore a Smith’s watch and a Rolex Oyster and Norgay wore a Rolex reference 6350. At the time of the climb, no Rolex watch was marketed as the Explorer, however Rolex had registered the name as a trademark in Geneva on January 26th of that year. After their successful climb, Rolex commemorated the feat by naming the 6350 the “Explorer.” This was the official start of the model line.
The 6350 gave way quickly to the 6150, which was then replaced by reference 6610. The 6150 and 6610 are very similar but are distinguished by the newer 1030 movement in the 6610, which led to a flatter case back. The 6610 itself was short-lived, only lasting for a few years in the late 1950’s. Rolex continued to advance their technology rapidly and, with the new 15xx calibre movement line, they introduced new Submariners, GMTs, and Explorers in 1959 — models 5512, 1675, and 1016, respectively. With these references Rolex would settle down and concentrate on small tweaks over the next 20-30 years. These models would become the workhorses of the sports line and, because of their excellence in design and construction, have become sought after by collectors.
Side Note: Rolex produced some great advertisements over the years that are icons in their own right. My friend Nick loves these vintage ads and sources them for collectors to match with their watches. If you’re interested, check some of them out here (I don’t get anything from you going there or buying ads, I just think they’re cool).