In an appendix to John Napier's work on logarithms in 1618, it served as the base for a table that gave the natural logarithms of various numbers, but it's role was not recognized.
In 1661, Huygens investigated the relation between the area under the rectangular hyperbola yx = 1 and the logarithm...but did he ask the value of x if the area from 1 to x is 1...a very defining moment if he had done so!
In 1683, Jacob Bernoulli investigated continuous compounding interest, trying to find the limit of (1 + 1/n)n as n goes to infinity...but was only able to show that this limit lay between 2 and 3.
In 1690, Leibniz's letter to Huygens used the notation b for what we now call "?"...which was its start as a special number.
The actual letter "?" made its first appearance in a 1731 letter from Euler to Goldbach...and it was not used because it was the first letter of Euler's name or even the first letter of the word exponential.
This is a repeat from seven years ago....